Advisories

Guidance on Improving Ventilation and Indoor Air Quality in Buildings amid the COVID-19 situation

1. Introduction
1.1 COVID-19 is mainly transmitted by close contact and respiratory droplets which are released when an infected person coughs, sneezes, talks, or sings. It can also be spread through virus aerosols in the air under certain settings, such as enclosed environments which are poorly ventilated. Hence, it is critical to mitigate this risk by improving ventilation and air quality in indoor environments.

1.2 This Guidance Note provides building owners and facilities managers with updated recommended measures to enhance ventilation and air quality in indoor spaces, through the proper operations and maintenance of air-conditioning and mechanical ventilation (ACMV)1 systems. It is aimed at reducing disease transmission amid the current COVID-19 situation and resurgence of community cases. Earlier versions of this document were issued on 25 May 2021 and 29 May 2020.

1.3 This Guidance Note applies to non-residential premises where air-conditioning is used intermittently or continuously, as well as to naturally ventilated premises, with the exception of specialised premisessuch as certain factory production areas, hospitals, polyclinics, and laboratories. Advice from subject matter experts and specialists should be sought for specialised premises.

1.4 Occupants of residential homes may improve home ventilation by opening doors and windows, especially when hosting non-household guests. Fans can be used to promote air circulation when needed.

1.5 The recommended measures in this Guidance Note should be accompanied by other key measures to reduce disease transmission, such as requiring building occupants to practise safe distancing, wearing masks, and carrying out regular disinfection of high-touch points within the building3.

1.6 A comprehensive plan should be created for safe management of indoor spaces, including developing communication plans to garner support from occupants, increasing ventilation, and ensuring supply of critical items such as filters. Managers should tailor the measures to each space, maximising ventilation while taking factors such as the type of ventilation system in the indoor space, the area of the indoor space, occupants’ thermal comfort, and relative humidity into consideration. Indoor relative humidity should be closely monitored and maintained according to Singapore Standard SS554:2016 Code of Practice for Indoor Air Quality for Air-Conditioned Buildings.
 

2. Measures for air-conditioned premises with mechanical ventilation provision (e.g. centralised air-conditioning system)
2.1 Ensure that ventilation systems are in good working order:

a. Check ACMV systems to ensure adequate ventilation in all occupied spaces, based on at least the minimum outdoor air supply rates specified in Singapore Standard SS553:2016 Code of Practice for Air Conditioning and Mechanical Ventilation in Buildings4. Use of sensors and systems for monitoring outdoor air supply rate should be considered.
b. Check AHUs/FCUs/PAUs/FAFs/EAFsdaily to ensure continuous operation, especially in occupied spaces.
c. Check all supply air diffusers and exhaust grilles to ensure airflow movement is in the correct direction.
d. Maintain ACMV systems regularly. This includes inspecting and cleaning supply fans and exhaust fans to ensure optimal operation, checking air ducts and dampers to ensure no air leakages or blockages, and checking filter seals to avoid air bypass. Recommended maintenance frequencies are specified in SS554:2016 Annex H. Replacement of filters should be done during non-operational periods, with the ACMV system turned off. Used filters should be properly disposed of in sealed bags. Maintenance staff should wear the appropriate PPE, comprising at least N95 masks, eye protection, and gloves, especially when replacing filters.
e. Check other systems to ensure there is no undesired air leakage into occupied spaces, including water seals in the sanitary system, cracks in pipes and ducts, and wall gaps. Rectify faults detected.

2.2 Maximise ventilation for indoor air dilution:

a. Maximise outdoor air intake and supply by setting AHUs/FCUs/PAUs/FAFs/EAFs to maximum speed and capacity with all air dampers (e.g. volume control dampers) opened fully. Increase outdoor air supply rate to at least 10 L/s/person if the system can accommodate this6.
b. Deactivate demand control systems, such as those with CO2 sensors, to avoid automatic reduction of outdoor air supply.
c. Open all air dampers and do not block air ducts to ensure optimal provision of outdoor air to all occupied spaces.
d. The air distribution system should be balanced to ensure outdoor air provision to all intended spaces.
e. Operate exhaust fans (e.g. in toilets, kitchens) at full capacity to expel air from indoor spaces. Keep windows and other openings (e.g. back door) around exhaust fans closed to avoid short-circuiting of air flow.
f. Install additional supply and/or exhaust fans if the existing system does not deliver sufficient outdoor air7.
g. If the system does not allow for increasing the ventilation up to the recommended minimum per person requirement, consider reducing the maximum room occupancy.

