Keeping My Home's Air Safe to Breathe - Aerosol Transmission Precautions

By Brian T. Lynch, MSW

According to the Centers for Disease Control (CDC):


DISCLAIMER: Aerosol transmission by the SARS-CoV-2 virus isn’t well understood by many people that I know because it really isn’t easy to understand. What follows is my understanding of how breath aerosols transmit air-borne viruses. It is based on what I have read in the literature on this topic over the past 18 months. My understanding may be flawed, but it is the basis on how I will try to reduce the risks of infection for my family over the holidays. I am not a scientist or a medical professional. These are my questions and how I have answered them. Don’t rely on my answers without first reading about aerosol transmission yourself and doing your own due diligence.

Question: What explains why the number of people getting Covid-19 is on the rise now in late November 2021?



My Short Answer: Notice that the states with the fastest increase in cases are the northern states. One likely explanation is that people in the colder states have started moving indoors where the risk of infection is greater.

My Long Answer: Think about how you can see your breath when it is cold outside. That frozen cloud of air is caused by the moist aerosols you expel when you exhale. If you happen to be positive for Covid-19 and shedding the SARS-CoV-2 virus (or any air-borne virus), each of those tiny particles of water would contain a small amount of virus. These tiny aerosols stay afloat in the air. They aren’t immediately pulled down by gravity, as are the larger breath droplets we avoid by social distancing.

When you are indoors, every breath sends more and more aerosols into the air. They float about and eventually disperse throughout the volume of air in the room. When the concentration of virus-laden breath aerosols becomes too high, or if you are exposed to them for too long, you may eventually breathe in enough virus to trigger an infection. This initial infection stage can happen whether you are fully vaccinated or not. Aerosol transmission of Covid-19 IS THE PRIMARY WAY people become infected with SARSpCoV-2. Indoor spaces are the places where most people become infected. So, as people move indoors when the weather gets colder it increases their risk of infection.

Question: Are infected aerosols from a person who has COVID-19 uniformly distributed in a room?

My Answer: Yes, eventually, but not at first. The breath cloud around an infected person, for example, will always have a higher concentration of virus than the rest of the room. This fact is true even if they are wearing a mask. Some masks are better at capturing some of the aerosols from your breath, but no mask can stop all of it.

Beyond that, how aerosols flow around a room while dissipating can be complex. There are always currents and eddies in the air that can create hot spots in a room. Think of human activities' impact on the air as you would imaging fish activities' impact in the water in a fishbowl. The fish create water currents as they move, push water through their gills, blow the water out their mouths, or sprint across the bowl. Likewise, people sitting and talking quietly don’t create much air disturbance, but when people shout, sing, run around, or sneeze, they create lots of are currents that can temporarily create uneven patterns of breath aerosol in a room. A sneeze can travel up to 200 miles per hour, sending a cloud of potentially infected aerosols across a room in seconds. Masks help diffuse breath aerosols more uniformly than talking or singing without a mask. This can help to lower aerosol concentrations in the portion of the air column where people are breathing and prevent hotspots.

But there are also the airflow characteristics in any given room from HVAC air handling systems, open windows, doorways, and more. Most heating and air conditioning systems in the United States recirculate the air within a building carrying aerosols throughout all the rooms. Few HVAC systems provide fresh air exchangers or HEPA-grade filters to lower the concentrations of potentially infected aerosols. And as one study of infection patterns in a public restaurant found, when air flows across a room from an infected person towards people seated nearby, those people can become infected. So, airflow patterns in a room are an important factor. 

A HEPA filter is capable of removing over 95% of the aerosols in the air but the blower motors on most heating and air conditioning systems are not built to handle the extra pressure it takes to move air through these thick, denser filters. Portable home and office HEPA filter machines are widely available and some are relatively inexpensive, but most homes and offices have not purchased them. 

In my home, we have five machines which we can move about to handle different situations. As a rule of thumb, whether you have an air exchange system are HEPA filters, the volume of air in a room should be exchanged about six times per minute to keep the levels of breath aerosols at a safe level. 

Question: Are there practical ways to keep aerosol concentrations down when having guests over to my home?

My Answer: The most obvious way to lower the risk of aerosol transmissions when entertaining is for everyone in your house to be fully vaccinated or have everyone wear a good quality N95 mask. Leaky surgical or cloth masks have little effect on aerosols being released by our breath. People with cold or flu symptoms should stay home.  But, breakthrough cases and asymptomatic infections can happen. Additional precautions should always be taken. 

If your home isn't equipped with air exchangers or HEPA filters, the most uncomplicated strategy is to crack open the windows in the rooms where you are entertaining your guests, and if you have bathroom or kitchen vents that exhaust outside, turn them on. Leave these vents running. Start before your guests arrive and continue until well after they have gone. Seat guests who are fully vaccinated (or least likely to be infected) nearest the open windows if practical. If you have a whole house fan and outside temperatures permit, turn on the fan and open the windows. These fans quickly exchange the air in the room, keeping breath aerosol levels low.

Another strategy is to create a positive pressure air exchange arrangement in your house with the use of window fans placed in a vacant room (such as a bedroom, perhaps) and set it to pull in the fresh air. Then open the windows a crack in rooms where you will be entertaining guests. The advantage of this method is there are no strong cross-currents. With this method, bathroom vents should only be used when the room is occupied.

If you have purchased portable HEPA-grade air filter machines, turn them on full in the rooms where you are entertaining. These machines can filter out about 95% of the aerosols in the air. If they are of the proper size for a room, they should recycle the volume of air in the room about six times per hour. Be sure to place them in a way that the filtered air blowing out faces along the wall and not across the room.

Please keep in mind that these strategies only reduce the risk of aerosol transmission of the virus and not transmission risks by other modes of transmission. Social distancing and mask should also be in place if guests’ vaccination status or health status is in doubt.