What We Know About Airborne Coronavirus Transmission

Uh oh. Photo: CSA Images/Getty Images/CSA Images RF

As the country lurched its way through an uneven reopening process — and the coronavirus spikes that came from people clustering together inside bars, for example — a group of doctors issued a plea to the World Health Organization in July. The virus, according to 239 scientists from around the globe, could not only linger in the air as an aerosol; it could also travel much farther distances than health authorities had previously acknowledged.

At the time, Dr. Benedetta Allegranzi — W.H.O.’s technical lead for infection prevention and control — told the New York Times that the argument lacked “solid or even clear evidence.” Now, however, the Centers for Disease Control and Prevention have officially acknowledged the possibility of airborne transmission, even when people stand six feet apart.

In classic COVID fashion, though, this update arrived amid confusion, the CDC posting guidance on airborne transmission and then abruptly deleting it, saying the information went up in error. So what is the actual deal here: Can respiratory droplets hang around in the air even after a screamer finishes screaming, to potentially infect any unwitting souls who wind up wandering through the spray zone? Let’s take a closer look.

To be clear: the CDC has admitted that airborne transmission is possible.

The CDC released its revised “How COVID-19 Spreads” guidance on Monday, for the first time adding airborne transmission — which the agency defines as “exposure to virus in small droplets and particles that can linger in the air for minutes to hours” — as a potentiality. Other viruses, like tuberculosis and measles and chicken pox, spread this way, yet the CDC remains adamant that when it comes to the new coronavirus, the largest culprit is close interpersonal contact. Still, it concedes that “under certain conditions, people with COVID-19 seem to have infected others who were more than 6 feet away.” Typically, this would occur in a poorly ventilated, enclosed space, particularly where people are “breathing heavily” — see: gyms, karaoke parties, and houses of worship.

Of course, it’s also possible that a good deal of the spread attributable to these contexts comes from maskless people spraying droplets in tight proximity to others. Still, the CDC says that when very fine droplets disperse, they may pose a problem:

Under these circumstances, scientists believe that the amount of infectious smaller droplet and particles produced by the people with COVID-19 became concentrated enough to spread the virus to other people. The people who were infected were in the same space during the same time or shortly after the person with COVID-19 had left.

The agency does not say how often airborne transmission might occur, simply that the available evidence indicates it “sometimes” does.

So why did the CDC backtrack on airborne transmission?

Admittedly, this new warning feels fairly intuitive, given what we know about person-to-person infection via respiratory droplets, and given the aforementioned warnings about airborne transmission some experts have been issuing for months. So why would the CDC backtrack?

Apparently, the agency posted a “draft” version of its guidance on September 18 and removed it within a day or two, telling CNN that the change “wasn’t ready to be posted.” According to the Washington Post, the CDC felt its language — which mentioned aerosols, or teeny droplets that stay suspended in air and may fan out quite significantly — implied that the virus mainly spread via airborne transmission. Again, the agency believes people exchanging droplets at close range pose the biggest risk.

But the possibility of airborne transmission, even when people are standing more than six feet apart, is significant, right?

Some scientists say definitely, yes. On Monday, a group of them published an open letter, urging public health officials to shift “the balance of attention … to protecting against airborne transmission” and make clear size distinctions between droplets and aerosols. “Viruses in droplets (larger than 100 μm) typically fall to the ground in seconds within 2 m of the source and can be sprayed like tiny cannonballs onto nearby individuals,” the letter states. Therefore, social distancing may adequately guard against droplet exposure. But meanwhile, “viruses in aerosols (smaller than 100 μm)” act more like “smoke,” they argue, sitting in the air “for many seconds to hours,” and potentially also traveling more than two meters.

If all of that holds true, consider a few hypothetical scenarios: Say two high-ranking officials are standing together, indoors and on a stage, for an extended period of time. Say that, although they maintain maybe 12 feet of distance, one spends an hour and a half screaming — only to receive a coronavirus diagnosis ~48 hours later. That does not seem so safe, in light of the CDC’s update. Nor does the prospect of the infected man’s deputy assuming the same position one week later, even as a growing list of his colleagues report their own infections.

We’ve already seen bars and restaurants drive huge spikes, and then, the president’s recent Rose Garden ceremony and Oval Office reception are looking like possible super-spreader events. So even if airborne transmission does not prove to be a primary infection driver, as the CDC currently maintains, why not cut out risks where we can?

So what can I do to better protect myself from airborne transmission?

The CDC says its longstanding recommendations — scrub your hands and clean high-tough surfaces regularly — still apply. But at this point, we also know that masks are an effective means of keeping our spray to ourselves — the CDC would like us to please keep wearing them, over our noses and our mouths. If avoiding enclosed spaces is simply not an option, make sure the mask stays on while you’re in there.

Kimberly Prather, the University of California San Francisco atmospheric chemist who wrote the aforementioned letter, told the Post that aerosols and airborne transmission “are the only way to explain super-spreader events we are seeing,” and therefore, people should “wear masks at all times indoors when others are present.”

In so far as the weather permits, socializing outdoors and at a distance still seems most advisable: The CDC has not dropped its warnings against crowded indoor spaces, especially those that aren’t properly ventilated, and maintains that people should stay a minimum of six feet apart. But according to Prather, the concept of a “safe social distance” is a myth when you’re hanging out inside.

What We Know About Airborne Coronavirus Transmission