Aerosol transmission has emerged as a COVID-19 infection mechanism with very small particles carrying the virus, emitted by infected people breathing, hanging in the air possibly for hours, pushed around by air flows in rooms. These are then potentially drawn into recirculating HVAC systems – possibly being reintroduced into occupied areas.
In deciding what to do it is important to think about the infection mechanism, what is known and what is not yet certain.
There is still much to discover and understand about aerosol transmission of SARS-CoV-2.
There is not yet a clear and accepted method for measuring the effectiveness of a HVAC system in mitigating the transmission of SARS-CoV-2.
Every building and its HVAC system is different, both in its design and operational state (for an explanation of typical HVAC system operation and potential viral airborne transmission see a previous advisory not
There are things that can be done with commercial built-up HVAC systems in both the base building system and in tenancy fit outs that will improve their role in mitigating the transmission of the virus. In this it is also important to remember that anything done with the HVAC system to provide a healthier indoor environment is generally a positive outcome for occupants.
The primary objectives are to dilute potential viral aerosol concentrations and move these aerosols away from people. Key issues are:
- Ventilation rates – the total amount of introduced air that is travelling through a space over time – is very important. In the first instance ventilation rates should be as-designed or as-commissioned unless internal occupancy requirements have changed (due to changes in space such as layout/size, number of people, heat emitting equipment). Have they been changed over time?
- Can ventilation rates be practically increased? This sounds straightforward however in practice may be challenging to achieve – Variable Air Volume Control (controlling temperature by changing ventilation rates), system capacity, the balancing of air systems, the potential for localised drafts can all introduce difficulties when increasing ventilation rates
- How is the air moving through the space? Are the outlet grilles in the best locations with respect to occupant locations? Is the design of the grilles optimal? Can the grilles accommodate additional air flow? Is the air mixing in a turbulent way potentially assisting in dilution or distributing aerosols more widely? Are the return air pathways ideal? Is air in the space being drawn across multiple occupants towards the return air grille?
These elements often get neglected in tenancy modifications, such as, did the door grilles get removed in the last fitout, restricting air flow?
- Can more outside air be brought in to further increase the dilution rate in the system? Many HVAC systems have modulating control of outside air proportions to maximise “free cooling” when the conditions are favorable, or to manage minimum CO2 levels. For the majority of the time these systems provide higher ventilation rates than the minimum requirement however controls may be overridden to further increase the outside air intake amount
Increasing outside air amounts may challenge the ability of the system to achieve the desired indoor temperature and relative humidity levels in hot or cold weather
Additional heat loads due to increased outside air rates will also increase system heat rejection rates and associated cooling tower water usage.
- Can the building be ventilated after daily occupancy, to flush it, ideally with maximum outside air?
Increasing ventilation rates, increasing outside air rates and increasing run times will all have an impact on energy usage, both fan energy and thermal energy. If not balanced with energy efficiency measures this will likely materially increase building energy consumption and compromise sustainability ratings.
Air Filtration or Air Cleaning
The objective is to remove contaminated particles and or cleanse the recirculating air.
If aerosols are caught up in recirculating HVAC systems can they be pushed back into an occupied space and infect people? In theory this is possible and while there is not substantive evidence to date to suggest this is a significant transmission risk, improving filtration performance will generally improve indoor air quality.
What can be done to improve the air filtration of recirculated air?
- Bag type air filters are typically installed in large commercial HVAC systems. In the first instance it is important to review the standard of the existing filtration. If less than as-designed, then as a minimum return to this standard.
- There will be options to improve filter selection to increase the arrestance level, removing more smaller particles. Increasing the filtration standard generally increases the resistance on the fan and thereby increases energy usage.
However, filter technology has improved over time; newer filters with more pockets and more surface area woven into the pockets can reduce resistance whilst improving filter arrestance.
There may be the opportunity to make a worthwhile improvement in filtration standard without too much impact on fan energy. Upgrading to (Minimum Efficiency Reporting Value) MERV 13 (F7) standard as a minimum is recommended if practical.
Increasing the filter standard may also require an increase in filter replacement frequencies. Generally, air filters in commercial HVAC systems are disposable and not cleaned and reused. Replacement and or cleaning of air filters should only be undertaken by experienced service providers using appropriate Personnel Protective Equipment and approved processes.Other types of air filter are available however are often costly or impractical to retrofit without significant system modification and may also have energy usage and other operational implications. These include:
- HEPA (High-efficiency particulate air) filters that will filter 99.97 per cent of particles whose diameter is equal to 0.3 micron have a very high air resistance and will usually require fan replacement and system changes. These are commonly used in critical healthcare situations.
