Thursday, July 23, 2020

Did That Public Toilet Stink? Then Its Plume Aerosol Blew Infected Feces Right Through Your Little Mask!

Slate |  In early July, a group of scientists authored an article outlining the evidence for COVID-19 being an airborne disease. It made plenty of headlines, which was, frankly, a bit confusing. Didn’t the public already know that COVID-19 was a respiratory illness? And didn’t medical providers already know that COVID-19 could be transmitted by aerosols in some situations, not just droplets? Why was this news, exactly?

To understand the confusion, we have to go back to the definition of airborne. In medical parlance, an “airborne” disease is one that is spread primarily by the distribution of aerosols—tiny particles, less than 5 microns in size, that can linger in the air and travel long distances. They can also travel lower into your respiratory tract. Classic examples are chicken pox, measles, and tuberculosis. In contrast, a “droplet disease” is one that is primarily transmitted by much larger droplets (20 microns or larger) that don’t linger in the air and don’t travel long distances—they typically fall to the ground within about 3 feet of the source. Classic examples are influenza, mumps, and whooping cough. 

These droplets can land in your eyes, nose, or mouth, and infect you, or be transferred from fomites (surrounding objects) to hands, and thereby to the face, infecting the respiratory tract by direct contact with mucus membranes in the eyes, nose, or mouth. But that doesn’t mean you can think of a droplet disease as requiring direct contact—this kind of disease can infect you either when you inhale it or when you have direct contact with it.

Which underscores the problem. In real life, what comes out of a COVID-infected patient when they breathe, cough, or sneeze doesn’t neatly fit exactly into one category or the other—particles can exist along a size continuum. And just to make things more confusing, not everyone even uses the term airborne to mean aerosol only—sometimes it means only that the disease is spread by any size infective particle that is inhaled. On top of that, while the World Health Organization hasn’t disputed that the disease can be spread by inhaled droplets, it has focused mainly on direct contact with droplets, which is why, until recently, it’s mostly pushed hand-washing and distancing as ways to contain spread, while being slower to push masks, which are mainly protective against droplet inhalation. Sorting through these competing transmission ideologies, and trying to figure out if you are keeping yourself safe from aerosols or droplets, feels like canoeing through crabgrass.

What I have come to realize is that it really shouldn’t matter that much. Even as we’ve focused on droplets, in the clinical world, we’ve always known that a COVID-positive patient could generate aerosols and spread the disease that way. The WHO and the Centers for Disease Control and Prevention both acknowledged this, hence their recommendation that medical staff wear an N95 mask when performing a procedure considered “aerosol generating.” But we couldn’t agree on what these procedures were either, in practice. Placing a breathing tube into someone’s trachea before putting them on a ventilator is considered an aerosolizing procedure, that is certain. But scientists and physicians quibble about everything else that could be an aerosolizing procedure: nebulizer treatments for asthma, chest tubes inserted for collapsed lungs, suctioning, CPR. A patient just sitting quietly by themselves in a room might cough and generate an aerosol, as well as a spray of droplets capable of traveling up to 200 mph, a speed that could easily launch them further than 3 feet.


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