CIBSE - TM26 - Hygienic Maintenance of Office Ventilation Ductwork

Hygienic maintenance of office ventilation ductwork CIBSE TM26: 2000

The Chartered Institution of Building Services Engineers 222 Balham High Road, London SW12 9BS

The rights of publication or translation are reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior permission of the Institution. ©October 2000 The Chartered Institution of Building Services Engineers London Registered charity number 278104 ISBN 1 903287 11 1 This document is based on the best knowledge available at the time of publication. However no responsibility of any kind for any injury, death, loss, damage or delay however caused resulting from the use of these recommendations can be accepted by the Chartered Institution of Building Services Engineers, the authors or others involved in its publication. In adopting these recommendations for use each adopter by doing so agrees to accept full responsibility for any personal injury, death, loss, damage or delay arising out of or inconnection with their use by or on behalf of such adopter irrespective of the cause or reason therefore and agrees to defend, indemnify and hold harmless the Chartered Institution of Building Services Engineers, the authors and others involved in their publication from any and all liability arising out of or in connection with such use as aforesaid and irrespective of any negligence on the part of those indemnified. Note from the publisher: This publication is primarily intended to provide guidance to those responsible for the design, installation, commissioning, operation and maintenance of building services. It is not intended to be exhaustive or definitive and it will be necessary for users of the guidance given to exercise their own professional judgement when deciding whether to abide by or depart from it. Typeset by CIBSE Publications Department Reprinted in England by Intype Libra Ltd., London SW19 4HE

Cover: scanning electron micrograph of bacteria Micrococcus roseus © John Forsdyke/Oxford Scientific Films

Foreword Building ventilation systems are seldom at the forefront of owners’ or property managers’ concerns. They may only give them attention in response to problems or a need to consider statutory responsibilities for maintaining the workplace. However, a clean and effective ventilation system may help to maintain workplace productivity and avoid potential health problems. TM26 aims to provide building managers with a method of assessing the microbial cleanliness of mechanical ventilation and air conditioning air supply ductwork. This assessment will inform the manager of any need to inspect the ventilation systems further, or to clean them. It will also help to assess residual contamination after ductwork has been cleaned. This publication has been prepared by the Chartered Institution of Building Services Engineers (CIBSE) to complement the Building Services Research and Information Association (BSRIA) guidance on ventilation system hygiene, and advice given by the Heating and Ventilation Contractors Association (HVCA) in their Guide to Good Practice on ductwork hygiene. This publication is the outcome of a study commissioned by the CIBSE. Funding support from the DETR was provided under the Partners in Innovation (PII) scheme. This scheme provides partial support for industry-led initiatives such as this research project. PII initiatives provide both government and industry with valuable perspectives on important issues. The DETR contribution to funding does not imply that the views expressed in published outcomes are necessarily accepted or endorsed by the DETR. The study was also supported by the HVCA through the participation of members of the HVCA Ventilation Hygiene Group Branch. The work was carried out by BRE under contract to CIBSE.

Technical contractor Building Research Establishment Ltd.

Authors Dr C A Hunter (Building Research Establishment Ltd.) P F Grigg (Building Research Establishment Ltd.) J T Smith (Building Research Establishment Ltd.)

Steering Group B Franklin (Chairman) Dr H Davies (CIBSE Research Manager) Dr L Fothergill (DETR Project Officer) C Booth (System Hygienics Ltd., representing HVCA) G Nicholls (Swiftclean (UK) Ltd., representing HVCA) R Yarham (CIBSE Publishing Manager) (until September 1999)

Expert Panel Dr B Crook Dr B Flannigan Dr W D Griffiths Dr A Price Dr D Telford Dr K Brown

Contents 1

Executive summary

1

2

Legislation

1

3

The status of TM26

2

3.1

With respect to health and safety regulations

2

3.2

TM26 in the context of other guidance

2

4

How to use this guidance

2

5

Ductwork hygiene indicators

3

5.1

Approach

3

5.2

Sampling methods

4

Air quality in buildings

5

6.1

Indoor pollutants

5

6.2

Microbial contaminants

5

6.3

Airborne microbial contaminants

6

6.4

Microbial contamination in buildings and air distribution systems

6

6.5

Health aspects of microbes in buildings

7

6

7

8

9

10

Cleaning air distribution ductwork systems

7

7.1

Mechanical cleaning methods

7

7.2

Use of biocides

7

7.3

Reducing the admission of micro-organisms

8

Protocol for sampling air in occupied spaces

9

8.1

Personnel

9

8.2

Equipment

9

8.3

Laboratory

9

8.4

Safety

9

8.5

Methodology

9

Protocol for surface sampling

10

9.1

Personnel

10

9.2

Equipment

10

9.3

Laboratory

11

9.4

Safety

11

9.5

Methodology

11

Interpretation of sampling results

12

10.1

Air sampling microbial limits

12

10.2

Surface sampling microbial limits

12

10.3

Actions following interpretation of the sampling results

13

References

13

Appendix 1: Glossary of microbiological terms

14

Appendix 2: Results of micobial sampling in indoor air and the ventilation ductwork at a selection of UK buildings

15

Appendix 3: Air duct systems

21

Appendix 4: Expert Panel members

22

1

Hygienic maintenance of office ventilation ductwork 1

Executive summary

The hygiene of ventilation system ductwork is often given less attention than it should receive. Much of the system is inaccessible and usually invisible during day-to-day operations. The consequences of poor hygienic maintenance are not readily apparent and there is little knowledge about the impact of ventilation system hygiene on the performance and working efficiency of the building occupants. This publication aims to demonstrate to building managers the importance of ductwork maintenance and to provide practical guidance on the proper procedures for maintaining ductwork systems in a safe and effective state. Recent publications from the Heating and Ventilation Contractors Association (HVCA) and Building Services Research and Information Association (BSRIA) have provided a general review of the issues of ventilation system hygiene and ductwork cleaning. BSRIA has also produced a standard form of contract for ductwork cleaning, with associated guidance. These documents give specific guidance on good cleaning and maintenance practices for ductwork systems in terms of particulate (dust) contamination only. The guidance contained in this publication is intended to add to that good practice by providing guidance for managers of buildings that are air conditioned, or otherwise mechanically ventilated, on the issues of assessment and maintenance of the microbiological cleanliness of ductwork systems. For those who are unfamiliar with current practice in air duct systems, a separate section reviews this subject. This publication gives guidance on the methods to use when obtaining microbial samples from the air in occupied spaces and from the inside surfaces of the ducts. It also indicates the levels of microbial contamination that are likely to be found on uncleaned and cleaned duct surfaces, and draws on a body of expert opinion to help the building manager to interpret the results of sampling. The procedure may also be used to assess the residual microbial contamination following a commercial cleaning process, or following disinfection, and as a remote means of assessing the performance of filtration of ventilation air. Carrying out a programme of regular assessment of the microbial activity in the building and air distribution ductwork will help the manager to fulfil his/her duty of care to the building occupants. This guidance has been developed for conditions expected in office environments. The general principles may apply to other building sectors but it should be noted that some environments, such as certain hospital or clean room areas, would require more stringent hygiene standards. Also, environments with a higher occupancy rate or higher exposure to outdoor conditions, such as many retail environments, may naturally display higher airborne contaminant levels.

This publication, together with the HVCA and BSRIA documents, provides a comprehensive toolkit for the management and maintenance of most ductwork systems to satisfy current best practice requirements.

2

Legislation

Building services, and their operation, are expected by current legislation to provide a comfortable and safe indoor environment for occupants. Relevant legislation includes: —

The Health and Safety at Work Act 1974(1)

—

The Management of Health and Safety at Work Regulations 1999(2)

—

The Workplace (Health, Safety and Welfare) Regulations 1992(3)

—

The Control of Substances Hazardous to Health Regulations 1991(4) (COSHH)*

The legislation is increasingly being interpreted as addressing the quality of air provided for the occupants of buildings in terms of taking steps to prevent its contamination by potentially harmful gases and dusts. The Approved Code of Practice and Guidance to the Workplace (Health, Safety and Welfare) Regulations(5), for example, states that mechanical ventilation systems, including air conditioning systems, should be regularly and properly cleaned, tested and maintained to ensure that they are kept clean and free of anything which may contaminate the air. A number of studies world-wide have suggested that high levels of microbial contamination in air samples taken inside a building can be associated with microbial growths in the air distribution ducts or on the fabric of the building. Some countries have attempted to set standards for this component of air quality in terms of the numbers of either fungal or bacterial colony forming units (cfu) present in samples of indoor air. Such standards or working levels must be treated with caution, however, as there are little clinical or epidemiological data on the health implications of exposure to these levels. The United States Occupational Safety and Health Administration (OSHA) has proposed an indoor air quality standard that is protected under the OSHA Act but is based on proactive preventative maintenance. North American and European private professional and government agencies have recommended guidelines for microbes ranging from less than 100 to greater than 1000 cfu·m–3 air. These recommendations have usually been based on personal experiences or on the results of particular surveys. They either specify ‘absolute’ levels (a total number of * The 1999 COSHH Regulations came into effect on 25 March 1999, with all previous versions and amendments being revoked.

