Post #671: Yes! Aerosol transmission is finally hitting the mainstream

Posted on April 27, 2020

Source: COMMENTARY: COVID-19 transmission messages should hinge on science. March 16, 2020, Lisa Brosseau, ScD, author, on line at the University of Minnesota Center for Infectious Disease Research and Policy.  Used without permission here, under a claim of fair use.

Guarding against aerosol spread is the only rational approach

I first stumbled across the likelihood of aerosol (airborne) spread of COVID-19 about a month ago (Post #585)  That is, the fact that COVID-19 can probably be transmitted by tiny (under 5 micron) particles that float on the air and can, in theory, travel significant distances.  In large part, that’s why I remain fixated on making masks capable of filtering aerosols (e.g., Post #648), until such time as ordinary citizens are allowed to buy commercially-made N95 respirators.

At this point, the likelihood of aerosol transmission seem so be working its way into mainstream discussions.  This has important implications for prevention actions that we need to take, particularly as more states open up larger segments of retail commerce and other businesses.

The US CDC still does not overtly acknowledge that aerosol (airborne) transmission is a likely route of infection for COVID-19.  This, despite the fact that at least one expert summary prepared for the government concluded just that, issued a few days prior to the change in CDC guidance to suggest that everybody wear masks when in public.  (Yo can see the original National Academies of Sciences summary letter at this URL.)

You don’t have to cough or sneeze to generate aerosols.  Talking generates a large number of such aerosol particles.  Not quite as much as coughing, but close.  Talking loudly generates more than talking softly, and singing generate aerosols at the same rate that coughing does (likely the ultimate cause of the Mount Vernon, Washington choir event).  Finally, some individuals are superemitters, generating vastly more aerosols than others.  All of this can be found in this very readable article in Nature.

It’s not as if aerosol transmission of disease is a radically new idea.  Many disease are known or thought to spread via aerosols, including tuberculosis, measles, and chickenpox.  In fact, those critical N95 respirators?  I believe that standard was literally established for dealing with tuberculosis (and, later, SARS 2003).  NIOSH and CDC literally mandated N95 respirators as a way to cut down on transmission of tuberculosis within the hospital. Expert opinion at this point is that COVID-19 is not as infectious, in this manner, as either measles or TB (op cit).

By contrast, the CDC has focused on droplet transmission of COVID-19.  This is transmission of disease by droplets (over 5 microns in size).  These largely drop out of the air within six feet of the person emitting them.  Droplet transmission is the basis of the 6-foot “social distancing” rule. 

Likely, the CDC’s position will not change because they don’t have evidence of adequate scientific quality to allow them to make that conclusion with certainty.  But here’s the problem:  Best case, it’ll take a couple of years of investigation before they can be sure that this was, in fact, spread in part by aerosol (airborne) transmission.  And so, as with so much of our federal public health infrastructure, they are hidebound by rigid adherence to rules that are completely out-of-place in the context of the current pandemic.

But if and where aerosol transmission is important, social distancing is not enough.  So I’d say it’s fairly important (and high time) that scientists get a handle on the situations where aerosol spread is more likely (outside of hospitals) and start offering some advice on how best to avoid it.

It’s all about the dose:  Hospitals are at one end of the spectrum of exposure to viral aerosols.  They end up with a high concentration of aerosol particles, due to a) high virus shedding rate by very ill people, b) small, confined spaces in hospital rooms, and c) staff who must spend considerable lengths of time in those spaces.  All three combine to raise the risk that hospital personnel will inhale a dose of aerosolized virus large enough to cause them to become infected.  Hence, a requirement for use of N95 respirators and use of negative-pressure rooms to restrict spread of aerosolized virus.

If that’s one end of exposure, where you get the highest dose of aerosolized virus, then I’d say that sparsely populated outdoor areas are at the other. And that is, in fact, what experts appear to say.  Like so:

"If you generate an aerosol of the virus with no circulation in a room, it's conceivable that if you walk through later, you could inhale the virus," Fineberg said. "But if you're outside, the breeze will likely disperse it."  (op cit).

And that’s not just a bunch of hot air.  Analysis of the Chinese experience showed that outdoor transmission of COVID-19 was rare, among all instances in which the transmission could be traced.  Quite rare.  Just one case, among more than 7,000 cases studied was attributed to transmission outdoors.  (That was from two guys have a conversation outside). That’s reported in this newspaper article.

