Respirators and Aerosol Transmission

A N95 respirator mask. (Robin Lubbock/WBUR)

I’ve commented in some of my posts here, on Facebook, and Twitter that we should be treating SARS-CoV-2 as a virus that is spread through both the airborne and droplet route and provided some primary literature to support that assessment. The distinction between the two has been treated as an almost binary relationship when in fact, they occur along a continuum. A number of different factors, both related to the person who is spreading it, but also to the local environment, determine which might be the more dominant form of spread in that area. These can be things such as viral load, point of infection, activity of the person who is infected, air changes per hour, humidity, etc. This should not be considered a completely list, but just a representative one. In addition, I have previously commented that we should not have eased up on the recommendations for mask use.

I came across a thread by an infectious disease physician on Twitter today that nicely summarizes everything I’ve been saying, but pulled it all together in one place. I asked her permission if I could post it year and she graciously agreed. Thank you Denise Dewald, MD.

“PSA: DO NOT TAKE YOUR MASK OFF INDOORS, even if there is no one else around.

The virus persists in the air for HOURS because #CovidIsAirborne Only take your mask off outdoors, and preferably use an N95 mask.

Aerosols that carry the virus can float in the air for HOURS. Long after an infected person has left the room, the air is still not safe to breathe, unless there is aggressive ventilation and filtration to keep it safe. Just because you are alone does not mean you are safe.

Do not take your mask off inside, even if you are alone. It’s not safe for you. And if you are infected, you will be making the air unsafe for others. Read on to understand this more fully.

Aerosols are tiny droplets produced in your respiratory tract during breathing, speaking, etc. Viruses attach to them, and this is how COVID is spread The virus is not free-floating in the air, so all the anti-maskers who say masks *can’t* work don’t understand the science. 4/n


The ones produced by breathing are the smallest. Small aerosols are most likely to persist in the environment and most likely to make it to the smallest of another person’s airways. Quiet breathing produces smaller amounts than other expiratory activities.


Talking produces a lot more aerosols that are larger, as well as droplets. These can quickly evaporate down to small aerosols in dry air. Masks help to contain the large aerosols and prevent this. Small aerosols are really bad because they can penetrate the lungs deeply.

If virus makes it to the smallest of your airways, it is more likely to cause severe disease. Virus that lands in your nasal passages is less likely to cause severe disease, and it will take more virus there to get you sick.

The more virus that gets to your lungs, the worse off you will likely be. You really want to avoid getting the virus deep down into your lungs.

N95 masks filter out these small infectious aerosols that carry the virus. Simple facemasks do not do this well because of leaks. Wear an N95 or the roughly equivalent FFP2, KF94, KN95, ASTM3. It’s critical to have good seal to the face.

What about taking my mask off in my personal office space? You might ask Don’t do it unless you know that the air in your office is not recirculated with air from other parts of the building. Recirculated air is chock full of other people’s aerosols.


How do you know that? Ideally talk with the building engineer. High ventilation can make a big difference by diluting out the aerosols with fresh air. (WHO recommends a ventilation rate of 10 liters per second per person). CO2 detectors can help.

Aerosols can easily spread through air ducts. SARS spread via the air ducts and defective sewage traps, leading to a large outbreak in an apartment building in Hong Kong. Buildings need to be designed better for better health and future pandemics.


Now for toilet plumes. Lidless toilets in public bathrooms contaminate the air and all surfaces with feces and virus. We need serious mitigation here. So for God’s sake, DO NOT EVER TAKE YOUR MASK OFF IN THE BATHROOM.

Recirculated air is an aerosol orge, where everyone’s lungs are bathing in everyone else’s aerosols. Practice safe breathing in the time of COVID. Avoid pulmonary promiscuity. Don’t let other people’s aerosols into your lungs, unless they are in your bubble.


Special message to educators: Do not take off your mask to eat lunch in your classroom! Do not take your mask off before or after class And most of all, do not take your mask off to speak This is for your safety, and the safety of your students.

Eat lunch outside or in your car. Avoid being downwind of other people. Maintain at least 6’ distance outside. Do not eat inside. It is too hard to know if the inside air is truly safe. We need to break the chain of transmission.

Many figures are from this review article on the airborne spread of disease. This should be required reading for all people in public health, infectious disease, hospital infection control, school superintendents, architects, and HVAC engineers.”

Thank you again Dr. Dewald.

I would also like to add that should we come up against supply chain shortages and face a respirator shortage, I am one of the coauthors to guidelines on respirator reuse. These may become helpful at some point.

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