1

Hygienic maintenance of office ventilation ductwork 1

Executive summary

The hygiene of ventilation system ductwork is often given less attention than it should receive. Much of the system is inaccessible and usually invisible during day-to-day operations. The consequences of poor hygienic maintenance are not readily apparent and there is little knowledge about the impact of ventilation system hygiene on the performance and working efficiency of the building occupants. This publication aims to demonstrate to building managers the importance of ductwork maintenance and to provide practical guidance on the proper procedures for maintaining ductwork systems in a safe and effective state. Recent publications from the Heating and Ventilation Contractors Association (HVCA) and Building Services Research and Information Association (BSRIA) have provided a general review of the issues of ventilation system hygiene and ductwork cleaning. BSRIA has also produced a standard form of contract for ductwork cleaning, with associated guidance. These documents give specific guidance on good cleaning and maintenance practices for ductwork systems in terms of particulate (dust) contamination only. The guidance contained in this publication is intended to add to that good practice by providing guidance for managers of buildings that are air conditioned, or otherwise mechanically ventilated, on the issues of assessment and maintenance of the microbiological cleanliness of ductwork systems. For those who are unfamiliar with current practice in air duct systems, a separate section reviews this subject. This publication gives guidance on the methods to use when obtaining microbial samples from the air in occupied spaces and from the inside surfaces of the ducts. It also indicates the levels of microbial contamination that are likely to be found on uncleaned and cleaned duct surfaces, and draws on a body of expert opinion to help the building manager to interpret the results of sampling. The procedure may also be used to assess the residual microbial contamination following a commercial cleaning process, or following disinfection, and as a remote means of assessing the performance of filtration of ventilation air. Carrying out a programme of regular assessment of the microbial activity in the building and air distribution ductwork will help the manager to fulfil his/her duty of care to the building occupants. This guidance has been developed for conditions expected in office environments. The general principles may apply to other building sectors but it should be noted that some environments, such as certain hospital or clean room areas, would require more stringent hygiene standards. Also, environments with a higher occupancy rate or higher exposure to outdoor conditions, such as many retail environments, may naturally display higher airborne contaminant levels.

This publication, together with the HVCA and BSRIA documents, provides a comprehensive toolkit for the management and maintenance of most ductwork systems to satisfy current best practice requirements.

2

Legislation

Building services, and their operation, are expected by current legislation to provide a comfortable and safe indoor environment for occupants. Relevant legislation includes: —

The Health and Safety at Work Act 1974(1)

—

The Management of Health and Safety at Work Regulations 1999(2)

—

The Workplace (Health, Safety and Welfare) Regulations 1992(3)

—

The Control of Substances Hazardous to Health Regulations 1991(4) (COSHH)*

The legislation is increasingly being interpreted as addressing the quality of air provided for the occupants of buildings in terms of taking steps to prevent its contamination by potentially harmful gases and dusts. The Approved Code of Practice and Guidance to the Workplace (Health, Safety and Welfare) Regulations(5), for example, states that mechanical ventilation systems, including air conditioning systems, should be regularly and properly cleaned, tested and maintained to ensure that they are kept clean and free of anything which may contaminate the air. A number of studies world-wide have suggested that high levels of microbial contamination in air samples taken inside a building can be associated with microbial growths in the air distribution ducts or on the fabric of the building. Some countries have attempted to set standards for this component of air quality in terms of the numbers of either fungal or bacterial colony forming units (cfu) present in samples of indoor air. Such standards or working levels must be treated with caution, however, as there are little clinical or epidemiological data on the health implications of exposure to these levels. The United States Occupational Safety and Health Administration (OSHA) has proposed an indoor air quality standard that is protected under the OSHA Act but is based on proactive preventative maintenance. North American and European private professional and government agencies have recommended guidelines for microbes ranging from less than 100 to greater than 1000 cfu·m–3 air. These recommendations have usually been based on personal experiences or on the results of particular surveys. They either specify ‘absolute’ levels (a total number of * The 1999 COSHH Regulations came into effect on 25 March 1999, with all previous versions and amendments being revoked.

2

Hygenic maintenance of office ventilation ductwork

specific fungi) that is considered acceptable or not acceptable, or ‘relative’ levels, which are based on the relationship between indoor and outdoor levels for simultaneously collected samples. In the latter case it is assumed that an indoor level lower than the outdoor level is acceptable. At the time of publication, the only governmental agency known to produce binding quantitative regulations for bioaerosols is that of the Russian Federation. The USA, which has played a significant role in generating research interest in sick building syndrome (SBS), has no official quantitative standard. Based on work in Canada, Miller et al. have suggested that as some pathogenic or toxigenic fungi are not acceptable in indoor air, counts higher than 50 cfu of a single species per cubic metre are a concern, requiring further investigation. Counts less than 150 cfu·m–3 would be acceptable if there is a mixture of species other than pathogens and certain toxigenic species and counts up to 300 cfu·m–3 would be acceptable if the species present are mainly Cladosporium and other common phylloplane fungi. It is worth noting that these levels are lower than would be found in many homes in the UK.

3 3.1

considered by the contributors to represent good practice in executing a duty of care for building occupants by monitoring the microbial contamination in the ductwork systems. It must be considered, however, that contaminants in the air in occupied spaces may not all originate from the air distribution systems. Also, in very exceptional circumstances, the growth of micro-organisms can be unpredictable and rapid and could be overlooked by even the most rigorous routine testing and inspection procedures. The probability that such circumstances would ever occur is normally minimised by ensuring the mechanical and functional integrity of the ductwork system and any components associated with it, such as filters, cooling coils, humidifiers, eliminators and attenuators.

3.2

In addressing the microbiological contamination of office ventilation ductwork systems, TM26 complements: —

HVCA Guide to Good Practice TR17: Cleanliness of ventilation systems(8), which proposes standards of cleanliness in ventilation ductwork in terms of visible dust deposition, and

—

BSRIA Facilities Management Specification FMS 1/97: Guidance and the standard specification for ventilation hygiene(9), which proposes forms of contract to assure ventilation hygiene.

The status of TM26 With respect to health and safety regulations

Current legislation often states that ‘reasonable’ provisions should be made to ensure the achievement of comfortable and safe environments, and refers the reader to professional guidance, including the CIBSE, to interpret the requirements. CIBSE Guidance Note GN2: Healthy Workplaces: Guidance on complying with the 1992 health and safety regulations(7) provided much general guidance on the interpretation of the legislative requirements. However, the hygienic maintenance of air distribution ductwork in respect of microbial cleanliness can affect the air supplied to office spaces but there has previously been no consensus professional guidance on the subject. TM26 provides professional guidance for building managers. It sets out a procedure for addressing the microbiological cleanliness of ductwork systems that can be carried out by the building manager or his/her agent. When included within the manager’s normal good practice provisions, such as those set out in the HVCA Guide to Good Practice TR17(8), to ensure (visible dust) cleanliness, the procedure is itself intended to represent good practice in the maintenance of the microbiological hygiene of the ductwork systems. This publication proposes a method of sampling for microbiological contaminants in office spaces and ductwork systems and proposes acceptable targets for microbiological activity which may be used in inspection programmes to decide whether cleaning is necessary on microbiological grounds. In maintenance programmes it may be used to assess residual contamination following commercial cleaning operations. The sampling procedures proposed are

TM26 in the context of other guidance

In addition, a number of agencies have proposed standards for the (visible dust) cleaning of air supply ductwork, notably: —

the North American Duct Cleaning Association (NADCA): Mechanical cleaning of non-porous air conveyance system components(10)

—

the Swedish National Board of Housing, Building and Planning: General Guidelines 1992:E: Checking the performance of ventilation systems(11).

4

How to use this guidance

The procedures described herein are intended to help a manager to assess the microbial content of the air entering spaces served by air distribution ductwork, and the degree of microbial contamination of the inner surfaces of the ductwork. Such assessment is intended to complement the normal good practice regime of regular inspection and maintenance described in HVCA Guide to Good Practice TR17: Cleanliness of ventilation systems(8). This recommends annual inspection and testing of the physical condition and cleanliness of air distribution systems and components in terms of such factors as dust deposition and the presence of moisture. The guidance contained herein assumes that filtration is installed to at least EU7 standard. Inferior levels of filtration would not remove many of the microbial contaminants present in outdoor air, and are thus unlikely to provide sufficiently low levels of microbial activity in ductwork systems.

2

Hygenic maintenance of office ventilation ductwork

specific fungi) that is considered acceptable or not acceptable, or ‘relative’ levels, which are based on the relationship between indoor and outdoor levels for simultaneously collected samples. In the latter case it is assumed that an indoor level lower than the outdoor level is acceptable. At the time of publication, the only governmental agency known to produce binding quantitative regulations for bioaerosols is that of the Russian Federation. The USA, which has played a significant role in generating research interest in sick building syndrome (SBS), has no official quantitative standard. Based on work in Canada, Miller et al. have suggested that as some pathogenic or toxigenic fungi are not acceptable in indoor air, counts higher than 50 cfu of a single species per cubic metre are a concern, requiring further investigation. Counts less than 150 cfu·m–3 would be acceptable if there is a mixture of species other than pathogens and certain toxigenic species and counts up to 300 cfu·m–3 would be acceptable if the species present are mainly Cladosporium and other common phylloplane fungi. It is worth noting that these levels are lower than would be found in many homes in the UK.

3 3.1

considered by the contributors to represent good practice in executing a duty of care for building occupants by monitoring the microbial contamination in the ductwork systems. It must be considered, however, that contaminants in the air in occupied spaces may not all originate from the air distribution systems. Also, in very exceptional circumstances, the growth of micro-organisms can be unpredictable and rapid and could be overlooked by even the most rigorous routine testing and inspection procedures. The probability that such circumstances would ever occur is normally minimised by ensuring the mechanical and functional integrity of the ductwork system and any components associated with it, such as filters, cooling coils, humidifiers, eliminators and attenuators.

3.2

In addressing the microbiological contamination of office ventilation ductwork systems, TM26 complements: —

HVCA Guide to Good Practice TR17: Cleanliness of ventilation systems(8), which proposes standards of cleanliness in ventilation ductwork in terms of visible dust deposition, and

—

BSRIA Facilities Management Specification FMS 1/97: Guidance and the standard specification for ventilation hygiene(9), which proposes forms of contract to assure ventilation hygiene.

