skip to Main Content

COVID Is Airborne — Are You Feeling Lucky?

Is Exercising In The Gym Safe When We Know COVID-19 Is Airborne?

The two kinds of people in the world are those who can extrapolate from incomplete data.  Regarding how COVID-19 spreads, the data are becoming more and more complete now.  And the facts and analysis all point to this deadly virus spreading through particles that float in the air for hours.  Technically, these are called aerosols, particles smaller and lighter than droplets.

Is it safe to go to the gym or the dentist?

I haven’t been to the gym or the dentist in over a year now.  They’re both open and many other people are still going to those places.  If you believe you’re safe as long as you’re six feet away from others, you may be among those who spend time in indoor environments with other strangers breathing the same air as you.  The odds are that you did that and didn’t contract COVID-19.  If so, it’s mainly because you were lucky.

I’ve watched the video my gym put out to reassure people that exercising in their facility is safe.  It shows people on the elliptical machines and treadmills, huffing and puffing with no masks on.  But, hey, they’re at least six feet apart.  The video discussed the temperature checks and entry requirements, how everyone must wear a mask in the lobby and halls, and the steps they’ve taken to disinfect everything.  One thing that video did not say anything about, however, is any steps they’ve taken to clean the air with ventilation and filtration.

Imagine one person in the gym who has COVID-19 but doesn’t know it yet.  They get on the treadmill and run for 30 minutes, expelling into the air a lot of virus-laden particles.  If it were just a matter of droplet and surface transmission, you’d probably be safe as long as you weren’t within a meter or two and didn’t touch anything their droplets fell onto.

But COVID-19 is airborne, so some of those virus-laden particles float around the air in the gym for a long time.  You could be on the other side of the room and still get sick.  Or maybe the ventilation and filtration in that space are good enough to keep the virus count of the air you breathe low enough for safety.  Do you feel lucky?

Thinking about that, as I said, has kept me from going to the dentist as well.  Yes, the dental hygienist and dentist will be masked up and everything disinfected.  But I’ll be lying in that chair for a while with my mouth open while another patient is in the room next door also lying in the chair with their mouth open.  Would I be lucky enough to be there when no infected person is there?  Probably.  And maybe they have a ventilation and filtration system up to the task.  But I haven’t wanted to take that risk.

COVID-19 is airborne

The evidence for airborne transmission of COVID-19 has been growing for a year now.  The aerosol scientists, epidemiologists, and indoor air quality researchers I follow on Twitter and LinkedIn have been unequivocal about this for a long time.  The main health organizations, WHO and CDC, have been slow put out the warning, though.  Finally, they seem to be changing course.

ASHRAE, on the contrary, has done a good job of stressing the importance of good indoor air quality.  Here’s their current statement:

Airborne transmission of SARS-CoV-2 is significant and should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures.

Professor Bill Bahnfleth last year wrote an article about airborne transmission in a restaurant in China, based on a scientific paper (in preprint at the time but now peer-reviewed).  The authors of the paper concluded, “Aerosol transmission of SARS-CoV-2 due to poor ventilation may explain the community spread of COVID-19.”

The CDC has finally come around and now says:

There is evidence that under certain conditions, people with COVID-19 seem to have infected others who were more than 6 feet away. These transmissions occurred within enclosed spaces that had inadequate ventilation. Sometimes the infected person was breathing heavily, for example while singing or exercising.

The medical journal The Lancet has just published an article titled, Ten scientific reasons in support of airborne transmission of SARS-CoV-2.  They justified my fear of going to the gym or dentist with their second reason:  “…long-range transmission of SARS-CoV-2 between people in adjacent rooms but never in each other’s presence has been documented in quarantine hotels.”  Also, “transmission of SARS-CoV-2 is higher indoors than outdoors and is substantially reduced by indoor ventilation.”  Read the whole paper for the other eight reasons.

Aerosol transmission of COVID-19 is far greater than transmission by droplets. [Image from Harvard Chan School of Public Health]
Aerosol transmission of COVID-19 is far greater than transmission by droplets. [Image from Harvard Chan School of Public Health]
What about the relative risk of transmission via aerosols versus transmission via droplets?  Dr. Joseph Allen, professor in the Harvard Chan School of Public Health, recently published a paper in the Proceedings of the National Academy of Sciences  estimating the split at 85% aerosol, 15% droplet.  (See chart above.)

How to avoid getting COVID-19

As mentioned above, ventilation and filtration are critical to keeping the concentration of any virus-laden particles low.  But the other  guidance available from the CDC and ASHRAE are also valuable.  Wear a mask anytime you’re in an indoor public space.  Limit the number of visitors to your home, and wear a mask as much as possible when you do have guests.  Keep your distance from people.  Wash your hands.  And keep the indoor relative humidity between 40% and 60%.

Things we’ve been doing that aren’t as important as first thought:  Disinfecting everything, installing plexiglass, and all the other types of “hygiene theater.”  Surface transmission can happen but it’s rare.

And then there’s the stuff some people are doing that can be downright bad for health.  Not all air-cleaning devices are safe.  Stick with mechanical filtration and ventilation.  I’ll have an article next week about this topic, particularly regarding ionization devices.  If you want to learn why you shouldn’t get that electronic air cleaner you’ve been thinking about, read this letter by Dr. Marwa Zaatari and Dr. Marcel Harmon.

The data on airborne transmission of COVID-19 are now complete enough that even if you’re the second kind of person I talked about in the first sentence, you should exercise caution about breathing indoor air.  Otherwise, at some point your luck may run out.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.