2.3 Purge indoor air before occupancy:

a. Perform air purging at least once a day and for at least two hours before each occupancy. Increased purging frequency can be considered for spaces with high disease transmission risk8.
b. For buildings without air purging systems, extend operation of ACMV systems with outdoor air intake for two hours before and after each occupancy.

2.4 Minimise indoor air recirculation; use efficient filters in AHUs to treat recirculated air:

a. Set recirculation air dampers to a minimum.
b. Use efficient filters (at least MERV14, F8, or ePM1 70-80% is recommended) in AHUs to treat recirculated air. Filters should be properly installed and maintained according to manufacturers’ recommendation.
c. Use of air-cleaning technologies such as ultraviolet germicidal irradiation (UVGI) in AHUs may be considered to augment efficient filters. Efficacy and safety of all air-cleaning devices under the operating conditions must be considered.
d. Switch off rotatory heat changers or heat recovery wheels to reduce risk of carry-over leakage from exhaust air.

2.5 Increase ventilation in premises with limited ventilation and air filtration provision (e.g. meeting rooms with only FCU):

a. Open operable windows and doors as frequently as possible9, unless outdoor/outside air quality is poor. Air-conditioning should be reduced or turned off when doors and/or windows are opened.
b. Consider positioning fans to blow air out of windows to increase air exchange.
c. Consider adding dedicated outdoor air supply and/or exhaust7.
d. Where there is a high risk8 of disease transmission, localised air cleaning may be considered as an interim measure, in line with the recommendation for spaces without mechanical ventilation (please see sections 3.3 and 3.4).
 

3. Measures for enclosed air-conditioned premises without mechanical ventilation provision (e.g. split-unit air-conditioners or FCUs without fresh air supply)
3.1 Increase ventilation and enhance air exchange:

a. Open operable windows and doors as frequently as possible9, unless outdoor air quality is poor. Air-conditioning should be reduced or turned off when doors and/or windows are opened.
b. Consider adding dedicated outdoor air supply and/or exhaust7.
c. Operate exhaust fans (e.g. in toilets, kitchens) at full capacity to expel air from indoor spaces. Keep windows or other openings (e.g. back door) around exhaust fans closed to avoid short-circuiting of air flow.

3.2 Consider installing window-mounted exhaust fans to enhance ventilation:

a. The fan systems should at least provide the minimum air changes specified in SS553:2016.
b. Air supply and exhaust system can be aligned to provide uni-directional airflow in a poorly ventilated space.
c. To cater for occasions of poor outdoor air quality (e.g. haze), consider getting fans with coverings that can cover up the fans when needed. As a longer-term solution, supply air fans fitted with efficient filters (at least MERV14, F8, or ePM1 70-80% is recommended) can also be considered to maintain good ventilation during haze episodes. Such systems should be installed by professionals to ensure good filtration and supply air distribution.

3.3 In enclosed spaces with high risk8 of disease transmission, portable air cleaners for localised air cleaning may be considered as an interim measure: 

a. Portable air cleaners should be equipped with high-efficiency air filters such as HEPA filters, which are effective at removing virus aerosols.
b. The clean air delivery rate (CADR) or equivalent of a portable air purifier should be used to determine the size and number of portable air cleaner devices needed in a space10.
c. If present, the portable air cleaner’s ozone generation function should be turned off to avoid potential exposure to excessive levels of ozone and undesirable by-products, which may be hazardous to health.
d. The use of portable air cleaners should be considered an interim measure. Air cleaning does not replace the need for adequate ventilation. Regular surface cleaning and disinfection3 should also continue, as portable air cleaners do not remove surface contamination.
e. Please refer to NEA’s Technical Advisory on Use of Air-Cleaning Technologies to Mitigate COVID-19 Aerosol Transmission Risk for more information on the use of portable air cleaners.

