Two recent interviews addressed the relationship between COVID-19 and air conditioning. An episode of Indoor Air Quality Radio featured Bill Bahnfleth, Chairman of the Epidemic Task Force of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The radio program Here and Now interviewed Shelly Miller, professor of environmental engineering and expert in indoor air quality at the university of Colorado at Boulder.

Both experts emphasized that indoor environments pose a greater risk of infection than outdoor environments. “There’s good reason to believe that most people are infected in indoor environments, whether they’re building or mobile environments.”, Bahnfleth said. Miller pointed out, “When we’re indoors a lot, with poor ventilation inside and crowding, and no masks, then it’s a perfect recipe for transmission.” Both also said there are no findings to indicate that that the virus can be transmitted through central air conditioning. According to Bahnfleth, “No one that I’ve talked with who’s been following this has seen any evidence that even if the virus gets through an air conditioning system, that there’s been space to space transmission.”

However, the relationship between HVAC systems and COVID-19 is a complicated issue that appears to require considering pros and cons.

For instance, Miller highlighted the ability of air conditioning to limit outdoor air from mixing indoors. With respect to central air conditioning she said, “You very rarely bring in much more than 20% or less outside air because it’s hot and it takes a lot of building energy to cool that air. And so my concern is that in air conditioned buildings you do not have enough outside air and don’t have enough adequate filtration in your re-circulation system to suppress and decrease concentrations of airborne virus that may be circulating in your environment…The transmission is happening locally inside the space where you are with lots of other people and there’s not enough outside air because you’re trying to keep the air cool.”

On the other hand, Bahnfleth pointed out there can be protective effects of central air conditioning. “Turning off your air conditioning system is not necessarily a good idea…the thing that has to be remembered is that AC systems do a lot of different things. They not only circulate air in a space, which is necessary to control temperature and humidity and contaminants, they also provide ventilation. They should provide ventilation and they filter they air. They do things that are highly protective like dilute contaminants and remove them from the air. So in general it’s a good thing to have the system running.” ASHRAE has developed recommendations for optimizing HVAC systems as a tool to fight the transmission of COVID-19. These include increasing ventilation, reducing re-circulation, improving filtration, adding air cleaning technologies, and opening windows.

Read more about ASHRAE recommendations for HVAC systems and access additional COVID-19 resources from ASHRAE HERE.

Listen to the full interview with Dr. Miller on Here and Now, which includes her insights on considerations for opening schools HERE.

Listen to the full conversation with Dr. Bahnfleth on IAQ Radio HERE.

On July 21, from 11:00 to 12:00 PM EDT, ASHRAE will be offering a free online course, Analysis of Airflow Patterns and Flow Path of Airborne Contaminants. The course is designed to provide valuable insights to HVAC design engineers, facility managers, infection prevention personnel, and building owners regarding the role of airflow patterns and resulting flow path of airborne contaminants. Read more and register HERE.

ROCIS insights on high MERV filtration

What did we find as we have explored the feasibility to modify existing residential HVAC systems to provide effective particle filtration? The graph below summarizes the problems/barriers identified. NOTE: Most of these HVAC systems were located in a basement.

Participants in ROCIS cohorts monitor carbon dioxide ( CO ₂) levels in their homes, and ROCIS provides interpretation and feedback regarding potential solutions for high indoor CO ₂. Concerns about a CO ₂ levels of a different kind recently gained attention on social media . The claim states that a severe health problem called hypercapnia - too much CO ₂ in the blood - can occur due to rebreathing your own exhaled CO ₂ by wearing a mask continually.  This claim has been repeatedly debunked by medical professionals and news organizations, including via the Cleveland Clinic, the Associated Press, and the BBC.

"This simply won't happen unless there is an air-tight fit and you rebreathe your air," Prof Keith Neal, an infectious disease expert, told the BBC. Sarah Stanley, associate professor of infectious diseases and vaccinology, told the Associated Press, “Keep in mind that many people —for example surgeons or certain kinds of scientists—have routinely worn masks for long periods of time without clear adverse effects. With how common mask wearing has always been, even before COVID-19, we would know if hypercapnia was a problem with wearing masks.”

Carbon dioxide molecules are much smaller than the droplets containing coronavirus which the masks are designed to stop. Breathable material cannot trap CO ₂ at dangerous levels, especially during relatively short periods like a trip to the grocery store.

However, it is possible for masks to pose a risk to some vulnerable populations. According to the Cleveland Clinic, people who should not wear cloth masks include kids under age two, anyone who ordinarily has trouble breathing, or anyone who can’t take the mask off without assistance. N95 respirators are far more restrictive than the surgical masks and cloth masks being worn in response to COVID. People with chronic respiratory, cardiac, or other medical conditions that make breathing difficult should check with their health care provider before using an N95 respirator.