Related Articles

How to Make a High-MERV DIY Portable Air Cleaner

Do UV Lamps Really Improve Indoor Air Quality?

Can Your HVAC System Filter Out Coronavirus?


Photo of people in gym by UNE Photos from, used under a Creative Commons license.


NOTE: Comments are moderated. Your comment will not appear below until approved.

This Post Has 12 Comments

  1. As one of the original petitioners with this shift in acknowledging the airborne nature of transmission is a breath of fresh air…its been an epic battle and source of frustration for our citizen scientists group that has scarred many of us as we have witnessed the damage that dogma and cognitive dissonance has unnecessarily spread across the world…many of us were buying N95s, making MERV13/fan assemblies and UV treated humidifiers in January of 2020….and advising through outreach that the advise by the WHO and CDC and PHO were flawed…several of the authors in the Lancet publication are in our group and have suffered ridicule for being vocal against these and other organizations of authority…anyways thank you for raising the awareness!

  2. “I’ve known for a long time that aerosol transmission is dominant, causing about 96% of the cases, but I recently found out that some people close to me didn’t understand that.”

    I wrote an article indicating aerosol was the main culprit, but this is the first time I’ve seen 96% or any percent.
    If you have a link on that, it would be great to have it.

    1. Paul, I’ll have to dig that up. I should have put in the reference when I wrote the article, but I’ll find it and come back with the source.

  3. Allison
    Your articles are a great education.
    Here is a doozy of a challenge: I have noodled over the aerosol vs. HVAC problem for six months or so. (Retired and no, I’m _not_ going to “get a real job”!) In particular, how air flow could be controlled in occupied space in ways to make it possible to reduce indoor air born contagion by 95+% while allowing full occupancy without physically chopping up a space. Assuming the HVAC system can provide necessary minimum flow w/in the system and also any filters needed… How can the air in the occupied spaces be controlled with the goal of carrying away exhalations immediately and without drawing them through/past other people?

    Say I’m a restaurant developer planning, oh, 12 tables in a room of, say, 400SF. Make it easy and allow full access below and 15-20′ ceiling height above. Registers, ducts, maybe a few <4' high dividers or some sort… Room penetrations (doors, windows) controlled anyway you like. Not limited to existing HVAC fixtures, but if you make up something new, it must be manufactureable with existing tech. Central system can be augmented with micro control by in-room fans. Monitoring needed and archived to document room's design performance and reduce legal liability from any customer illness… What monitoring would be helpful and possible? What room airflow plan is possible? "Active" system adjustments for different crowds. Minimize air flow velocity in room (no drafts…).

    Maybe just one big down draft spread over the whole floor space, like a huge Jennair downdraft stove top. Or a room sized air hockey board in reverse? Or should the flow be up? Or individual table registers?

    Or tweek the example. Say a hospital waiting room. ER. Whatever.

    Ideas? All this technology piling up in the world, seems like this might be a worthy problem.


  4. Thx for that link. Great to seem some timely smart efforts.

    It sounds like the common wisdom says that up-drafting is the way to go, moving used air up to the ceiling and then removing and processing it. I wonder what air velocities are in the room. If doing a new install with this type system in mind, what would be the maximum spacing for makeup air registers in the floor? For collectors in the ceiling?

    Tables would presumably be something of a dead space both because of obstruction but also because most people might get squirrelly about a strong draft blowing up their legs. With fully adequate central air volume (fans, ducts, registers, etc), would the flow around the tables be enough to carry upwards exhalations from customers or would there need to be some kind of fan at each table to move the air column above the table upwards? Like the little table filters in that example.

    12 tables, four sensor locations if I read it right. That’s’ not too high a bar. Sounds like monitoring might be relatively easy.

    This might impact wall-to-wall carpeting in public areas…


  5. Thanks…we have also found aerosol transmission most likely (for over a year, due to studies, etc)
    My associates, however, are loath to consider doing more than CDC’s inane guidelines recommend, and believe they have proven the ‘droplets’ theory because they have yet to contract Covid.

    Your article will not persuade them, but it gives my co-habitants, who must toe the aerosol line or else (or else I have to move) some measure of comfort.

    We are forgoing (and insulting) relatives who poo-poo our personal safety requirements…and in doing so prevent our close association.

    Funny, now we are divided not only along race, wealth, and political lines, but split into aerosol vs droplets ‘scientists’….because our government has found it inconvenient to follow science.

    Thanks for the work.

  6. This is very late but I want to scream from the rooftops “YES!!!”. I’ve always thought of transmission like smoke. If smoke can’t leave an area quickly, you shouldn’t be there. For example: walled tents at restaurants. All they did was build an unconditioned room. Doesn’t count.

    The plexiglass barriers are ridiculous too since they impede airflow. Dirty air just lingers instead of mixing & dispersing. Same goes for staff who put a mask on as soon as you walk into the business like they didn’t just fill the room with virus loaded air.

  7. Allison

    My awesome dentist didn’t reopen until they upgraded their ventilation. Each room is sealed off and he tells me that by the time I walk to the door the air I was breathing has been evacuated. HEPA filtration.

    I just can’t stop myself from going to my favourite lunch spot though. Mask off to eat. I keep my fingers crossed and hoping the huge volume of space keeps any viral count low. The vaccine passport system only ended March 2022 though, so there’s that.

Comments are closed.

Back To Top