In that Chinese study, for “clusters” of cases (where there were several cases that could be shown to be related), the prevalence of transmission site (may add to more than 100%, as multiple routes may be involved) was:

  • Within the home (79.9%)
  • Public transport (34%)
  • Followed by:  restaurants and cafeterias, entertainment (gyms, teahouses, barbershops), shopping (malls and markets) and miscellaneous.
    “This study shows that the individual indoor environments in which we live and work are the most common venues in which the virus of the once-in-a-century-pandemic is transmitted among us,”  ( same newspaper article.).

Note that the rates above are raw prevalence data, and don’t adjust for how frequently individuals were exposed to those environments.  I.e., gyms might be particular hotspots for spread of disease among those who use them, but they would show up low on the list if relatively few people used gyms.

Here’s a readable summary of evidence of aerosol transmission of COVID-19, mostly in China and Japan.  They cite a 20-times-higher transmission rate indoors as compared to outdoors, and cite several examples from restaurants and other indoor settings where aerosol spread was the only plausible mechanism.  All of this is for the obvious reason that, for a given density of persons, aerosols will disperse far faster outdoors than indoors.

Finally, within the category of “indoors”, research spotlights high-traffic areas where moisture is present They point to bathrooms and changing rooms (pay attention if  you are going back to your gym!).  But if I had to identify a high-density high-moisture area, the first place I’d point to is a meat-packing plant.  So, once again, I’m betting that (eventually) they’ll figure out that the spate of superspreader-type events in meat plants is not a concidence, but is part-and-parcel of the work environment.   Let’s get our meat packers some N95 masks.

That research above also seconds the China findings regarding public transportation.  Apparently, in Boston, the prevalence of viral particles on subway turnstyles correlates well with the prevalence of COVID-19 in the surrounding neighborhoods.

Farmer’s markets as a model of safe commerce.

What get me thinking about this was the proposed re-starting of the Vienna farmers’ market later this year.  Someone casually asked me if I thought that was safe, and my immediate response was, that’s safer than shopping in a grocery store.  And it was exactly because it was out-of-doors.  As long as it doesn’t get too crowded, I’d far rather shop at a farmers’ market than indoors at the grocer.

In fact, I had already said (Post #600):

“If aerosol transmission of this disease actually does occur frequently, and vendors won’t wear masks, then an open-air market on a windy day is probably your safest bet for shopping, and long as the density of individuals is kept low.”

Now that research is starting to come out, I’m standing by that.  I don’t think the Town should hesitate even one bit about this.  Because the research now strongly suggests that if this is an alternative to shopping indoors, it’s probably safer than the alternative.

But that got me thinking.  Are there other pieces of commerce that can be done this way?

The first obvious candidate is outdoor dining.  Assuming this research is right, the obvious implication here is that the Commonwealth should have more relaxed rules for resumption of outdoor dining than indoor.

The second obvious candidate is organized exercise classes.  To the extent possible, if the Commonwealth is going to allow (e.g.) gyms, yoga studios, and similar to re-open, they again should have more relaxed rules for appropriately-spaced outdoor exercise.

Beyond that, it’s hard to think of major classes of commerce that could be done this way.

And, separately, I’d say that no commercial construction built in the last half-century was designed with any significant natural ventilation in mind.  Because, more-or-less, a strong breeze blowing through an area puts your ventilation on a par with being outdoors.  But I would bet that few-to-none of the commercial establishments in Vienna have enough free ventilation area (screened and openable windows and doors) to make that happen.

Barring that, the best thing you can do is wear the best mask you can, at all times where you share indoor spaces with others, outside of the home.  Near as I can tell, a lot of the remaining transmission is likely aerosol transmission by pre- or non-symptomatic individuals.  Any time you are sharing an indoor space, you are somewhat at risk for that mode of transmission.

The CDC appears dead-set against explicitly mentioning aerosol transmission.  And so, all of its rules are geared to something that simply does not happen now:  coughing and sneezing by symptomatic individuals.  In short, their public hygiene guidance is … well, useful, but obsolete.  It addressed a situation that might have existed two months ago, but no longer exists.  (Not-being-sick-in-public is now rigidly socially enforced).

So it’s incumbent on you to do what’s right, even if the CDC won’t tell you.  This becomes even more important as larger segments of retail commerce are again opened up.  My advice:

  • Do what the CDC says, and:
  • Wear the best mask you own, particularly if it will filter aerosol particles.
  • Do your business out-of-doors when you can.
  • Don’t talk unless you need to, and then, talk softly.

If this now is all about indoor aerosol transmission by asymptomatic individuals, then the rules need to change.  Please try to get ahead of the curve.