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  • Art Goguen

Latest Recommendations on Indoor Air Transmission of COVID-19 in Schools and Workplaces

The latest American Society for Heating Refrigeration and Air Conditioning Engineers (ASHRAE) COVID-19 Epidemic Task Force Core recommendations for Reducing Airborne Infectious Aerosol Exposure have recently been released.[1] They state that “within limits, ventilation, filtration and air cleaners can be deployed flexibly to achieve exposure reduction goals.” The Task Force states that subject to constraints like comfort, energy use, and cost, these goals are achievable by setting certain targets.[1]

Some of these ventilation, filtration, and air cleaning goals also tie to state and local building codes that set minimum outdoor airflow rates for building ventilation.[2] Merging outdoor airflow rates in combination with building and portable air filtration rates that achieve MERV 13 or better for air recirculated by HVAC systems will provide desired indoor exposure reduction.[2] These filtration rates should be checked periodically by testing the airflow rates and measuring the filter static pressure differential against the maximum operating resistance set by the filter manufacturer. Portable air cleaners for which efficacy evidence is clear can be used in combination with the building HVAC systems when properly set up.

How do you know if your efforts are working?

The CDC position on using portable HEPA air cleaners state that air filter testing is complex requires specialized skill and equipment, and no individual test can adequately evaluate all filters. However, there are ways to checking to see if filters are performing effectively. Spot checks can be performed using discrete optical particle counters and differential pressure monitors.[3]

The US Department of Energy recommends that where directional airflow is not specifically required, or not recommended as a result of a risk assessment; promote mixing of space air without causing strong air currents that increase direct transmission from person-to-person.[4] The reason is that stratified ventilation can trap infectious aerosols in inversion layers and increase risk.[4] Studies on airflow patterns within hospital patient isolation rooms indicate that airborne pathogens tend to follow airflow currents from the supply air diffusers to the room exhaust.[5]

Other factors to consider:

Placement of supply registers and room exhausts are not the only influences in controlling airborne transmission of viruses. Transmission of viruses via airborne routes may be affected by ambient humidity, which affects not only the virus’ stability but also respiratory droplet size, as water content evaporates.[3] Another environmental factor that may influence viral transmissibility is precipitation.[3] Though relatively few data exist, airflow (the speed of air currents flowing through indoor spaces) and ventilation (the degree of mixing between indoor and outdoor air) seem to play a role in respiratory virus infectivity and transmission.[3] Airflow visualization analysis can locate undesired airflow patterns, visualize turbulence, obstructions, potential sources of contamination, to ensure proper airflow.[6]

Need help? We have you covered.

For more than a decade, Higgins and Associates, LLC has consulted with both contractors and healthcare facility directors to provide in-depth environmental infection control training and construction oversight support.

If you need help, we can assist with your program development, training programs, containment design reviews, specialized ICRA training, and environmental infection control air and water sampling during construction projects within hospitals and healthcare facilities of any size and any location.

For more information, contact Art Goguen, Director of EHS, at (303) 994-8122. You can also visit our website to learn more about all of our healthcare EHS services.


1. American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE), ASHRAE Epidemic Task Force, Core Recommendations for Reducing Airborne Infectious Aerosol Exposure, 2020,

2. Mudarri, David H., US Environmental Protection Agency (EPA), Building Codes and Indoor Air Quality, 2010,

3. Pica, Natalie and Bouvier, Nicole M., Environmental factors affecting the transmission of respiratory viruses, 2012, Science Direct,

4. US Department of Energy, Minimizing Infectious Disease Spread in Buildings: How HVAC Guidance has Evolved, January 25, 2021

5. U. Ghia,, Assessment of health-care worker exposure to pandemic flu in hospital rooms, ASHRAE Transactions(Vol. 118, Issue 1), Jan 2012,

6. Mousavi, Ehsan S.,, COVID-19 Outbreak and Hospital Air Quality: A Systematic Review of Evidence on Air Filtration and Recirculation, August 26, 2020,

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