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.

Important potentially lifesaving thread. Please RT. 1/n

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. 2/n

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. 3/n

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. 5/n

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. 6/n

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. 7/n

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. 8/n

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! 9/n

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 10/n

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. 11/n

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. 12/n

Now for toilet plumes. Lidless toilets in public bathrooms contaminate the air and all surfaces with feces and virus. We need serious mitigation here.


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. 14/n

🚨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 15/n

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. 16/n

Many firgures 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. 17/n

More info on how to make our schools and buildings safe can be found in this phenomenal thread by the esteemed aerosol scientist @kprather88 18/n

Also check out the recent video conference organized by Dr. Kim Prather on opening schools safely. I am very honored to have been a part of it along with
@DataDrivenMD and many more
#DeltaSchoolsAirborne 19/fin

Sun Aug 29 22:47:53 +0000 2021