The status of TM26 With respect to health and safety regulations

Current legislation often states that ‘reasonable’ provisions should be made to ensure the achievement of comfortable and safe environments, and refers the reader to professional guidance, including the CIBSE, to interpret the requirements. CIBSE Guidance Note GN2: Healthy Workplaces: Guidance on complying with the 1992 health and safety regulations(7) provided much general guidance on the interpretation of the legislative requirements. However, the hygienic maintenance of air distribution ductwork in respect of microbial cleanliness can affect the air supplied to office spaces but there has previously been no consensus professional guidance on the subject. TM26 provides professional guidance for building managers. It sets out a procedure for addressing the microbiological cleanliness of ductwork systems that can be carried out by the building manager or his/her agent. When included within the manager’s normal good practice provisions, such as those set out in the HVCA Guide to Good Practice TR17(8), to ensure (visible dust) cleanliness, the procedure is itself intended to represent good practice in the maintenance of the microbiological hygiene of the ductwork systems. This publication proposes a method of sampling for microbiological contaminants in office spaces and ductwork systems and proposes acceptable targets for microbiological activity which may be used in inspection programmes to decide whether cleaning is necessary on microbiological grounds. In maintenance programmes it may be used to assess residual contamination following commercial cleaning operations. The sampling procedures proposed are

TM26 in the context of other guidance

In addition, a number of agencies have proposed standards for the (visible dust) cleaning of air supply ductwork, notably: —

the North American Duct Cleaning Association (NADCA): Mechanical cleaning of non-porous air conveyance system components(10)

—

the Swedish National Board of Housing, Building and Planning: General Guidelines 1992:E: Checking the performance of ventilation systems(11).

4

How to use this guidance

The procedures described herein are intended to help a manager to assess the microbial content of the air entering spaces served by air distribution ductwork, and the degree of microbial contamination of the inner surfaces of the ductwork. Such assessment is intended to complement the normal good practice regime of regular inspection and maintenance described in HVCA Guide to Good Practice TR17: Cleanliness of ventilation systems(8). This recommends annual inspection and testing of the physical condition and cleanliness of air distribution systems and components in terms of such factors as dust deposition and the presence of moisture. The guidance contained herein assumes that filtration is installed to at least EU7 standard. Inferior levels of filtration would not remove many of the microbial contaminants present in outdoor air, and are thus unlikely to provide sufficiently low levels of microbial activity in ductwork systems.

2

Hygenic maintenance of office ventilation ductwork

specific fungi) that is considered acceptable or not acceptable, or ‘relative’ levels, which are based on the relationship between indoor and outdoor levels for simultaneously collected samples. In the latter case it is assumed that an indoor level lower than the outdoor level is acceptable. At the time of publication, the only governmental agency known to produce binding quantitative regulations for bioaerosols is that of the Russian Federation. The USA, which has played a significant role in generating research interest in sick building syndrome (SBS), has no official quantitative standard. Based on work in Canada, Miller et al. have suggested that as some pathogenic or toxigenic fungi are not acceptable in indoor air, counts higher than 50 cfu of a single species per cubic metre are a concern, requiring further investigation. Counts less than 150 cfu·m–3 would be acceptable if there is a mixture of species other than pathogens and certain toxigenic species and counts up to 300 cfu·m–3 would be acceptable if the species present are mainly Cladosporium and other common phylloplane fungi. It is worth noting that these levels are lower than would be found in many homes in the UK.

3 3.1

considered by the contributors to represent good practice in executing a duty of care for building occupants by monitoring the microbial contamination in the ductwork systems. It must be considered, however, that contaminants in the air in occupied spaces may not all originate from the air distribution systems. Also, in very exceptional circumstances, the growth of micro-organisms can be unpredictable and rapid and could be overlooked by even the most rigorous routine testing and inspection procedures. The probability that such circumstances would ever occur is normally minimised by ensuring the mechanical and functional integrity of the ductwork system and any components associated with it, such as filters, cooling coils, humidifiers, eliminators and attenuators.

3.2

In addressing the microbiological contamination of office ventilation ductwork systems, TM26 complements: —

HVCA Guide to Good Practice TR17: Cleanliness of ventilation systems(8), which proposes standards of cleanliness in ventilation ductwork in terms of visible dust deposition, and

—

BSRIA Facilities Management Specification FMS 1/97: Guidance and the standard specification for ventilation hygiene(9), which proposes forms of contract to assure ventilation hygiene.

The status of TM26 With respect to health and safety regulations

Current legislation often states that ‘reasonable’ provisions should be made to ensure the achievement of comfortable and safe environments, and refers the reader to professional guidance, including the CIBSE, to interpret the requirements. CIBSE Guidance Note GN2: Healthy Workplaces: Guidance on complying with the 1992 health and safety regulations(7) provided much general guidance on the interpretation of the legislative requirements. However, the hygienic maintenance of air distribution ductwork in respect of microbial cleanliness can affect the air supplied to office spaces but there has previously been no consensus professional guidance on the subject. TM26 provides professional guidance for building managers. It sets out a procedure for addressing the microbiological cleanliness of ductwork systems that can be carried out by the building manager or his/her agent. When included within the manager’s normal good practice provisions, such as those set out in the HVCA Guide to Good Practice TR17(8), to ensure (visible dust) cleanliness, the procedure is itself intended to represent good practice in the maintenance of the microbiological hygiene of the ductwork systems. This publication proposes a method of sampling for microbiological contaminants in office spaces and ductwork systems and proposes acceptable targets for microbiological activity which may be used in inspection programmes to decide whether cleaning is necessary on microbiological grounds. In maintenance programmes it may be used to assess residual contamination following commercial cleaning operations. The sampling procedures proposed are

TM26 in the context of other guidance

In addition, a number of agencies have proposed standards for the (visible dust) cleaning of air supply ductwork, notably: —

the North American Duct Cleaning Association (NADCA): Mechanical cleaning of non-porous air conveyance system components(10)

—

the Swedish National Board of Housing, Building and Planning: General Guidelines 1992:E: Checking the performance of ventilation systems(11).

4

How to use this guidance

The procedures described herein are intended to help a manager to assess the microbial content of the air entering spaces served by air distribution ductwork, and the degree of microbial contamination of the inner surfaces of the ductwork. Such assessment is intended to complement the normal good practice regime of regular inspection and maintenance described in HVCA Guide to Good Practice TR17: Cleanliness of ventilation systems(8). This recommends annual inspection and testing of the physical condition and cleanliness of air distribution systems and components in terms of such factors as dust deposition and the presence of moisture. The guidance contained herein assumes that filtration is installed to at least EU7 standard. Inferior levels of filtration would not remove many of the microbial contaminants present in outdoor air, and are thus unlikely to provide sufficiently low levels of microbial activity in ductwork systems.

Ductwork hygiene indicators

3

It should be stressed that this assessment of microbiological contamination is intended to add to, but in no way replace, the good practice assessment of other nonmicrobial contaminants described in HVCA TR17(8). Such good maintenance practice is essential and will generally reduce the likelihood of microbial contamination becoming a concern, as micro-organisms generally require moisture and nutrient to grow. However, the building manager should obtain the additional reassurance that ductwork systems are clean in terms of microbial contamination, and that the microbial content of the air supplied to spaces is within acceptable limits. An assessment of the microbial hygiene of ductwork surfaces may be carried out at the same time as the physical inspection described. This will often be convenient as the system will be stopped and access points opened to determine dust deposition levels. An alternative would be to undertake air contaminant assessment on a regular basis

Have the working spaces supplied by AHU been assessed for microbiol loading in the last 12 months?

and, where the microbial content of the air indicates a need for further inspection, follow up with inspection and assessment of ductwork surface contamination. It is important that clear records are maintained of the results of microbial sampling both in the occupied spaces and in the ductwork. This will allow the manager to observe natural fluctuations in the levels of contaminant and help to identify abnormal variations which may require further investigation. The flowchart (Figure 1) demonstrates how the microbial assessment would form part of the risk assessment procedure proposed in the BSRIA Facilities Management Specification Guidance and the standard specification for ventilation hygiene(9). Microbial sampling may be carried out following mechanical cleaning of ductwork systems. In this instance it is

Yes

No Is the microbial monitoring being conducted at the same time as inspection of ductwork to HVCA TR17 requirements?

Yes

Is the AHU working correctly?

No

Correct AHU fault

Decide on next microbial review date. Ensure HVCA TR17 procedures are followed

No

No

Yes

Is the AHU working correctly? Yes

Is the filtration in the AHU effective?

No

Repair housing and/ or replace filter

No

Yes

Is the filtration in the AHU effective? Yes

Conduct air and surface sampling

Conduct air sampling Ensure HVCA TR17 surface deposit limits are not exceeded and decide on next microbial review date

Are the air and surface counts acceptable? Refer to section 10.2

Yes

Yes

No Conduct surface sampling of AHU and duct work

No Refer to Section 10.3 for interpretation, including the need for further investigation or cleaning

Are the counts acceptable? Refer to section 10.1

Yes No

Note: For reasons of economy it is recommended that the HVCA TR/17 and CIBSE TM26 procedures are normally carried out at the same time.

Are the surface counts acceptable? Refer to Section 10.2

Figure 1 Flowchart for the hygienic maintenance of office ventilation ductwork

4

Hygenic maintenance of office ventilation ductwork

important to note that the presence of microbial contaminants on duct surfaces does not necessarily indicate that the normal cleaning process has been unsatisfactory. Mechanical cleaning is specifically a process of loosening and removing particulate dust contamination, and HVCA Guide to Good Practice TR17(8) describes ways of determining the effectiveness of that cleaning operation. Where mechanical cleaning leaves an unacceptable level of microbiological contamination, further investigation would be required to determine the source of the contaminant. This may, for instance, prove to be some mechanical failure in the air handling plant which causes outdoor air to by-pass the installed filtration. If the source is considered to be microbial growth inside the ductwork, then an alternative decontamination process is required which may include the use of biocides.

