Coronavirus in the Air

As research evolves and we learn more about coronavirus SARS-CoV-2 (the virus which causes COVID-19), CDC scientists have learned the virus is often spread through respiratory droplets produced when a person coughs, sneezes, or even talks. These droplets can end up in the mouths or noses of people who are nearby, or possibly inhaled into the lungs when people are close to one another (within 6 feet).  

Droplets vs. aerosols

There are differing opinions as to at what size a particle is considered a droplet vs. an aerosol. The WHO and CDC identify particles of more than 5 μm as droplets, and those less than 5 μm as aerosols. The unit of measurement is in microns, or 1/1000th of a millimeter. The particle size of the COVID-19 virus ≈0.1 μm in diameter.

Large droplets tend to fall to the floor faster, while small droplets are light enough to stay somewhat “suspended” or buoyant in the air. The condition of the air – factors like temperature and relative humidity – can also affect how long and what size particles remain airborne.

How particles behave

Droplets of all sizes are expelled from our mouths during everyday activities like breathing or talking, not just through coughs or sneezes. Research shows most expelled particles, even those greater than 30 μm, tend to hang in the air (see Table). But how? Airborne droplet nuclei, or dried residuals of large droplets, develop as the fluid of pathogenic droplets evaporates. These new droplet nuclei, which are still infective, and the previously aerosolized particles are so lightweight they can all be disbursed over long distances by air currents and may transmit infections to susceptible individuals who may not have ever been in the same room.

A recent study evaluated the stability of SARS-CoV-2 in aerosols and found only a slight reduction in infectivity during a 3-hour period of observation. “Aerosols from infected persons may therefore pose an inhalation threat even at considerable distances and in enclosed spaces, particularly if there is poor ventilation,” Harvard University biologist Matthew Meselson wrote in a commentary accompanying the study.

The role of HVAC in virus transmission

With these findings, a building’s HVAC system can play an important role in reducing the risk of the virus spreading indoors. The key aspects of the HVAC system which can impact the spread of the virus are:

• Ventilation – includes the percent of outside air entering the building, the flow of air within spaces in the building and temperature.
• Humidity – an important aspect as the virus has a higher survival rate at certain humidity settings, so it is important for this to be in the right range.
• Filtration – the rate-level of filters in your HVAC system can help keep the coronavirus from entering or staying in the air at your facility.  However, some HVAC systems are not designed to handle higher rating filters, so it is important to consider the HVAC system.

These aspects all impact each other and the building systems themselves, so it is best to have a professional TAB (Testing, Adjusting, Balancing) firm come to your facility to ensure all of these aspects are set correctly and working most effectively to reduce the spread of the virus.