3.4 Where ceiling height allows, upper-room UVGI may be considered for air cleaning. As viral disinfection efficacy and safety (UV-C radiation can cause injury to the eyes and skin) are greatly dependent on the system and how it is installed and maintained, professional services must be used. Please refer to NEA’s Technical Advisory on Use of Air-Cleaning Technologies to Mitigate COVID-19 Aerosol Transmission Risk for more information.

3.5 Check other systems to ensure there is no undesired air leakage into occupied spaces, including water seals in the sanitary system, cracks in pipes and ducts, and wall gaps. Rectify faults detected.
  

4. Measures for naturally ventilated premises
4.1 Natural ventilation is weather dependent. It could be limited to the areas around windows and doors (perimeter zones), with little air exchange in the inner portion of the space (internal zones). Cross ventilation is significantly more effective than single-sided ventilation (e.g. when the opening exists only at one side of the space).

4.2 Increase natural ventilation with enhancement by fans:

a. Keep windows and/or doors open at all times, unless outdoor air quality is poor or the weather condition does not allow.
b. Position fans at windows to blow air outwards and increase air exchange
c. Operate exhaust fans (e.g. toilet, kitchen) at full capacity to expel air form the indoor space. Keep windows or other openings (e.g. back door) around exhaust fans closed to avoid short-circuiting of air flow.

4.3 Consider installing window-mounted exhaust fans to enhance ventilation:

a. The fan system should at least provide the minimum air changes specified in SS553:2016.
b. Air supply and exhaust system can be aligned to provide uni-directional airflow in a poorly ventilated space.

4.4 Check other systems to ensure there is no undesired air leakage into occupied spaces, including water seals in the sanitary system, cracks in pipes and ducts, and wall gaps. Rectify faults detected.
 
5. Carbon dioxide (CO2) levels as a proxy for ventilation

5.1 To assess the adequacy of ventilation, measurement of ventilation rate is required. If this is not possible or difficult to determine, CO2 levels in occupied areas may be used as a proxy for ventilation adequacy.

5.2 Managers may use CO2 readings to identify pockets of under-ventilated spaces or overcrowding, so that prompt action can be taken to improve the situation. Quick assessment can be done by taking readings over a period of at least 5 minutes per sampling location, in occupied areas at breathing zones. In spaces with a dynamic crowd, managers could implement constant monitoring by installing CO2 sensors with visible displays in occupied areas or take spot measurements especially during high occupancy.

5.3 CO2 levels exceeding 1100 ppm indicate inadequate ventilation or potential overcrowding. SS554:2016 recommends an indoor CO2 limit of not more than 700 ppm in excess of outdoor levels. With ambient outdoor CO2 levels at about 400 ppm, the limit is therefore approximately 1100 ppm.

5.4 In view of the current COVID-19 situation, managers are advised to aim for CO2 levels below 800 ppm over the measurement period11,12,13.

5.5 While high CO2 levels indicate poor ventilation and/or overcrowding, CO2 levels may not be directly correlated to risk of exposure to virus, as other risk factors may be involved. Good ventilation and existing measures (masks, safe distancing, cleaning) are still needed even if CO2 levels are within the recommended limits.
 

6. More information
6.1 Further reference can be made to SS553:2016 Annex D and similar guidance documents published by ASHRAE14, REHVA12, and WHO6.
 
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6. Frequently Asked Questions

Scope of Guidance Note
Q1. Is it mandatory to comply with the guidelines in the Guidance Note?
No. While the recommendations are not mandatory by law, building owners and facilities managers should take active steps to improve ventilation, as this would reduce the risk of COVID-19 transmission through the dilution and removal of virus aerosols from indoor spaces. Building owners and facilities managers should develop their individual operation plans, taking reference from the Guidance Note and considering the existing building condition, to best meet the needs of their own buildings.

Q2. Do the guidelines apply to non-commercial buildings such as schools and community centres?
The guidelines apply to non-residential premises, including non-commercial buildings, schools and community centres, except for specialised premises such as certain factory production areas, hospitals, polyclinics and laboratories.