5

Ductwork hygiene indicators

5.1

Approach

This publication proposes protocols for microbial sampling and measurement inside ductwork and the occupied spaces it serves. The protocols are designed to provide, ideally, early warning of changes in the microbial loading of air supply ductwork that may adversely influence air quality. It is anticipated that such sampling would be carried out in addition to the good practice inspection, maintenance and cleaning of systems recommended in HVCA TR17(8). It is beyond the scope of this publication, and is not normally considered possible or practical, to define particular levels of microbial contamination that are considered safe or unsafe for building occupants. In general, however, specialists asked to investigate particular suspected microbiological problems employ two approaches. One is qualitative and the other is quantitative. 5.1.1

Qualitative

The qualitative approach involves sampling, culturing (growing) and identifying specific micro-organisms in a search for those known to be associated with particular problems. If the number of organisms is low, or there are none of the specified organisms present then it is commonly assumed that a microbial cause is unlikely. If the specified organisms are present and above critical levels, then they should be regarded as potential causes of the known problems. However, the presence of the organisms is not sufficient to indicate that there will be a problem. Sampling and monitoring for particular micro-organisms known to indicate a hazard to occupants would normally be impractical as a procedure for use on a regular (routine) basis. Samples would need to be analysed for the presence of an extensive list of particular micro-organisms, which would be prohibitively expensive. The risks would remain that the list of microbes may be incomplete, and that such microbes may become apparent only after a hygiene problem has already manifested itself. Even if there is a problem micro-organism, it may be present in low or

undetectable amounts so that negative results would not always offer reassurance. 5.1.2

Quantitative

The quantitative approach involves sampling and counting the total number of microbial cells or spores present, normally referred to as the total viable content (TVC) in the sample. As the main objective of this publication is to propose a system of sampling which provides an indication of the hygiene of ductwork under normal operating conditions, it is considered appropriate to set guidelines that are based on the TVC. Qualitative analysis may still be useful to follow up high TVC measurements, or if an air quality problem is known to exist. It must be noted that airborne sampling can only give a snapshot view of conditions during that sampling period. Outside, airborne micro-organism counts can vary by several orders of magnitude from hour to hour, day to day, and season to season. A general equipment inspection is recommended as part of good practice and may detect faults and indicate conditions (such as the presence of moisture or excessive dust contamination) in which poor air hygiene may arise.

5.2

Sampling methods

When selecting a sampler for collecting microbial material from air, it is important to consider the particular circumstances in which it will be used. In obtaining the series of samples from buildings which formed part of the background study for this publication, it was clear that the sampler selected must be battery powered as many of the samples would be taken at locations where there is no electricity supply. It is also necessary to take a number of samples during each site examination, and to minimise the need for handling in the laboratory. Samplers that collect onto agar plates or strips were therefore considered the most appropriate. At least ten battery powered portable samplers that collect onto agar are currently available in the UK. Battery powered samplers that collect into liquid, or onto filters, were not considered suitable for the reasons outlined above. It is clear from manufacturers’ information that there is little consistency either in the flow rate of the samplers or in the size of the collection plate. The sampler used for the study was selected because the 55 mm collection plates can also be used to monitor surface microbial loading. Site requirements inhibit the opportunity to use a number of current ‘laboratory quality’ sampler types that might have been suitable for use under less demanding operating conditions. Sampler design and technology is continuously developing, however, and other alternative samplers may become suitable for the purposes described in these protocols. The use of an appropriate sampler is important: results obtained with different samplers cannot be compared. For compatibility of results and consistency with the figures contained herein, sampling should be undertaken using a single-stage portable multi-orifice impactor operating at a flow rate of 100 litres per minute.

4

Hygenic maintenance of office ventilation ductwork

important to note that the presence of microbial contaminants on duct surfaces does not necessarily indicate that the normal cleaning process has been unsatisfactory. Mechanical cleaning is specifically a process of loosening and removing particulate dust contamination, and HVCA Guide to Good Practice TR17(8) describes ways of determining the effectiveness of that cleaning operation. Where mechanical cleaning leaves an unacceptable level of microbiological contamination, further investigation would be required to determine the source of the contaminant. This may, for instance, prove to be some mechanical failure in the air handling plant which causes outdoor air to by-pass the installed filtration. If the source is considered to be microbial growth inside the ductwork, then an alternative decontamination process is required which may include the use of biocides.

5

Ductwork hygiene indicators

5.1

Approach

This publication proposes protocols for microbial sampling and measurement inside ductwork and the occupied spaces it serves. The protocols are designed to provide, ideally, early warning of changes in the microbial loading of air supply ductwork that may adversely influence air quality. It is anticipated that such sampling would be carried out in addition to the good practice inspection, maintenance and cleaning of systems recommended in HVCA TR17(8). It is beyond the scope of this publication, and is not normally considered possible or practical, to define particular levels of microbial contamination that are considered safe or unsafe for building occupants. In general, however, specialists asked to investigate particular suspected microbiological problems employ two approaches. One is qualitative and the other is quantitative. 5.1.1

Qualitative

The qualitative approach involves sampling, culturing (growing) and identifying specific micro-organisms in a search for those known to be associated with particular problems. If the number of organisms is low, or there are none of the specified organisms present then it is commonly assumed that a microbial cause is unlikely. If the specified organisms are present and above critical levels, then they should be regarded as potential causes of the known problems. However, the presence of the organisms is not sufficient to indicate that there will be a problem. Sampling and monitoring for particular micro-organisms known to indicate a hazard to occupants would normally be impractical as a procedure for use on a regular (routine) basis. Samples would need to be analysed for the presence of an extensive list of particular micro-organisms, which would be prohibitively expensive. The risks would remain that the list of microbes may be incomplete, and that such microbes may become apparent only after a hygiene problem has already manifested itself. Even if there is a problem micro-organism, it may be present in low or

undetectable amounts so that negative results would not always offer reassurance. 5.1.2

Quantitative

The quantitative approach involves sampling and counting the total number of microbial cells or spores present, normally referred to as the total viable content (TVC) in the sample. As the main objective of this publication is to propose a system of sampling which provides an indication of the hygiene of ductwork under normal operating conditions, it is considered appropriate to set guidelines that are based on the TVC. Qualitative analysis may still be useful to follow up high TVC measurements, or if an air quality problem is known to exist. It must be noted that airborne sampling can only give a snapshot view of conditions during that sampling period. Outside, airborne micro-organism counts can vary by several orders of magnitude from hour to hour, day to day, and season to season. A general equipment inspection is recommended as part of good practice and may detect faults and indicate conditions (such as the presence of moisture or excessive dust contamination) in which poor air hygiene may arise.

5.2

Sampling methods

When selecting a sampler for collecting microbial material from air, it is important to consider the particular circumstances in which it will be used. In obtaining the series of samples from buildings which formed part of the background study for this publication, it was clear that the sampler selected must be battery powered as many of the samples would be taken at locations where there is no electricity supply. It is also necessary to take a number of samples during each site examination, and to minimise the need for handling in the laboratory. Samplers that collect onto agar plates or strips were therefore considered the most appropriate. At least ten battery powered portable samplers that collect onto agar are currently available in the UK. Battery powered samplers that collect into liquid, or onto filters, were not considered suitable for the reasons outlined above. It is clear from manufacturers’ information that there is little consistency either in the flow rate of the samplers or in the size of the collection plate. The sampler used for the study was selected because the 55 mm collection plates can also be used to monitor surface microbial loading. Site requirements inhibit the opportunity to use a number of current ‘laboratory quality’ sampler types that might have been suitable for use under less demanding operating conditions. Sampler design and technology is continuously developing, however, and other alternative samplers may become suitable for the purposes described in these protocols. The use of an appropriate sampler is important: results obtained with different samplers cannot be compared. For compatibility of results and consistency with the figures contained herein, sampling should be undertaken using a single-stage portable multi-orifice impactor operating at a flow rate of 100 litres per minute.

Air quality in buildings The selection of appropriate collection media is critical since each will be selective* for a small or large range of microbes depending on the media used. It should therefore be stressed that no one medium will allow the entire range of bacteria and fungi to be picked up. The best option is therefore to select a broad spectrum agar suitable for the collection of most bacteria or fungi. It is essential that antibiotics (or inhibitory agents) are added to the collection to prevent the growth of bacteria on plates designed to collect fungi, and vice versa. Failure to incorporate such compounds may lead to an underestimate of the total numbers of the microbe of interest due to inhibition or swamping. Alternative media include the following. For fungi: —

malt extract agar

—

potato dextrose agar

—

dichloran rose bengal chloramphenicol agar (Note: this agar must be stored in the dark to prevent the build-up of materials that are toxic to the fungi).

For bacteria: —

nutrient agar

—

plate count agar.

It must be stressed that these may give different results to the media recommended although all are said to be general purpose. Results from air samples taken using the protocol described herein can only be directly compared with the figures cited as low, medium and high where the two media specified in this publication are used (see section 9). Results obtained using other types of media will probably give results that differ from those obtained with the agars specified here. However, provided that commercial broadspectrum agars are used, results obtained with another medium may be used for comparisons within a particular sample series. Broad comparison might be made with the figures cited for the three categories of counts for the specific media (i.e. if counts are high on one medium they are likely to be high on the others). Unless counts have been obtained in side-by-side tests on the specified and alternative medium, a direct correlation cannot be made. The selection of nutrient rich media such as Sabouraud agar, which encourages fungal vegetative growth, should be avoided. The medium recommended here is less nutritionally rich than many, adequate for the majority of fungi, and reduces the risk of swamping by fast growing fungi.

* Selective implies that the nutrients, pH (acidity) and inhibitory agents are such that they support the growth of moulds (say) while inhibiting bacteria. By the correct formulation of the media, we select those which are capable of growth on that medium from the whole population, i.e., we are being selective. By way of analogy, the environment of deserts is selective for cacti but not for roses.

