Chuan He, Ph.D.
Recent guidelines from a top HVAC (heating, ventilation, and air conditioning) organization, provide actionable suggestions on how you can put your HVAC system to work in reducing the transmission of Covid-19. REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, represents a network of over 100,000 engineers from 25+ countries. In response to the coronavirus pandemic, REHVA created a guide for building operation services to prevent the spread of the virus. Although these guidelines were initially written for workspaces, which admittedly do have some differences from homes (e.g., in space volume, equipment capacity or control complexity), the principles of maintaining good indoor air quality are overall the same for office and home. The full document is available here.
The Role of Air in SARS-CoV-2 Transmission
To understand the benefits that your HVAC system can provide with Covid-19, you first need to understand the role that air plays in the transmission routes of SARS-CoV-2, the virus that causes Covid-19. Given the recent emergence of the virus and continuing research, we are still learning about how it spreads. But here’s what we know or suspect:
- The virus is released along with carrier particles (droplets >10 microns in diameter) when infected people sneeze, cough, talk or breathe. These droplets can travel 1-2 meters or around 6 feet, and can deposit on nearby surfaces. Most people are exposed to the virus when they come into close contact with an infected person.
- These large droplets that remain in the air aren’t able to maintain their size for long, and soon evaporate and break down into smaller particles (<5 microns). Although no confirmed cases of this type of airborne transmission have been reported, there are several reasons many scientists think it is possible. Early evidence indicates that these smaller particles in aerosols can remain active in common indoor conditions for up to 3 hours in the air. This suggests that these small particles could also travel long distances away from initial aerosolization, possibly even along the exhaust air ducts of ventilation systems or other airflow systems. Airborne transmission is thought to have contributed to cases of SARS, a closely related illness, which could indicate that the same is true for Covid-19.
- Particles that sit on surfaces indoors can survive for up to 3 days (depending on surface type); if people touch contaminated surfaces and then touch their mouth, nose or eyes, it may be possible that they can become infected.
- The virus might also be spread through the fecal-oral route: flushing the toilet might spread the aerosol, potentially exposing others to the virus if the air has not been fully exchanged.
Thus while the route may vary, air is a major player.
With this in mind, REHVA has provided some practical recommendations to help control the spread of the virus through the air in office buildings. Since many things about the novel coronavirus are still not fully understood, and the science is still emerging, exercising caution may be the best approach. REHVA’s recommendations use a principle common in engineering and toxicology, called ALARA (As Low As Reasonably Achievable). The ALARA principle means that we should try to avoid even small doses of potentially harmful substances if they are of no direct benefit and if the cost of avoiding them is acceptable. REHVA’s suggested guidelines are designed to be practical and not burdensome.
The Delos Labs team has distilled REHVA’s technical advice into several easy-to-implement practices relevant for homes while you spend the most of your time indoors:
- Ventilation is a very efficient way to reduce airborne pathogen levels in your home. The influx of fresh air works to dilute contaminated air and thus reduce the levels of infectious agents. Consider using the 100% fresh air mode in your central AC if you have one, or simply keeping your windows open (as long as there aren’t high levels of outdoor air pollution or pollen. You can view the outdoor air quality in your area using an air quality reporting service, such as breezometer.com)
- The use of highly efficient particle filtration in centralized HVAC systems reduces the transport of infectious particles, especially when these areas share the same central HVAC system through supply of recirculated air; Avoid using recirculated air without fully validating your air cleaning equipment.
- Use a portable air purifier equipped with a HEPA filter, and place the device close to where you and/or members of your household are frequently present.
- In the bathroom, use the exhaust as much as possible to release the air. However, as opposed to the rest of your home, avoid opening bathroom windows as incoming air might push the contaminated air from the bathroom into the rest of your home.
- After using the bathroom, flush with the toilet lid closed in order to help minimize the risk of droplet release and residue.
Edited by Radhika Singh, Carolyn Swope and Regina Vaicekonyte