Q3. Are there plans to enforce against operators whose premises have high CO2 readings?  Would measures to improve ventilation be mandated under legislation?
The recommended indoor CO2 limits of 800ppm and 1,100ppm are guidelines under SS554:2016 Code of Practice for Indoor Air Quality for Air-Conditioned Buildings*, which is a voluntary standard and is not currently mandated. Notwithstanding, building owners, facility managers and occupants all have a role to play in maintaining good indoor CO2 levels and adequate ventilation to reduce indoor transmission of COVID-19.

While high CO2 levels are an indicator of inadequate ventilation or potential overcrowding, the direct correlation between CO2 levels and risk of exposure to the COVID-19 virus is not established, as risk can also be affected by other factors such as compliance with safe distancing, proper masks-wearing, and the presence of infected persons. Besides good ventilation, measures such as mask-wearing, safe distancing, cleaning are still needed even if CO2 levels are within the recommended limits.

[*Note: Under SS554, CO2 levels of >700 ppm in excess of outdoor level (approx. 1,100ppm), indicate poor ventilation or overcrowding. In COVID-19 situation, it is recommended that CO2 levels should not exceed an average of 800ppm over the measurement period.]

Ventilation
Q4. As different premises may be configured differently, does BCA/NEA/MOH have specific guidance to premises with CO2 levels higher than 800ppm to help them bring down the CO2 levels at their premises?  Who can the premises owner/operators reach out for clarification and technical advice on how to improve ventilation at their premises?
The revised Guidance Note provides building owners and facilities managers with updated recommended measures to enhance ventilation and air quality in indoor spaces.

For more detailed assessments, building owners and facilities managers may also engage consultants or professional engineers to provide technical advice and recommendations according to your needs. A list of mechanical professional engineers and their contact details can be found here: https://go.gov.sg/professional-engineer. A list of BCA-registered contractors offering air-conditioning, refrigeration and ventilation works can be found here: https://go.gov.sg/bca-registered-contractors.

Q5. My premises is enclosed and air-conditioned without mechanical ventilation provision. What should I do to improve ventilation?
It is recommended to install supply or exhaust fans to introduce outdoor air into the enclosed spaces.

Opening operable doors and windows, if it is safe to do so, can also be considered to increase natural ventilation. Air-conditioning systems should be reduced or turned off when doors and windows are opened to minimise energy wastage. If there are existing air exhaust systems, such as toilet exhaust fans, the exhaust fans should be operating at full capacity continuously.

In the event that these options are not possible or feasible, you may consider localised air cleaning as an interim measure. Portable air cleaners with HEPA filters may be used to help remove virus aerosols.

Nevertheless, localised air cleaning will not improve ventilation in the indoor space. Building owners of such spaces should make plans for mechanical ventilation where possible. You may wish to engage consultants or professional engineers to provide technical advice and recommendations according to your needs. A list of mechanical professional engineers and their contact details can be found here: https://go.gov.sg/professional-engineer. A list of BCA-registered contractors offering air-conditioning, refrigeration and ventilation works can be found here: https://go.gov.sg/bca-registered-contractors.

Q6. How can I determine if my workplace is sufficiently ventilated?
While there is currently no international standard for a minimum ventilation rate that would sufficiently prevent the risk of COVID-19 disease transmission in indoor spaces, WHO recommends a ventilation rate of at least 10L/s per pax. Ventilation rates can be calculated or measured based on the ventilation systems selected.

Safe distancing measures for building occupants, wearing of masks and regular cleaning of high-touch points are proven key measures that would significantly reduce the risk of COVID-19 transmission. Increasing ventilation further reduces the risk of disease transmission through dilution and removal of infectious agents from an indoor space.

Q7. Does the virus spread via the ACMV system? Will the increased ventilation spread the virus from one zone to the other?
It is commonly accepted that well-controlled ventilation systems incorporating high efficiency filtration of recirculated air will help to reduce the risk of aerosol transmission through air dilution and air cleaning. Directionality of ventilation air should be considered to facilitate air movement from clean-to-less-clean areas. Increased ventilation brings in more outdoor air, and will dilute any contamination, thus reducing the risk of aerosol transmission in an indoor environment. Notwithstanding the importance of increasing ventilation, other key measures to reduce disease transmission should continue, such as safe distancing, wearing masks properly, and carrying out regular disinfection of high-touch points within the building.