5

6

Air quality in buildings

6.1

Indoor pollutants

The air quality within a building is influenced in part by contaminants in the form of particles and gases that are generated within the building envelope and those brought in from outdoors. The impact on indoor air quality of the siting of fresh air inlets in relation to local sources of pollution, and in relation to the risk of re-admitting exhaust air, are discussed in detail in CIBSE Technical Memoranda TM21: Minimising pollution at air intakes(12). Contaminant particles may enter the building with the outdoor air. These can include carbon produced by combustion and vehicles, and particles of biological origin. Contaminant gases generated within the building include the volatile organic compounds (VOCs) emitted by some construction materials, fabrics and adhesives, and fumes emitted from photocopiers and laser printers. Gases admitted from outdoors may include vehicle exhaust gases. These pollutants are outside the scope of this publication. Biological agents such as bacteria, fungal spores and pollen grains can enter buildings from outside. Particles generated indoors can include human skin scales, bacteria, viruses and fungi, faecal material from the house dust mite and paper dust. Settled deposits in ductwork may cause contamination of supply air by means of the release of chemicals such as odorous VOCs, produced either microbiologically or chemically. Heavily microbiologically contaminated deposits may release microbes into the airstream, for instance by the release of spores. Many of these indoor pollutants can be harmful to human health, either through their potential to cause infection, their toxic nature, or their potential to cause allergic reactions in susceptible individuals. BSRIA Technical Note TN 18/92: Ventilation system hygiene — a review(13) includes further discussion of contaminants, and a World Health Organisation table of the main sources of biological contaminants in indoor air. Many microbes can survive for long periods in dry conditions, but they require moisture to grow. The combination of free water on the bottom of ducts, or elevated induct relative humidity, together with dust to provide nutrient can provide the ideal conditions for microbial growth.

6.2

Microbial contaminants

Like many exposed materials, mechanical ventilation ductwork can be subject to colonisation by a range of microorganisms (bacteria and fungi). The main requirements for the growth of these micro-organisms are a source of food, oxygen (although some bacteria can grow without oxygen), temperatures ideally between 15 oC and 25 oC, and moisture. Bacteria and fungi can grow both in illuminated areas and where there is no light. Since enough nutrient and oxygen, and suitable temperatures, are almost always available in buildings the dominant controlling factor is the availability of moisture.

Air quality in buildings The selection of appropriate collection media is critical since each will be selective* for a small or large range of microbes depending on the media used. It should therefore be stressed that no one medium will allow the entire range of bacteria and fungi to be picked up. The best option is therefore to select a broad spectrum agar suitable for the collection of most bacteria or fungi. It is essential that antibiotics (or inhibitory agents) are added to the collection to prevent the growth of bacteria on plates designed to collect fungi, and vice versa. Failure to incorporate such compounds may lead to an underestimate of the total numbers of the microbe of interest due to inhibition or swamping. Alternative media include the following. For fungi: —

malt extract agar

—

potato dextrose agar

—

dichloran rose bengal chloramphenicol agar (Note: this agar must be stored in the dark to prevent the build-up of materials that are toxic to the fungi).

For bacteria: —

nutrient agar

—

plate count agar.

It must be stressed that these may give different results to the media recommended although all are said to be general purpose. Results from air samples taken using the protocol described herein can only be directly compared with the figures cited as low, medium and high where the two media specified in this publication are used (see section 9). Results obtained using other types of media will probably give results that differ from those obtained with the agars specified here. However, provided that commercial broadspectrum agars are used, results obtained with another medium may be used for comparisons within a particular sample series. Broad comparison might be made with the figures cited for the three categories of counts for the specific media (i.e. if counts are high on one medium they are likely to be high on the others). Unless counts have been obtained in side-by-side tests on the specified and alternative medium, a direct correlation cannot be made. The selection of nutrient rich media such as Sabouraud agar, which encourages fungal vegetative growth, should be avoided. The medium recommended here is less nutritionally rich than many, adequate for the majority of fungi, and reduces the risk of swamping by fast growing fungi.

* Selective implies that the nutrients, pH (acidity) and inhibitory agents are such that they support the growth of moulds (say) while inhibiting bacteria. By the correct formulation of the media, we select those which are capable of growth on that medium from the whole population, i.e., we are being selective. By way of analogy, the environment of deserts is selective for cacti but not for roses.

5

6

Air quality in buildings

6.1

Indoor pollutants

The air quality within a building is influenced in part by contaminants in the form of particles and gases that are generated within the building envelope and those brought in from outdoors. The impact on indoor air quality of the siting of fresh air inlets in relation to local sources of pollution, and in relation to the risk of re-admitting exhaust air, are discussed in detail in CIBSE Technical Memoranda TM21: Minimising pollution at air intakes(12). Contaminant particles may enter the building with the outdoor air. These can include carbon produced by combustion and vehicles, and particles of biological origin. Contaminant gases generated within the building include the volatile organic compounds (VOCs) emitted by some construction materials, fabrics and adhesives, and fumes emitted from photocopiers and laser printers. Gases admitted from outdoors may include vehicle exhaust gases. These pollutants are outside the scope of this publication. Biological agents such as bacteria, fungal spores and pollen grains can enter buildings from outside. Particles generated indoors can include human skin scales, bacteria, viruses and fungi, faecal material from the house dust mite and paper dust. Settled deposits in ductwork may cause contamination of supply air by means of the release of chemicals such as odorous VOCs, produced either microbiologically or chemically. Heavily microbiologically contaminated deposits may release microbes into the airstream, for instance by the release of spores. Many of these indoor pollutants can be harmful to human health, either through their potential to cause infection, their toxic nature, or their potential to cause allergic reactions in susceptible individuals. BSRIA Technical Note TN 18/92: Ventilation system hygiene — a review(13) includes further discussion of contaminants, and a World Health Organisation table of the main sources of biological contaminants in indoor air. Many microbes can survive for long periods in dry conditions, but they require moisture to grow. The combination of free water on the bottom of ducts, or elevated induct relative humidity, together with dust to provide nutrient can provide the ideal conditions for microbial growth.

6.2

Microbial contaminants

Like many exposed materials, mechanical ventilation ductwork can be subject to colonisation by a range of microorganisms (bacteria and fungi). The main requirements for the growth of these micro-organisms are a source of food, oxygen (although some bacteria can grow without oxygen), temperatures ideally between 15 oC and 25 oC, and moisture. Bacteria and fungi can grow both in illuminated areas and where there is no light. Since enough nutrient and oxygen, and suitable temperatures, are almost always available in buildings the dominant controlling factor is the availability of moisture.

6 6.2.1

Hygenic maintenance of office ventilation ductwork Bacteria

Bacteria are unlikely to betray their presence by clearly recognisable surface growths, but they can be seen as slimes or films in areas where standing water occurs. They can be divided into two different groups, autotrophic or heterotrophic, depending on their mode of obtaining nutrition. Autotrophs are a very specialised group of bacteria which use reduced sulphur and nitrogen compounds as energy providers and release strong inorganic acids (sulphuric and nitric acids) as the metabolic end product of these reactions. These are rarely, if ever, found in building air distribution systems. Heterotrophs are bacteria which make use of organic carbon compounds as their chief source of energy and, as a byproduct of this process, produce a wide range of organic acids and metabolites. The heterotrophic mode of nutrition is the most common means of gaining energy among bacteria and is the only means used by the fungi and higher organisms. It is this group of bacteria that is often present on ductwork surfaces. 6.2.2

Table 1 Typical numbers of airborne biological particles per cubic metre of outdoor air in summer Particle type

Numbers of airborne particles (m–3)

Fungal spores Bacteria Pollens

Up to 1 000 000 About 100 to 10 000 viable cells At most a few hundred, accounting for 1% of the total number of particles

6.4

Microbial contamination in buildings and air distribution systems

Fungi

Fungi are simple heterotrophic micro-organisms existing as single cells (yeasts) or as multicellular filamentous structures (moulds). Fungi can live on any substrate where adequate nutrients are available. They do not use the duct surface itself as a source of nutrients, but live on the accumulated dirt or dust and bird-droppings, which may be considerable in some circumstances. They may also use the metabolites of other organisms or the decaying remains of other colonisers. This requirement for organic material probably accounts for the observation that, where fungi are isolated from duct surfaces, other micro-organisms are usually also present.

6.3

mainly released from February to May, grass pollens mainly in June and July, nettle pollen from June to August and weed pollens throughout the summer. Table 1 gives typical numbers of airborne microbial particles per cubic metre of air outdoors in summer.

Airborne microbial contaminants

Outdoors, bacteria are largely picked up with dry soil particles by the wind or by rain splash. Bacteria may be present indoors in the aerosols caused by activities such as toilet flushing and the use of humidifiers. Indoors, bacteria are dispersed by the normal shedding of skin scales and hair. They may also be dispersed by the occupants breathing, talking, coughing and sneezing. Fungi may develop on the damp patches caused by condensation or other surface water sources. Concentrations of outdoor airborne micro-organisms vary widely through the year. They are generally fewest during winter and most abundant in the spring and summer. Fungal spores are almost always present in the air. Their numbers and types vary with time of day, weather, season, geographical location and the presence of local spore sources. Outdoor fungal spore numbers are usually dominated by Cladosporium by day and by Sporobolomyces or ascospores by night. The presence of pollen grains is determined by the flowering seasons of the plants they come from. Tree pollens are

The dust found in indoor air, on horizontal surfaces in rooms, and inside air distribution ducts, is generally a mixture of particles from inside and outside the building, and contains both inorganic and organic material. The microbial contaminants found in air include particles, such as pollens and fungal spores, and less readily observed bacteria. The presence of dust does not necessarily mean that microbial contaminants will be present, although it would be very likely that they are. Conversely, a surface that appears visibly clean may still be contaminated with microbes. It is quite possible, for example, for bacteria to grow on glass which appears to be clean, provided sufficient moisture is present. It has been found that micro-organisms are capable of colonising surfaces such as the galvanised steel and flexible ductwork commonly used in air distribution systems. There are a number of circumstances which can make such materials susceptible particularly to colonisation by fungi. As noted earlier, surface dust will often contain the required nutrients, and the oxygen and temperature levels are normally ideal for growth. The moisture needed to allow growth to continue should not normally be available inside ductwork. However, surface water can occur as a result of condensation inside the ducts, or as a consequence of cooling or humidifying the flow of air. It may also be introduced as a result of a failure of some part of an air treatment process, or from nearby pipework or equipment that has suffered a failure. Fungi have been found on dry surfaces in air distribution systems, including the fan chamber housing, duct surfaces and in some parts of the exhaust air duct. A recent two-year study(14) of an air conditioning system in a newly constructed, large, modern office building with no history of water damage, found most surfaces associated with the air handling unit and thermal acoustic fibreglass populated with micro-organisms, with growth heaviest close to the air handling units (AHUs). Patches of fungi were also visible on the fibreglass facings of the thermal insulation.

