Global research is being conducted to combat COVID-19. Classified as a coronavirus, this disease behaves much the same way that other coronaviruses do in terms of the way it spreads.

While a cure for this pandemic has not yet been discovered, the virus is not without its limitations. Optimal (and non-optimal) environments exist for air contaminants like the COVID-19 virus, and although adjusting the air quality inside a home or business won’t stop the infectious disease, it could impede its transmission.

A research report was released on March 9, 2020 from a team of scientists out of two Beijing universities — Beihang University and Tsinghua University. Supported by the National Key R&D Program of China and the National Natural Science Foundation of China, the "High Temperature and High Humidity Reduce the Transmission of COVID-19" document concluded that high temperature and high humidity reduce the transmission of COVID-19.

“Using the daily R values from January 21 to 23, 2020 as proxies of non-intervened transmission intensity, we find, under a linear regression framework for 100 Chinese cities, high temperature and high relative humidity significantly reduce the transmission of COVID-19, respectively, even after controlling for population density and GDP per capita of cities,” stated the report. “Our finding is consistent with the evidence that high temperature and high humidity reduce the transmission of influenza, which can be explained by two possible reasons: First, the influenza virus is more stable in cold temperature, and respiratory droplets, as containers of viruses, remain airborne longer in dry air. Second, cold and dry weather can also weaken the hosts' immunity and make them more susceptible to the virus. These mechanisms are also likely to apply to the COVID-19 transmission. Our result is also consistent with the evidence that high temperature and high relative humidity reduce the viability of SARS coronavirus.”

The 20-page document went through the scientists’ methodology and findings in great detail. At one point, it drew attention to what it called a noteworthy phenomenon.

“Rough observations of outbreaks of COVID-19 outside China show in the early dates of the outbreak, countries with relatively lower air temperature and lower humidity (e.g. Korea, Japan, and Iran) see severe outbreaks than warmer and more humid countries (e.g. Singapore, Malaysia, and Thailand) do.”


This doesn’t mean that optimal temperature and humidity stops the transmission of COVID-19 in its tracks, but there is evidence that air quality will affect how long a viral contaminant like COVID-19 lingers in the air, as well as the radius that the contaminant spreads while in the air.

“The only place where humidity can actually affect the virus is during transmission, while the person is expelling the virus as a droplet infection,” said Jim Bridgeman, chief operating officer, DewAir Corporation. “Once it lands on the surface, humidity no longer has a role in its transmission.”

For those viruses that respond to humidity levels in the air, the theory is that drier air allows the airborne droplets to absorb quickly and linger longer in the air, as compared to higher humidity situations where the air is unable to absorb the infection droplets and they move quickly to the surface.

Not all scientists and researchers are in agreement. A COVID-19 guidance for buildings document was released from The Federation of European Heating, Ventilation, and Air Conditioning Associations (REHVA) on March 17, 2020, in which humidity was discounted as a viable option in the reduction of transmission.

“Transmission of some viruses in buildings can be limited by changing air temperatures and humidity levels,” stated the document. “In the case of COVID-19, this is unfortunately not an option, as the SARS-CoV-2 virus is quite resistant to environmental changes and is susceptible only for a very high relative humidity above 80 percent and a temperature above 30˚C (86°F), which are not attainable and acceptable in buildings for other reasons (e.g. thermal comfort).”

In general, ASHRAE promotes keeping humidity between 30-60 percent to minimize overall virus survival rates, and the Environmental Protection Agency (EPA) agrees that home and building owners should keep indoor humidity in an optimal range. Specific ranges for every virus, however, has not been determined. (See Figure 1)


Research on COVID-19 is still in its early stages, and the conclusions have been a mixed bag and directly impacted by innumerable variables. That being said, there is still a role for HVAC contractors to play in the prevention of health issues and the boosting of the public’s immune systems.

“For instance, if a person is living in a house that is full of mold, mildew, dust mites, and they have allergies and other respiratory conditions, it is even worse for them if they become exposed to COVID-19,” said Bridgeman. “Contractors need to focus on keeping the environment for any living space as healthy as possible and minimize all these other pathogens and stressors on customers’ immune systems.”

Humidity holds a direct link to human health, as it supports many of the body’s natural defenses.

“When the humidity is too low, your natural defenses start to break down,” said John Bloemer, executive advisor, engineering fellow, Aprilaire. “Your nasal passages get dry, your skin might get dry, and your throat gets dry. The lack of moisture is breaking down the body's natural immunity systems, or natural prevention systems. That's why being too dry is not good.”

Aprilaire released a statement addressing airborne viruses on March 19, where the company explained that having a properly functioning whole-home ventilation system is critical to maintaining a healthy home environment and that ventilation helps dilute and remove contaminants. 

“Contractors can tell their customers to turn on their ventilators and let them run; don’t try to run them at a minimum level” said Bloemer. “That's probably the best thing you can do, then controlling humidity which helps the body fight infections; then add good air filtration, which helps remove particulates. Make sure the HVAC system’s running so the air is moving through the air cleaner.”

He cautioned that the position contractors take with customers should not be that there is an environmental silver bullet, but that there are multiple things that can be done to create a better environment for the homeowner.

“I think the message really is that a better environment isn't just for today, but it's also for months from now when things come back to normal,” said Bloemer.


The COVID-19 virus has brought IAQ into the limelight again as links between indoor environment and health continue to surface. HVAC contractors are front and center in this dialogue.

“In essence, they are ground zero for this discussion,” said Caroline Blazovsky, national healthy home expert and CEO of My Healthy Home®. “The HVAC contractors are the ones on the forefront to look at indoor environmental problems. There is a lot of opportunity for them to be working with mold remediation specialists, certified indoor environmental consultants, and environmental investigators.”

Her company provides testing and investigation services for homeowners across the country. To date, she has inspected over 30,000 homes. When people are sick or a doctor cannot determine what’s happening from an environmental standpoint, Blazovsky is sent in and works with a team of professionals, from doctors to HVAC contractors, to help find the answers.

“I look at the home like a doctor,” she said. “I look at everything that's wrong in the house and how that correlates to human health and how we can improve our wellness at home and then improve our relationships with the doctors. I've trained doctors to get involved in this and understand that when you're looking at a patient, you have to look at the whole person, and their environment is a huge key.”