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Smoke on the Wind: the Aerosol Spread of Covid-19

posted by Jason Kottke   Aug 26, 2020

Chemist Jose-Luis Jimenez writing for Time magazine: COVID-19 Is Transmitted Through Aerosols. We Have Enough Evidence, Now It Is Time to Act. In it, he argues that while much of the early attention has been on fomites (surface contamination) and droplets as pathways for spreading Covid-19, tranmission by aerosols may be more significant. His analogy of smoke makes it quite easy to understand:

When it comes to COVID-19, the evidence overwhelmingly supports aerosol transmission, and there are no strong arguments against it. For example, contact tracing has found that much COVID-19 transmission occurs in close proximity, but that many people who share the same home with an infected person do not get the disease. To understand why, it is useful to use cigarette or vaping smoke (which is also an aerosol) as an analog. Imagine sharing a home with a smoker: if you stood close to the smoker while talking, you would inhale a great deal of smoke. Replace the smoke with virus-containing aerosols, which behave very similarly, and the impact is similar: the closer you are to someone releasing virus-carrying aerosols, the more likely you are to breathe in larger amounts of virus. We know from detailed, rigorous studies that when individuals talk in close proximity, aerosols dominate transmission and droplets are nearly negligible.

Thinking of smoke can help guide our actions:

The visual analogy of smoke can help guide our risk assessment and risk reduction strategies. One just has to imagine that others they encounter are all smoking, and the goal is to breathe as little smoke as possible. But COVID-19 is not very contagious under most situations, unlike, for example, measles: the CDC says that 15 minutes of close proximity to a COVID-19 infected person often leads to contagion, which provides an estimate of how much “exhaled smoke” one may need to inhale for infection. Inhaling a little whiff of “smoke” here and there is OK, but a lot of “smoke” for a sustained period of time and without a mask is risky. (To be clear, actual smoke does not increase the probability of infection.)

In thinking about aerosolized Covid, Jimenez recommends avoiding crowds & indoor spaces and cutting down on proximity & duration, among other things.

We should continue doing what has already been recommended: wash hands, keep six feet apart, and so on. But that is not enough. A new, consistent and logical set of recommendations must emerge to reduce aerosol transmission. I propose the following: Avoid Crowding, Indoors, low Ventilation, Close proximity, long Duration, Unmasked, Talking/singing/Yelling (“A CIViC DUTY”). These are the important factors in mathematical models of aerosol transmission, and can also be simply understood as factors that impact how much “smoke” we would inhale.

But what about masks? N95s can block most aerosols but cloth masks have “huge” holes in them relative to aerosol particles, right? They still work in lowering risk (but not eliminating it!) because your “smoke” doesn’t travel as far when you’re wearing a mask and it filters incoming smoke (if your mask fits correctly and you’re keeping your distance).

Second, masks are essential, even when we are able to maintain social distance. We should also pay attention to fitting masks snugly, as they are not just a parapet against ballistic droplets, but also a means to prevent “smoke” from leaking in through gaps. We should not remove masks to talk, nor allow someone who is not wearing a mask to talk to us, because we exhale aerosols 10 times as much when talking compared to breathing. Everyone should be careful to not stand behind someone with a poorly fitting mask, as the curvature of an ill-fitting mask can cause aerosols to travel behind the person wearing it.

Great article, full of common sense advice backed up by science.