Cleaning air distribution ductwork systems

6.5

Health aspects of microbes in buildings

Engineers do not often consider the health effects of microbes in ductwork systems, focussing rather on the attainment of specified operating conditions for comfort purposes. However, it is important to be aware of potential health issues arising from the presence of microbial material in ductwork. There are no environmental health criteria setting safe levels of microbial exposure. However, the possible harmful health effects on the occupants of buildings from microbial growth within the fabric can be divided into a number of categories. These are defined according to the health risk and, ultimately, the type of disease that might result. 6.5.1

Allergy

Allergies are perhaps the most well known reaction. One of the most common examples is hay fever, for which the main agent is pollen. Allergy to microbes may manifest itself as asthma (wheezing), allergic rhinitis (irritated runny nose) or, in exceptional circumstances, extrinsic allergic alveolitis (a diffuse lung inflammation — hypersensitivity pneumonitis). From the literature it is evident that asthma and rhinitis present the major indoor allergy problem. Extrinsic allergic alveolitis, requiring a much greater inhalation exposure, is more an industrial (especially agricultural) disease in the UK, although there have been some reports of cases caused by exposure to dry rot spores and Penicillium in dwellings. Exposure to microbes capable of inducing asthma or rhinitis may cause irritation or inflammation in the upper respiratory tract (i.e. the nose, throat and bronchial passages), sore eyes and can cause dermatitis when contaminated objects are handled. 6.5.2

Infection

Most of the bacteria and fungi growing on surfaces indoors in the UK are common environmental organisms and do not pose any threat to healthy people. However, some environmental fungi such as Aspergillus fumigatus can cause serious infections in more vulnerable people. Some harmful bacteria such as the agents causing Legionnaires' disease can be spread by the airborne route. Other bacteria may cause a range of illnesses from localised infections to potentially fatal septicaemia (blood poisoning). 6.5.3

Toxicosis

Some fungi produce toxic substances called mycotoxins. These toxins may be present in high concentrations in fungal spores (and in hyphal fragments) and can cause a response in the lungs. The inhalation of mycotoxins may interfere with the lung’s cell-mediated immune response and some are considered to be carcinogenic. The fungus Stachybotrys chartarum has received much press coverage, particularly in the USA, as it has been found growing on very wet wallpaper or other cellulosic materials in buildings. It has been implicated in fatal pulmonary

7 haemosiderosis in infants (although the evidence has been recently challenged by the discovery that there were serious problems with the exposure assessments) and in a variety of symptoms among adults living or working in damp, Stachybotrys-contaminated, buildings. BSRIA Technical Note TN 18/92(13) refers to a number of illnesses considered by the World Health Organisation to be caused by a variety of biological agents and their derivatives.

7

Cleaning air distribution ductwork systems

7.1

Mechanical cleaning methods

There are several process that cleaning contractors can use to remove dust, debris and other surface contaminants: —

vacuum

—

steam

—

compressed air

—

rotary brush.

Cleaning methods are more fully described in HVCA Guide to Good Practice TR17(8) and BSRIA Technical Note TN 18/92(13). Dust resulting from cleaning, particularly dust which may contain biologically active material, should be disposed of safely. The cleaning process involves loosening dust adhering to ductwork surfaces and its subsequent removal. The loosening can be carried out remotely by the use of compressed air or rotary brushing equipment in conjunction with removal using a large industrial vacuum collector. Dust may alternatively be removed more directly by a technician crawling along the ducts using a hand held vacuum cleaner. A mechanical ventilation system is normally cleaned in sections, starting at the AHU. The process may take several days or extend to weeks for large systems. Although some micro-organisms are able to grow on clean surfaces, removal of the dust reduces the opportunities for microbial colonisation, and is considered to be good practice. HVCA Guide to Good Practice TR17(8) specifies standards of industry practice recommended for the removal of dust deposits.

7.2

Use of biocides

In certain circumstances biocidal treatment or fogging is requested. This is normally indicated when microbial colonisation of the ductwork is considered to have occurred. However the routine use of biocides is not considered to be desirable practice (indeed, the World Health Organisation explicitly discourages their use) for the reasons outlined below. Biocides are, by their nature, dangerous chemicals. They represent a potential health hazard both to the cleaning

Cleaning air distribution ductwork systems

6.5

Health aspects of microbes in buildings

Engineers do not often consider the health effects of microbes in ductwork systems, focussing rather on the attainment of specified operating conditions for comfort purposes. However, it is important to be aware of potential health issues arising from the presence of microbial material in ductwork. There are no environmental health criteria setting safe levels of microbial exposure. However, the possible harmful health effects on the occupants of buildings from microbial growth within the fabric can be divided into a number of categories. These are defined according to the health risk and, ultimately, the type of disease that might result. 6.5.1

Allergy

Allergies are perhaps the most well known reaction. One of the most common examples is hay fever, for which the main agent is pollen. Allergy to microbes may manifest itself as asthma (wheezing), allergic rhinitis (irritated runny nose) or, in exceptional circumstances, extrinsic allergic alveolitis (a diffuse lung inflammation — hypersensitivity pneumonitis). From the literature it is evident that asthma and rhinitis present the major indoor allergy problem. Extrinsic allergic alveolitis, requiring a much greater inhalation exposure, is more an industrial (especially agricultural) disease in the UK, although there have been some reports of cases caused by exposure to dry rot spores and Penicillium in dwellings. Exposure to microbes capable of inducing asthma or rhinitis may cause irritation or inflammation in the upper respiratory tract (i.e. the nose, throat and bronchial passages), sore eyes and can cause dermatitis when contaminated objects are handled. 6.5.2

Infection

Most of the bacteria and fungi growing on surfaces indoors in the UK are common environmental organisms and do not pose any threat to healthy people. However, some environmental fungi such as Aspergillus fumigatus can cause serious infections in more vulnerable people. Some harmful bacteria such as the agents causing Legionnaires' disease can be spread by the airborne route. Other bacteria may cause a range of illnesses from localised infections to potentially fatal septicaemia (blood poisoning). 6.5.3

Toxicosis

Some fungi produce toxic substances called mycotoxins. These toxins may be present in high concentrations in fungal spores (and in hyphal fragments) and can cause a response in the lungs. The inhalation of mycotoxins may interfere with the lung’s cell-mediated immune response and some are considered to be carcinogenic. The fungus Stachybotrys chartarum has received much press coverage, particularly in the USA, as it has been found growing on very wet wallpaper or other cellulosic materials in buildings. It has been implicated in fatal pulmonary

7 haemosiderosis in infants (although the evidence has been recently challenged by the discovery that there were serious problems with the exposure assessments) and in a variety of symptoms among adults living or working in damp, Stachybotrys-contaminated, buildings. BSRIA Technical Note TN 18/92(13) refers to a number of illnesses considered by the World Health Organisation to be caused by a variety of biological agents and their derivatives.

7

Cleaning air distribution ductwork systems

7.1

Mechanical cleaning methods

There are several process that cleaning contractors can use to remove dust, debris and other surface contaminants: —

vacuum

—

steam

—

compressed air

—

rotary brush.

Cleaning methods are more fully described in HVCA Guide to Good Practice TR17(8) and BSRIA Technical Note TN 18/92(13). Dust resulting from cleaning, particularly dust which may contain biologically active material, should be disposed of safely. The cleaning process involves loosening dust adhering to ductwork surfaces and its subsequent removal. The loosening can be carried out remotely by the use of compressed air or rotary brushing equipment in conjunction with removal using a large industrial vacuum collector. Dust may alternatively be removed more directly by a technician crawling along the ducts using a hand held vacuum cleaner. A mechanical ventilation system is normally cleaned in sections, starting at the AHU. The process may take several days or extend to weeks for large systems. Although some micro-organisms are able to grow on clean surfaces, removal of the dust reduces the opportunities for microbial colonisation, and is considered to be good practice. HVCA Guide to Good Practice TR17(8) specifies standards of industry practice recommended for the removal of dust deposits.

7.2

Use of biocides

In certain circumstances biocidal treatment or fogging is requested. This is normally indicated when microbial colonisation of the ductwork is considered to have occurred. However the routine use of biocides is not considered to be desirable practice (indeed, the World Health Organisation explicitly discourages their use) for the reasons outlined below. Biocides are, by their nature, dangerous chemicals. They represent a potential health hazard both to the cleaning

8

Hygenic maintenance of office ventilation ductwork

operatives who apply them and to building occupants. As hazardous materials, their use is subject to COSHH legislation(4) and precautions must be taken to ensure that operatives and building occupants are not exposed to the chemicals. The use of biocides is not necessarily an effective treatment against all microbial contaminants. It is therefore considered that biocidal fogging is not an appropriate treatment unless there is a known microbial problem which cannot be resolved by more routine cleaning processes.

7.3.2

Electrostatic precipitators are also capable of removing particles as small as 0.01 micron, and are also used where high standards of cleanliness are essential. Installation costs are high and safety precautions are necessary owing to the presence of high voltages, but resistance to flow and maintenance costs are low. Pre-filtration is again necessary.

7.3.3 Where biocides are applied over an existing dust layer, they may treat only the top surface leaving microbes at the duct surface unaffected. The application of biocides introduces moisture to the ductwork system which may persist after the active agent in the biocide has dispersed thus providing the conditions under which fresh contamination might flourish. Note: biocidal fogging should never be used as a substitute for mechanical cleaning The EU Biocidal Products Directive(15) of February 1998 requires member countries to adopt a consistent scheme for assessing the range of biocides marketed and rating the suitability for their intended use in order to provide a high level of protection for humans and the environment. The Health and Safety Executive (HSE) will soon provide supporting Regulations for the UK, maintaining a list of biocides considered appropriate where exposure to humans is likely.

