The main transmission routes for SARS-CoV-2 are direct contact or respiratory droplets (aerosols) during close contact (within  six feet for 15 minutes) with an infected person.  However, according to an early release CDC report due out this July, whether SARS-CoV-2 can be transmitted by aerosols remains controversial.

The following discussion will try to shed some light on what can be a complicated question.  Should any special steps be implemented for a building’s heating, ventilating and air conditioning (HVAC) system in buildings during various stages of occupancy?  This question is asked more frequently as we start to move from community-wide stay at home orders to phased re-opening of buildings.

What is the Purpose of an HVAC System?

HVAC systems are designed to maintain areas of the building in comfortable temperature and humidity ranges and to dilute airborne contaminants (e.g. human bioeffluents, carbon dioxide, etc.) by replacing indoor air with “fresh” outdoor air. Out with the bad, in with the good. For indoor air, elevated levels of CO₂ can be an indicator of potential contaminants and the HVAC system becomes an engineering control to remove those contaminants.  In the hierarchy of hazard controls, the preferred option is to remove the hazard.  Since people are frequently the source of indoor air contaminants, removing them is not generally the option.  There will be more on source control in a bit.

In some instances, HVAC systems can also be designed to maintain pressure differentials between spaces and to create a directional flow of air.  Examples of these types of applications include operating rooms in hospitals and clean rooms in pharmaceutical or high-tech manufacturing areas where the pressure in the controlled room is higher than the pressure in adjacent spaces. This directs air movement from the controlled clean spaces to the less controlled adjacent space.  We sometimes call this moving from clean to dirty. Examples of negative pressure rooms include airborne infection isolations rooms (AIIR) in hospitals for patients such as those with SARS or Tuberculosis. Pressure differentials are accomplished for positive pressure rooms by delivering a higher volume of air into the room through the HVAC supply vents than the volume of air pulled out of the room through the exhaust vents. Just the opposite is done for negative pressure areas. Another example of a less stringently engineered negative pressure room might be the restroom in your home where the exhaust vent pulls air out at a rate greater than that which is brought in through the HVAC vent.

This topic can get very detailed and complicated in buildings with different climate control zones and needs and varying occupancy factors.  In most commercial buildings such as retail, offices, educational and in residential buildings the HVAC systems are still very important but less complex.  The flow of air through any building is a dynamic condition and can be changed dramatically with minor changes to how much air is delivered or removed, controlling the points of entry and removal, movement of physical objects, or people in the space, changes to indoor and outdoor temperatures and humidity and many other factors. One study discussed convectional airflow created by human body heat. As our bodies shed heat, air in our personal space moves gently upward.  This may provide some protection as exhaled breath is pulled up towards the ceiling and away from those around us.  It certainly plays a role in how air moves in a building.  But this is just one study, and many others such as CDCs Interim Guidance for Businesses and Employers to Plan and Respond Coronavirus Disease 2019 and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Position Document on Infectious Aerosols recommend that dilution ventilation be increased with additional, preferably 100%, outdoor air if possible.

HVAC Systems & COVID-19 – Do Changes Need to be Made?

When it comes to taking advantage of HVAC system design and function to minimize risks related to COVID-19, numerous entities provide general recommendations, but at this time there is no clear standard.

Following are some of the recommendations provided on the CDC site COVID-19 Employer Information for Office Buildings.

• Take steps to improve ventilation in the building:
  • Increase the percentage of outdoor air (e.g., using economizer modes of HVAC operations) potentially as high as 100% (first verify compatibility with HVAC system capabilities for both temperature and humidity control as well as compatibility with outdoor/indoor air quality considerations).
  • Increase total airflow supply to occupied spaces, if possible.
  • Disable demand-control ventilation (DCV) controls that reduce air supply based on temperature or occupancy.
  • Consider using natural ventilation (i.e., opening windows if possible and safe to do so) to increase outdoor air dilution of indoor air when environmental conditions and building requirements allow.
  • Improve central air filtration:
    • Increase air filtration to as high as possible (MERV 13 or 14) without significantly diminishing design airflow.