- Electrostatic Precipitators that use electrically charged screens to attract particles were common when smoking was permitted in buildings. These can have a filtration efficiency close to HEPA filters with less air flow resistance. They have particular spatial and maintenance requirements and need a power supply. These filters also typically create ozone, potentially at a level that requires the air to be treated with an activated Carbon filter or similar, introducing further resistance into the airflow and introducing an element that requires regular replacement.
- Standalone room air purifiers with a fan that recirculates room air through a HEPA filter can reduce levels of suspended aerosols and particulates in the air and therefore reduce the risk of SARS-CoV-2 virus transmission. These units can be considered for higher risk enclosed rooms such as meeting rooms where social distancing is difficult. Consideration should be given to their ongoing operational and maintenance requirements and associated costs.
In relation to disinfecting or purifying the recirculating air, there are a number of technologies that can be used, to date usually in healthcare or laboratory situations:
- Ultra-violet germicidal irradiation (UVGI) can be applied to kill viruses and bacteria. The effectiveness of this depends on the strength of the radiation and the period of time in which it has to impact on particles. This technology can be applied in ductwork and air handing plant, in rooms above the occupied zone, and stand alone in-room recirculating units are also available.
When installed in air handling units the risk of UV exposure to maintenance personnel must be managed with safety interlocks, signage and UV safe viewing portals. UV light can also cause premature degradation of some materials used in air handling units.
The potential effectiveness of UVGI technology will depend on the circumstance of the particular building and its systems, and the method of application. (For a more detailed explanation of UVGI systems refer to this advisory note.)
- Ionisation technology can be employed to charge airborne particles including aerosols, causing them to clump together to facilitate filtration or precipitation. Bi-polar ionisation systems can also provide a level of viral inactivation. Some ionisation systems produce ozone as a by-product at levels considered harmful to human health. These systems should be avoided.
The potential effectiveness of ionisation technology will depend on the circumstance of the particular building and its systems, along with the method of application.
Air disinfection or cleansing methods should be considered as part of a broader strategy that includes enhanced ventilation and improved filtration. Consideration should also be given to their ongoing operational and maintenance requirements and associated costs.
The objective is to create conditions that limit the formation of aerosols from larger particles.
Research indicates that particles emitted by breathing can rapidly evaporate and shrink in size when emitted creating smaller and more problematic aerosols. The relative humidity of the air in a space is believed to have an effect on the rate of evaporation. Current advice suggests indoor relative humidity be kept in the range of 40-60 per cent.
Commercial air conditioning systems typically do not directly control relative humidity and could require additional control functions and or adjustment to better assure this range is maintained.
The installation of humidification devices may also be considered.
Improved humidity control should be considered as part of a broader strategy that includes the initiatives described previously, some of which may detrimentally affect humidity.
See this advisory note for more detailed explanation of the role of humidity in HVAC.
This is also an important factor in how HVAC systems can be used to help to potentially mitigate the transmission of the virus. The objective is to improve occupant knowledge and understanding of the role that HVAC plays and provide confidence in the system’s performance.
- The key in this is to offer reliable and accurate information to build tenant knowledge and confidence that the HVAC system is being well managed by building management and the best it can be in providing a safe environment.
- Appropriate indoor air quality testing and a meaningful reporting regime will underpin the veracity of messaging to tenants.
- An effective HVAC maintenance regime with useful reporting from a proactive provider who understands the role of HVAC in providing a healthy indoor environment will also provide confidence to building occupants.
There are a number of useful initiatives that can be considered to assure and/or improve the ability of HVAC systems to assist in the mitigation of viral transmission:
- Increase the dilution and removal of infectious aerosols via appropriate ventilation rates and outside air rates. Audit ventilation rates and airflow patterns and pathways, are they as designed/commissioned? Can they be improved? Can outside air rates be increased in practice?
- Good or improved air filtration to remove particles in a recirculating system. A minimum filtration standard of MERV13 (F7) is recommended. Should an alternative high efficiency filtration system be considered? What will the effect be on fan suitability and system design?
- Treat the air to inactivate the virus, or to help remove particles. Is this a useful strategy for this system?
- Manage indoor humidity levels to reduce the formation of aerosols. What are the options?
- Provide proper maintenance together with appropriate testing and reporting to support accurate and valued communication to occupants.
Many of these initiatives will also increase energy usage and affect building energy and potentially water ratings. Likely implications should be determined ahead of changes along with other efficiency measures that can balance these negative effects.
The best approach for a setting will depend very much on the design and operation of the particular building, its systems and its occupancy. An audit of current conditions and a well-informed approach can deliver high value improvements whilst minimising any effects on energy or water usage.
Matthew Peacock is a senior mechanical engineer at A.G. Coombs Advisory. This article first appeared on the A.G. Coombs Advisory website, where the full list of references and resources are also available.
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