Q8. What should the building owners and facility managers look out for when checking the ACMV systems? Is there a checklist that we can refer to?
Building owners and facility managers should ensure that the ventilation systems are properly installed and operated according to specifications and relevant standards. For example, these systems should be designed in accordance with SS553:2016 Code of Practice for Air Conditioning and Mechanical Ventilation in Buildings (for building/occupancy types not listed in SS553:2016ASHRAE 62.1 should be used as reference) and SS554:2016 Code of Practice for Indoor Air Quality for Air-Conditioned Buildings. A checklist can be found in SS554 Annex G. In addition, additional measures to mitigate COVID-19 risk could be found in the Guidance Note, e.g. ensuring that demand control systems, such as those with CO2 sensors, are deactivated to avoid automatic reduction of outdoor air supply.

Q9. How often should maintenance of ACMV systems be carried out?
The recommended frequency of maintenance for various ACMV components can be found in SS553:2016 Code of Practice for Air Conditioning and Mechanical Ventilation in Buildings and SS554:2016: Code of Practice for Indoor Air Quality for Air-Conditioned Buildings. In addition to the recommended maintenance frequency, please also follow the recommendations specified in the latest Guidance Note.

Q10. My premises is located within a building (e.g. shopping mall) with mechanical ventilation system and I have no control over the building’s ventilation system. What can I do to improve ventilation within my premises?
Building owners and facilities managers should ensure that the building’s ventilation systems are properly maintained and in good working condition to provide adequate ventilation in all spaces, including worker resting areas. As tenants and occupants of the building, you may wish to engage the facilities managers to discuss ventilation issues and ensure that there is adequate ventilation within your premises. Where possible, you may also keep the windows or doors open to improve ventilation within your premises.

Air Cleaning
Q11. How effective are ultraviolet germicidal irradiation (UVGI) and high-grade filters in dealing with the virus?
The efficacy of UVGI against a range of microbes has been demonstrated by numerous studies, which have found that applying UV-C radiation can inactivate the virus. The effectiveness of this technology depends on many factors in a UVGI system’s design and operation, such as its structural design, lamp design, and effective contact time. When using UVGI, care needs to be taken to protect occupants from exposure to UV radiation.

Filters can remove particles of with varying efficiencies. Filters with rating of at least MERV 14, F8, or ePM1 70-80% are necessary to substantially remove fine particles, including most viruses and bacteria. To achieve optimal efficiencies, filters should be properly installed and maintained.

In spaces where there is a high risk of aerosol transmission, upper-room UVGI may be considered for air cleaning if the ceiling heights allow. UVGI systems must be installed by professionals, as viral disinfection efficacy and safety are greatly dependent on the system and how it is installed and maintained. More information on air cleaning is available in NEA’s Technical Advisory on Use of Air-Cleaning Technologies to Mitigate COVID-19 Aerosol Transmission Risk.

Q12. Does BCA/NEA/MOH approve or provide specifications for air-cleaning technologies that can be used to reduce airborne contaminants?
The agencies do not approve nor provide specifications for any air-cleaning technologies. Interested building owners and facility managers should seek professional services and consult various vendors for solutions of their interest. Building owners and facility managers are also encouraged to exercise caution and to ensure that product claims and the intended use of the product are supported by efficacy and safety data.

Please refer to NEA’s Technical Advisory on Use of Air-Cleaning Technologies to Mitigate COVID-19 Aerosol Transmission Risk for more information.

Q13. How can I select and size portable air cleaners for my premises?
Portable air cleaners or portable air-filtering devices are often equipped with a high-efficiency particulate air (HEPA) filter. Products with HEPA filters are recommended for use in environments with higher risk of COVID-19 transmission, while products with non-HEPA filters may be used in lower-risk environments.

When choosing a portable air-filtering device, select a unit that is appropriately sized for the space. The minimum smoke Clean Air Delivery Rate (CADR) that a unit should provide for a particular room size can be estimated as follows, according to AHAM AC-1 standard:

smoke CADR (cmh) ≥ room size (cubic meter) × 5

For example, a room with a floor area of 24 m2 and a ceiling height of 2.6 m (room volume of 62.4 m3) will need a portable air-filtering device with a minimum smoke CADR of 62.4 x 5 = 312 cmh (i.e. 184 cfm).