Note: MERV stands for Minimum Efficiency Reporting Value, which is a value assigned to filters and considers both the size and percentage of particles that are prevented from passing through the filter.  The MERV value is from 1 to 16. Higher MERV values correspond to a greater percentage of particles captured, with a MERV 16 filter capturing more than 95% of particles over the full range. Here is a link to more information on MERV ratings.

• Inspect filter housing and racks to ensure appropriate filter fit and check for ways to minimize filter bypass

• Consider running the building ventilation system even during unoccupied times to maximize dilution ventilation.
• Generate clean-to-less-clean air movement by re-evaluating the positioning of supply and exhaust air diffusers and/or dampers and adjusting zone supply and exhaust flow rates to establish measurable pressure differentials. Have staff work in areas served by “clean” ventilation zones that do not include higher-risk areas such as visitor reception or exercise facilities (if open).

• Increase circulation of outdoor air as much as possible by opening windows and doors, using fans (note that fans placed near occupants may distribute respiratory droplets), and other methods. Do not open windows and doors if doing so poses a safety or health risk for current or subsequent occupants, including children (e.g., allowing outdoor environmental contaminants including carbon monoxide, molds, or pollens into the building).Consider using portable high-efficiency particulate air (HEPA) fan/filtration systems to help (especially in higher risk areas).
• Ensure exhaust fans in restroom facilities are functional and operating at full capacity when the building is occupied.

The CDC Guidelines has published a table the shows the impact of air exchange rates on the removal of airborne contaminants from a controlled space.  It looks at the time it takes to remove 99% and 99.9% of airborne contaminants based on the number of air changes per hour (ACH).  Air changes per hour is a description of how many times the full volume of room air is exchanged.  Increasing the ACH reduces the time needed.

There are some important limitations to the data in the table though.  First, it presumes perfect air mixing which is considered unlikely.  Second, and most importantly in this scenario, it presumes no new contaminants are being introduced. Following the concept that increasing ACH reduces the time to remove airborne contaminants, it seems to be common sense to increase ventilation in occupied buildings as an environmental engineering control measure.

Numerous agencies, associations and businesses have been providing webinars and other educational content during the COVID-19 Pandemic presenting research papers regarding the role of HVAC systems.  Many arrive at the same general recommendations.  However, there are some important things to consider.  First, as with most everything related to COVID-19, papers and presentations often state something to the effect of, the findings and conclusions of this study were based on the information available at the time of the study. Since the research related to SARS-CoV-2 is ongoing and evolving rapidly, additional research is needed and continued diligence in watching for updates to these findings is recommended.  Next it is important to consider that much of the research is related to other pathogens such as SARS-CoV-1, Influenza viruses and MERS, which may not be directly applicable to SARS-CoV-2.  Some studies discussing filtration suggest that high efficiently particulate air (HEPA) filters, which are tested for particles larger than 0.3 microns in diameter, are useful but do not filter out smaller particle sizes such as the SARS-CoV-2 virus. While other reports on filtration suggest that these filters are more efficient for smaller particles than they are for 0.3 micron sized particles. And to further complicate the issue, some conclude that increasing ventilation is likely to reduce risk while at least one suggested increasing airflow could increase the risk of short distance airborne transmission.

If the people are the source, direction of flow can be important.  Below is a diagram showing the potential for improving conditions through directional air flow and the possibility of increasing risk. Blue arrows indicate the direction of air movement, red arrows indicate exhaled breath.

While directing airflow from back to front could reduce risk of short-distance airborne transmission, moving air from right to left may have an undesirable effect. It may be difficult to place people in a convenient, single-file line and even if it can be done, their behaviors and movements can be unpredictable.

Can the HVAC System be the Source of the Contaminant?

Available research has not yet concluded that SAR-CoV-2 enters the building from outdoors through HVAC systems.  Using influenza or tuberculosis as examples, it has not historically been considered a risk that these pathogens, shed by people outdoors enter buildings through HVAC systems.

