Medical Examiner

Why the COVID-19 Debate Between Aerosols and Droplets Matters Less Than You Think

For most people, the way to avoid the coronavirus will stay the same, no matter how it’s spread.

A man sneezing and spraying aerosol and droplets everywhere.
Photo illustration by Slate. Photo by Getty Images Plus.

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.

But even though clinicians have understood from the beginning that the risk of contracting COVID-19 both in the hospital and in public could involve inhaling the virus, the contention that COVID-19 is mostly aerosol-borne does make us pause for just a moment. Aerosolized particles are potentially more dangerous for two reasons: One, they can travel further, and two, they are tinier—so not kept out quite as well by a surgical or cloth mask. Still, given the facts of the droplet-aerosol continuum, and the knowledge that most aerosol transmitted illnesses are much, much more infectious than COVID-19 seems to be, the idea that aerosols are not a significant means of COVID-19 spread seems a reasonable assumption to make. But even if aerosols account for some COVID-19 spread, do we need to act differently?

Not really. We already know that crowded spaces are bad, for droplet or aerosol spread. Crowded bars. Indoor restaurants. Unmasked church choirs. Family gatherings indoors. (And as colleges are close to reopening, frat parties.) We should still be avoiding all of these.

Should the public be wearing N95s to protect from the smaller particles? Not as a general rule, for a variety of reasons. First of all, if the people around you are wearing masks, the combination of your mask and theirs should be sufficient to filter out most viral particles. Secondly, N95s can be incredibly uncomfortable. While some of my colleagues wear one for an entire shift, I prefer to limit myself to using one only when going to see COVID-suspected patients or performing a risky procedure, and otherwise use a surgical mask. And thirdly, N95s should ideally be tested for correct fit. Easier than wearing an N95 would be to avoid crowded spaces with unmasked people. As we learn more about this disease, it’s possible some non–health care workers with a higher risk of exposure may be recommended to wear N95s, but for the general public, it still doesn’t seem to make sense.

What about rejiggering the ventilation system in buildings to decrease the risk of aerosols causing infection? This is a costly, time-consuming, and in many instances, and on a global level, impractical solution to a problem that the available evidence says can be combatted with simple, cost-effective, and easy to implement measures: physical distancing, mask wearing, hand-washing. The average person, and public health authorities, would do best to maximize these three, before investing in retrofitting a ventilation system.

If further research does indeed show a significant degree of aerosol spread, there will need to be long-term ventilation solutions to combat this problem, that’s true. But for now, there’s no reason to intentionally stray from the path we are having a hard time getting people to stay on. The debate isn’t over yet. There will be an aerosol camp and an anti-aerosol camp, and there will be modeling and published studies that support each, and there will be policy changes—such as have already littered the road as we navigate COVID-19. But for right now, the airborne controversy doesn’t change anything we all need to be doing to try to get this thing under control.

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