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Do HVAC filters block Covid-19?

The Covid-19 pandemic has led some to wonder: does my HVAC filter block out the virus?

In short, it depends on the grade of HVAC filter you have.  Filters with a high MERV rating (ASHRAE recommends 13 or higher) have a greater chance of blocking the spread of the virus from room to room.  HEPA filters, which are considered the best of the best, would do the best job at blocking the spread of the virus from room to room.  HVAC filters coupled with well-designed ventilation and UVGI systems can be effective in blocking the flow of the virus through a building.

How does the virus spread?

While new research continues to be done on the virus, it is known that the coronavirus can spread from person to person through virus-infused aerosols. Wherever an infected person speaks, coughs, or even breathes, they release aerosols. Aerosols are “infectious viral particles that can float or drift around in the air.” Aerosols can remain in the air for three hours upon emittance. These aerosols spread throughout the air after exiting the body of the infected person. Another person can contract the same infection when they come into contact with those same aerosols.

Before we get into HVAC ventilators, it is important to review a bit of physics.  

Infectious aerosols released into a space immediately begin to spread evenly throughout that space through a process called diffusion.

Diffusion is a natural process by which a substance moves from an area of high concentration to an area of low concentration.  

Buy why does this happen?  

All matter is in constant motion.  The air we breathe, the atoms that makeup our laptop, and even the water molecules in our glass are constantly moving.  This constant motion causes substances to spread evenly throughout a container.  The greater the energy a particular set of particles possess, the faster they will diffuse.

Check out this video below:

Notice how much faster the dye spread throughout the hot water.

As the volume of the container that the substance in question enters increases, the average concentration of that substance decreases.  In other words, if you dropped ten drops of dark blue dye in a cup of water, you would likely see the whole vat of water darken.  If those same ten drops were added to a bathtub, the color of the water would not become as dark as the cup.

The fact above can be expanded to increase our understanding of coronavirus aerosols.  

Just like the dye in the video, when coronavirus aerosols are released into a container of air, e.g. a room, they immediately begin to diffuse within that container. The larger the room, the lower average concentration of the virus (think cup vs. bathtub). This is why contracting the virus is much more likely indoors than outdoors. When the aerosols are released within a room indoors, they have a comparatively smaller space to spread out in. When released outdoors, they have comparatively much greater space to spread out.

ASHRAE recommends a multi-faceted approach for controlling the spread of infectious disease within a building. Increasing a building’s ventilation reduces the concentration of the infections aerosols within a structure (this is kind of like increasing the size of the container, e.g. going from a cup to a bathtub). Using high-grade air filters also reduces the spread of aerosols. Lastly, using ultraviolet light to kill microorganisms is another effective way to reduce the spread of the virus.

Here are some products that you may consider:

UVGI Systems

MERV-13 filters

Disclaimer: The article above represents an opinion based on research and not medical advice.


Harvard Health




Buy Ultraviolet

NY Times

Bryans United