7.3

Reducing the admission of micro-organisms

Methods currently used to eliminate micro-organisms from air ductwork concentrate on killing or filtering to remove micro-organisms on entry, before they can contaminate the ductwork system. Once present inside the ductwork, there is at present no practical method of disinfecting duct surfaces other than the use of biocides. Among the air filtration or precipitation devices only the high efficiency particulate absolute (HEPA) air filters and electrostatic units have been proven to be effective at removing particles in the size range of microbes. The others are more experimental and therefore the ability of these systems to remove or destroy microbes entering ducts is less well proven. 7.3.1

High efficiency particulate air (HEPA) filters

The guidance included herein is based on the use of filters to EU7 or better. HEPA filters capable of removing particles as small as 0.01 micron are used in some mechanical ventilation systems. Owing to their high resistance to flow, and consequent energy and cost penalty, their use is normally restricted to those areas where particularly high standards of cleanliness are essential. These would include hospital operating theatres, isolation and recovery rooms, and industrial clean rooms. Pre-filtration is also necessary.

Electrostatic precipitators

Anti-microbial filters

Under certain conditions air filters can support the growth of micro-organisms and act as a source of contaminants. Standard air filters are now available which include an antimicrobial coating reported to kill or inhibit the growth of micro-organisms on the filter material and any trapped dust and debris. Due to the potential for the active biocide to outgas from the surface, the user of such systems should, however, take steps to assure that they are safe for building occupants. Information about the long term performance of these filters is currently limited.

7.3.4

Ultraviolet germicidal irradiation (UVGI)

Ultraviolet (UV) lamps are sometimes mounted in air supply ductwork. The UV light emitted (wavelength normally between 240 and 280 nm) causes inactivation of microorganisms by disrupting the DNA. The damage is such that cell division (i.e. growth) can no longer take place. This system is claimed to be effective against all types of bacteria and fungi, as well as spores and viruses, which are normally found in the air. The user of such systems should ensure that staff are protected from exposure to the UV radiation. This approach does not remove biological material; it is only rendered non-viable.

7.3.5

Photocatalytic oxidation technology

Photocatalytic technology involves the action of low energy ultraviolet light on a catalyst in the presence of water vapour which generates hydroxyl radicals that destroy micro-organisms. The photocatalytic process is primarily an oxidation process and therefore, in theory, microbial hydrocarbons will be reduced to carbon dioxide and water. The range of micro-organisms destroyed by this technology include bacteria, fungi (including spores), viruses and allergens. As with UV treatment, the organisms are killed but biological material remains.

7.3.6

Anti-microbial coated duct surfaces

Anti-microbial ductwork coatings are available which are reported to inhibit the growth of micro-organisms within ductwork systems. Again, owing to the potential for the active biocide to outgas from the surface, the users of such systems should take steps to assure that they are safe for building occupants.

Protocol for sampling air in occupied spaces

8

Protocol for sampling air in occupied spaces

In the absence of a reliable method of directly measuring the content of micro-organisms in the moving air-streams inside ducts, the following method is recommended in an attempt to provide an indication of the airborne microbiological conditions. It is recommended that the monitoring protocol described should be carried out on an annual basis. While the microbial flora in mechanically ventilated buildings does not fluctuate greatly with season, the date, time and weather conditions should be noted so that comparisons can be made with samples taken on previous occasions.

8.1

Personnel

To minimise variation between samples it is recommended that the same person should collect all the samples required in a particular building. He or she should be familiar with the manufacturer’s operating manual for the air sampler.

8.2

Equipment

Sampling in the room air will require the following: —

—

Air sampler: a single-stage portable multi-orifice impactor operating at a flow rate of 100 litres per minute.

9 The sampler head should be cleaned with 70% ethanol and allowed to dry prior to any site visit. Particular attention must be paid to ensure that all orifices are open and free from debris. For each batch of growth medium purchased or prepared, between one and three plates should be randomly selected and retained. These plates are to be incubated or dispatched to the laboratory along with the site samples to confirm the initial sterility of the plates.

8.4

The air sampling equipment being used does not itself require specific safety procedures. Where necessary, access to any part of the duct system will need to be arranged with the relevant plant manager. Most plant-room situations will require the use of a hard-hat and protective footware. Single-person working in air handling units or plant rooms should be avoided, and access to the fan chamber should not be attempted while the fan is operational. Many plantrooms are accessed by ladders and stairs that require care in their use. The rooms themselves can contain trip hazards, changes in level and stored chemicals, apart from the expected electrical and gas fuelled plant. Stepladders should always be used with care.

8.5

2% (w/v) malt extract

—

1.2% Agar No. 3 or equivalent

—

20 units·ml–1 benzyl penicillin

—

50 µg·ml–1 streptomycin sulphate

Carry out a ‘walk through’ of the building to assess, visually and by smell, the occurrence and extent of any dampness, water staining and mould growth. This should also include the status of the AHUs of interest and the state of dust build-up on the air supply terminal, grille or diffuser in the rooms. Details of the building should be noted and recorded.

(2)

Samples should be taken from at least two spaces served by each AHU, one should be near the beginning (AHU) of the ductwork system and one near the end (furthest from the AHU). The spaces and activities should be representative of the spaces within the building.

(3)

Samples should be taken from two sampling locations in each space.

For bacteria the medium is tryptone soya agar (TSA) containing 10 µg·ml–1 amphotericin B. (Note: this agar must be stored in the dark to prevent degradation of the antibiotic.) Alternative antibiotics that can be used are chloramphenicol (100 µg·ml–1) for mould and yeast agar and cycloheximide (50 µg·ml–1) for bacterial agar. —

Timer: air samplers without built-in timers or preset volume control will require the use of a countdown timer capable of measuring time intervals between 10 seconds and 10 minutes.

—

Tripod: capable of holding the head of the sampler 100 cm above floor level.

—

Stepladder.

8.3

Laboratory

The air sampler should be calibrated at least annually, as specified by the manufacturer.

Methodology

(1)

Collection plates: disposable plastic petri-dishes containing appropriate agar. For mould and yeast counts the growth medium contains: —

Safety

—

Doors to the office and openings to outdoor air should be kept closed during sampling.

—

Ideally the office should be vacant at the time of sampling. However if this is not possible, occupants in the office should remain more than 2 m from the sampler while the sample is being taken. The operator should also keep more than 2 m distant from the sampler, except when taking ceiling level samples.

—

One set of samples should be taken in the centre of the room with the sampler intake orifice at least 100 cm above floor level and facing the ceiling. This can be achieved by the use of a tripod. Ideally the sampler should be placed at least 50 cm from any

10

Hygenic maintenance of office ventilation ductwork soft-furnishings, tables, display cabinets, or any other potential sources of dust particles. —

—

The second set of samples should be taken close to and facing, but not in the direct air stream from, an air supply grille or diffuser. When taking this set of samples no part of the operator’s body should be above the sampler, where contaminants might fall from the operator onto the sampling head. The sampler should be held at ‘armslength’ directly above the operator to avoid this.

(10)

A location map should be made on which the sampler locations and the positions of windows and doors are recorded.

The following sampling method is applicable either for assessing the microbial level of uncleaned office supply ductwork or for evaluating the effectiveness of the disinfection process. Samples to assess the effectiveness of disinfection should be taken as soon as practicable after cleaning to avoid surface contamination from uncleaned surfaces. Results obtained should be below the levels set out in section 10.

(4)

It is recommended that, for both fungi and bacteria, two separate 200 litre volumes of air should be sampled at each location. The volume of air sampled is defined by the sampling flowrate (litres/min) and the duration of sampling (min).

(5)

In sampling: —

one collection dish is placed centrally under the sampler head, the lid of which is placed on a clean surface

—

the operator should take care not to touch the agar surface or the inside of the lid of the petri-dish;

—

the sampler should be reassembled ensuring that the head fits correctly and that no air leakage occurs.

(6)

After exposure, the lids of the plates should be replaced aseptically and each plate labelled with the sample location, date and the length of sample (either in minutes or litres).

(7)

The number of occupants in the room and level of activity should be recorded separately. Ideally the room should be empty as human occupants add significantly to the microbial load.

(8)

(9)

will return the count for each sample as (a) the corrected value and (b) converted to counts per cubic metre of air.

For each building, or for every ten locations in a building, a field blank sample plate should be prepared. This requires that the sampler be loaded with a fresh sampling plate, reassembled, and the plate removed after 30 seconds — without having run the sampler. The plates (exposed, field and media blanks) should be transported to the laboratory with the minimum of handling or delay. The maximum recommended period between exposure and returning to the laboratory is 24 hours. The laboratory must comply with the EC Biological Workers Directive(16) and COSHH Regulations(4) and should be able to demonstrate competence in handling biological samples. oC

Plates should be incubated at 25 in the dark for 4–7 days and the number of colonies counted. Fungal colonies appearing on the bacterial plates (i.e. TSA plates) should be ignored and vice versa. Any adjustments to the total counts (i.e. positive hole correction) should be performed as per the manufacturer’s recommendations. The laboratory

For repeat visits, the sampler should be placed in the same locations and samples should be taken at the same time, which is either in the morning or afternoon.

9

Protocol for surface sampling

Sampling after any disinfection or biocidal treatment requires particular care. The surfaces of the air distribution system should be fully dry before the samples are taken. It must be noted that normal agars only partially, and inconsistently, neutralise biocides. The incorporation of thiosulphate, at an appropriate level, into the appropriate agars cited in this report should neutralise chlorine based biocides. While the microbial flora in mechanically ventilated buildings does not fluctuate greatly with season, the date, time and weather conditions should be noted so that comparisons can be made with samples taken on previous occasions.

9.1

Personnel

To minimise variations between samples the same person should collect all samples required in a particular building. Personnel should be familiar with the manufacturer’s operating manual for any equipment used.