Recirculating air through the HVAC system is another concern.  At this time, there are very few papers suggesting that viruses similar to SARS-CoV-2 have caused infections by being distributed through HVAC systems.  As noted, the primary route of concern is direct person-to-person transmission. That is not to say the risk of transmission by other means does not exist.  Until we have studies that conclude the risk or transmission through recirculated air is negligible, we should take steps such as reducing or eliminating recirculation or increasing filtration.  With regards to maintenance and repairs of HVAC systems, we should find ourselves using terms like “out of an abundance of caution”, or “as low as reasonably achievable.”  This means use of PPE to change filters is probably a good idea.

As for duct cleaning in response to concerns for contamination, it does not appear to be a consensus opinion that enhanced duct cleaning is necessary. In general, the EPA states that no evidence exists to suggest that any health hazards stem from light dust accumulation in ducts and does not recommend cleaning unless there is visible mold growth or pest infestation. Specific to COVID-19, one report out of Europe, REVHA’s Guidance Document Version 2 stated the following:

There have been overreactive statements recommending to clean ventilation ducts in order to avoid SARS-CoV-2 transmission via ventilation systems. Duct cleaning is not effective against room-to-room infection because the ventilation system is not a contamination source… Viruses attached to small particles will not deposit easily in ventilation ducts and normally will be carried out by the air flow anyhow. Therefore, no changes are needed to normal duct cleaning and maintenance procedures. Much more important is to increase fresh air supply, avoid recirculation.

As we have tried to show, the role of the HVAC system is complicated, the research can be confusing and the impact of changes to it on preventing COVID-19 is not yet conclusive. Some bundle or combination of environmental control options is likely to be the best approach, but actions taken should be based on actual risks and ability to control them.  Therefore, a site-specific risk assessment that considers both occupants and the building systems is necessary. In many buildings, some of the recommendations listed above may not be feasible, may increase costs or energy usage and may have limited comparative impact on controlling the risks present.

So…What Should we Do?

Conduct a site-specific risk assessment and consider the following:

• Inspect the HVAC system as noted in the recommendations provided by CDC.
• Isolate higher risk areas and place an emphasis for enhanced engineering controls on these areas. Create pressure differentials by adjusting supply and exhaust ventilation dampers and vents.
• Increase ventilation (outdoor air supplied to indoor spaces), set the HVAC system to 100% outdoor air if possible, eliminating the recirculation of indoor air and increase air changes per hour.
• If you must recirculate indoor air, consider upgrading filter efficiency to MERV 13 or better it the system will allow for it.
• Open windows and doors to let in more outside air if possible.
• Use indoor fans to help direct air flow in desired directions. Personal fans should be minimized or eliminated. If used, they must be part of the overall control plan.
• Use air scrubbing fans equipped with HEPA filtered exhaust to help rapidly reduce airborne particles and direct airflow in desired directions.
• Use positioning of people and temporary physical barriers such as portable cubicle walls, plexiglass walls or even shower curtains on PVC pipe frames to create air corridors.
• Test your changes. Use visible indicators to show how air is moving.  This can be done through a variety of low to higher tech methods.  An example of low tech could include the use of ventilation smoke pens that generate a visible, non-toxic smoke plume that can be observed as it moves up, down and laterally through the building spaces.  Instrument or laboratory-based methods, such as tracer gas studies or the use of manometers to measure pressure differentials or anemometers to measure direction and velocity of airflow are also available

All of these are environmental engineering controls.  Remember that source control is preferred to engineering controls.  That means that people are important in this control equation. Consider reducing the number of people present at any given time in some spaces and closing off other spaces.

Even if it is possible to effectively re-engineer how air moves around the building from clean to dirty or to quickly and efficiently replacing indoor air with fresh outdoor air, these are still supplemental to the more important control measures of stay home if you are sick; wash your hands; keep distance; clean surfaces regularly; know and follow the safe work practices that have been developed to protect you and others; and wash your hands again!