Please refer to NEA’s Technical Advisory on Use of Air-Cleaning Technologies to Mitigate COVID-19 Aerosol Transmission Risk for more information.

Energy Consumption
Q14. How will the recommended guidelines affect building energy efficiency/ use? Specifically, what is the impact of increased ventilation on energy?
We anticipate an increase in energy consumption by ACMV systems for occupied spaces, due to the recommendations for increased outdoor air supply and extended operating hours. However, the overall electricity consumption of buildings is expected to drop with the reduced building occupancy during COVID-19 period.

Design Standard
Q15. Is there any additional ventilation requirement that will be introduced in new construction for buildings in future? How should ventilation systems in new buildings be designed to support Business Continuity Plans while coping with potential pandemic situations?
There is currently no additional ventilation requirement due to COVID-19, other than what is recommended in SS553:2016: Code of Practice for Air Conditioning and Mechanical Ventilation in Buildings and SS554:2016 Code of Practice for Indoor Air Quality for Air-Conditioned Buildings. Managers are also advised to refer to SS553 Annex D and SS554 Annex K, which contain additional recommendations for operation of ACMV systems amid the pandemic, including maximising outdoor air supply, purging indoor air, and installing efficient filters in ACMV systems to treat recirculated air.  

In view of potential pandemic situations similar to COVID-19, it would be ideal for buildings to be designed with resiliency features that provide operational flexibility – normal operation mode vs epidemic mode.

For instance, systems could be designed to allow a wider operational range of outdoor air intake and accommodate high efficiency filters.

Notes and references

[1] ACMV systems include air handling units and fan coil units such as cassette- or wall-mounted types.

[2] Specialised premises refer to those with ACMV systems that fall outside the scope of SS553:2016 Code of Practice for Air Conditioning and Mechanical Ventilation in Buildings.

[3] National Environment Agency - Environmental Cleaning Guidelines. https://www.nea.gov.sg/our-services/public-cleanliness/environmental-cleaning-guidelines

[4] For building/occupancy types not listed in SS553:2016, ASHRAE 62.1 should be used as reference.

[5] AHU: air handling unit; FCU: fan coil unit; PAU: primary/pre-cool air handling unit; FAF: fresh air fan; EAF: exhaust air fan.

[6] World Health Organization - Roadmap to improve and ensure good indoor ventilation in the context of COVID-19. https://www.who.int/publications/i/item/9789240021280

[7] To avoid mould growth due to condensation caused by introduction of excessive humid outdoor air, outdoor air may be treated by a dedicated outdoor air processing system. The outdoor air system can be designed such that efficient filters (at least MERV14, F8, or ePM1 70-80% is recommended) can be fitted when necessary. The filters will be useful during times when windows cannot be opened due to poor outdoor air quality.

[8] Such spaces include those where bioaerosol-generating procedures are performed on people (e.g. nasopharyngeal swab taking and dental procedures), where COVID-19 patients may be present, or where masks must be removed.

[9] European CDC - Heating, ventilation and air-conditioning systems in the context of COVID-19: first update, Nov 2020. https://www.ecdc.europa.eu/en/publications-data/heating-ventilation-air-conditioning-systems-covid-19

[10] The size and number of portable air cleaner devices used shall be based on CADR. If the unit of CADR is in m3/h, the total CADR of the air purifier(s) should be at least five times the room volume in m3. In enclosed spaces where ventilation cannot be easily improved (e.g. no operable doors or windows), CADR of seven times the room volume should be considered.

[11] Centers for Disease Control and Prevention - Ventilation in Buildings. https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html

[12] Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) - REHVA COVID 19 Guidance version 4.1: How to operate HVAC and other building service systems to prevent the spread of the coronavirus (SARS-CoV-2) disease (COVID-19) in workplaces. https://www.rehva.eu/activities/covid-19-guidance/rehva-covid-19-guidance

[13] Draft Amendment No. 1 to SS554: 2016. https://www.enterprisesg.gov.sg/-/media/37324ECF5FF7448F9C43021E69EE0D78.ashx

[14] American Society of Heating, Refrigerating and Air-Conditioning Engineers https://www.ashrae.org/technical-resources/resources