9.2

Equipment

Sampling ductwork surfaces will require the following: —

Sample collection plates: which should normally be disposable plastic 55 mm contact plates containing appropriate agar. For mould and yeast counts the growth medium contains: —

2% (w/v) malt extract

—

1.2% Agar No. 3 (or equivalent)

—

20 units·ml–1 benzyl penicillin

—

50 µg·ml–1 streptomycin sulphate.

For bacteria the medium is tryptone soya agar (TSA) containing 10 µg·ml–1 amphotericin B. (Note: this agar must be stored in the dark to prevent degradation of the antibiotic.) Alternative antibiotics that can be used are chloramphenicol (100 µg·ml–1) for mould and yeast

10

Hygenic maintenance of office ventilation ductwork soft-furnishings, tables, display cabinets, or any other potential sources of dust particles. —

—

The second set of samples should be taken close to and facing, but not in the direct air stream from, an air supply grille or diffuser. When taking this set of samples no part of the operator’s body should be above the sampler, where contaminants might fall from the operator onto the sampling head. The sampler should be held at ‘armslength’ directly above the operator to avoid this.

(10)

A location map should be made on which the sampler locations and the positions of windows and doors are recorded.

The following sampling method is applicable either for assessing the microbial level of uncleaned office supply ductwork or for evaluating the effectiveness of the disinfection process. Samples to assess the effectiveness of disinfection should be taken as soon as practicable after cleaning to avoid surface contamination from uncleaned surfaces. Results obtained should be below the levels set out in section 10.

(4)

It is recommended that, for both fungi and bacteria, two separate 200 litre volumes of air should be sampled at each location. The volume of air sampled is defined by the sampling flowrate (litres/min) and the duration of sampling (min).

(5)

In sampling: —

one collection dish is placed centrally under the sampler head, the lid of which is placed on a clean surface

—

the operator should take care not to touch the agar surface or the inside of the lid of the petri-dish;

—

the sampler should be reassembled ensuring that the head fits correctly and that no air leakage occurs.

(6)

After exposure, the lids of the plates should be replaced aseptically and each plate labelled with the sample location, date and the length of sample (either in minutes or litres).

(7)

The number of occupants in the room and level of activity should be recorded separately. Ideally the room should be empty as human occupants add significantly to the microbial load.

(8)

(9)

will return the count for each sample as (a) the corrected value and (b) converted to counts per cubic metre of air.

For each building, or for every ten locations in a building, a field blank sample plate should be prepared. This requires that the sampler be loaded with a fresh sampling plate, reassembled, and the plate removed after 30 seconds — without having run the sampler. The plates (exposed, field and media blanks) should be transported to the laboratory with the minimum of handling or delay. The maximum recommended period between exposure and returning to the laboratory is 24 hours. The laboratory must comply with the EC Biological Workers Directive(16) and COSHH Regulations(4) and should be able to demonstrate competence in handling biological samples. oC

Plates should be incubated at 25 in the dark for 4–7 days and the number of colonies counted. Fungal colonies appearing on the bacterial plates (i.e. TSA plates) should be ignored and vice versa. Any adjustments to the total counts (i.e. positive hole correction) should be performed as per the manufacturer’s recommendations. The laboratory

For repeat visits, the sampler should be placed in the same locations and samples should be taken at the same time, which is either in the morning or afternoon.

9

Protocol for surface sampling

Sampling after any disinfection or biocidal treatment requires particular care. The surfaces of the air distribution system should be fully dry before the samples are taken. It must be noted that normal agars only partially, and inconsistently, neutralise biocides. The incorporation of thiosulphate, at an appropriate level, into the appropriate agars cited in this report should neutralise chlorine based biocides. While the microbial flora in mechanically ventilated buildings does not fluctuate greatly with season, the date, time and weather conditions should be noted so that comparisons can be made with samples taken on previous occasions.

9.1

Personnel

To minimise variations between samples the same person should collect all samples required in a particular building. Personnel should be familiar with the manufacturer’s operating manual for any equipment used.

9.2

Equipment

Sampling ductwork surfaces will require the following: —

Sample collection plates: which should normally be disposable plastic 55 mm contact plates containing appropriate agar. For mould and yeast counts the growth medium contains: —

2% (w/v) malt extract

—

1.2% Agar No. 3 (or equivalent)

—

20 units·ml–1 benzyl penicillin

—

50 µg·ml–1 streptomycin sulphate.

For bacteria the medium is tryptone soya agar (TSA) containing 10 µg·ml–1 amphotericin B. (Note: this agar must be stored in the dark to prevent degradation of the antibiotic.) Alternative antibiotics that can be used are chloramphenicol (100 µg·ml–1) for mould and yeast

Protocol for surface sampling

11

agar and cycloheximide (50 µg·ml–1) for bacterial agar. —

A volume of agar: which should be in accordance with the manufacturer’s guidance.

—

Stepladder.

—

Torch.

9.3

(2)

Laboratory

For each batch of medium purchased or prepared, between one and three plates should be randomly selected and retained. These plates are to be incubated or dispatched to the laboratory along with the site samples to confirm the initial sterility of the plates.

9.4

Safety

No specific safety procedures have been identified as necessary for collecting surface biological samples other than the normal expected use of thin protective gloves. Access to any part of the duct system will need to be arranged with the relevant plant manager. In most plantroom a hard-hat and protective footwear are required. Single-person working in AHU(s) or plant rooms should be avoided and access to the fan chamber should not be attempted while the fan is operational. Opening duct systems in operation should generally be avoided, particularly in the case of high velocity or high pressure systems, and particularly where the ductwork or building components are dirty. The AHU should not be accessed while running both for safety reasons and to ensure that occupants downstream are not subjected to increased levels of airborne dust caused as the resistance of AHU filters is by-passed and velocity increased.

(3)

Stepladders should always be used with care (see section 8.4).

9.5 (1)

—

immediately downstream of the AHU

—

inside the distribution ductwork the sample locations should be approximately every 50 linear metres; for ducts in excess of 300 linear metres, the sample locations should be approximately every 100 linear metres.

All locations should be sampled unless access is impractical. —

The fan should not be running when samples are being collected.

—

At each location samples for both media should be taken.

—

For ducts with a diameter or maximum dimension up to 250 mm, one surface sample should be taken.

—

For ducts with a diameter or maximum dimension up to 500 mm, two surface samples should be taken, with each sample randomly placed at least 150 mm apart.

—

For ducts with a diameter or maximum dimension exceeding 500 mm, three surface samples should be taken, with each sample randomly placed at least 150 mm apart.

—

Samples should be taken from the lowest horizontal surface of square or rectangular ducts or the lowest point of circular or oval ducts.

In sampling, the lid of the contact plate should be placed on a clean surface and the exposed agar gently pressed onto the selected surface. —

In circular ducts of diameter less than 700 mm, it may not be possible to use a standard 55 mm diameter contact plate due to the radius of curvature. As an alternative long thin agar samplers, like dip slides, can be used.

—

The operator should not touch the agar surface or the inside of the lid of the contact plate.

Methodology Detailed plans of the building should be obtained showing the AHU and the ductwork concerned together with an indication of the possible access points to the ducts. The locations of the samples will need to be determined in each case, and selected to allow full surface contact of the sample plates. All sampling should be carried out downstream of the bag filter and should usually include: —

the inner surfaces of the ductwork within the AHU downstream from the filter

—

on both sides of the cooling coils (where fitted and where access allows); the samples should be taken on the lower horizontal surfaces of the AHU within 15 cm of the cooling coils (or outside this limit if obvious signs of wetting is observed elsewhere)

(4)

After exposure, the lids of the plates should be replaced aseptically and each plate labelled with the sample location and date of sampling.

(5)

Descriptive information should be recorded on the specific location of each sampling point, including the following: —

the specific access location (so that this can be located again later, if necessary)

—

the part of the system monitored, for example: inside the AHU; on the coil surface; inside the duct

—

whether the sample is taken from a vertical or a horizontal surface

—

the nature of the surface, for example: plastic, stainless steel, concrete

—

the function of the duct (whether it carries supply, return or extract air)

12

Hygenic maintenance of office ventilation ductwork —

the proximity to any upstream change in direction of the duct; for example: less than 1 m; between 1 and 5 m; over 5 m

—

the distance to upstream dampers or other obstructions, for example: less than 1 m; between 1 and 5 m; over 5 m

—

the presence of any visible moisture

—

the presence of any visible rust

—

the presence of any viable mould growth

—

the presence of any unusual odour

—

whether the dust level is above the maximum recommended in HVCA Guide to Good Practice TR17(8)

— (6)

(7)

the dust type, for example: mainly fine dust; coarse dust (sand-sized particles)

For each building, or for every ten locations in a building, a field blank sample plate should be prepared. This requires that a fresh plate be removed from the transport bag or similar, and returned to the transport bag without being exposed.

This classification has been developed by the group of experts listed in Appendix 4, who had access to the results of sampling a number of air conditioned and mechanically ventilated buildings in the UK. Appendix 2 contains a detailed summary of these sampling results. The action level for requesting further microbial investigations should be set at ≥ 1000 cfu·m–3. As air samples for microbial counts can be influenced by several factors outside the direct influence of microbes growing in the ducts, one single high count should not be considered sufficient to trigger a further investigation in the ducts. 10.1.2

It is considered that, at each working space, the following criteria should apply: (a)

Where either of the desk level samples obtained and processed using the protocol described in section 8 is classified as ‘high’ then investigations on the efficacy of ventilation to that office should be considered. (b)

10

Interpretation of sampling results

10.1

Air sampling microbial limits

10.1.1

Classification of microbial limits

The individual fungal and bacterial counts for each location should be converted to counts per cubic metre (cfu·m–3), thus at each working space eight counts (four fungal and four bacterial) should be available and compared with the classification given in Table 2. Table 2 Classification of air sampling microbial limits Category

Colony forming units per cubic metre (cfu·m–3)

Low Medium High