Hundreds of scientists across the world have just petitioned the World Health Organization via an open letter with one demand: Acknowledge publicly that the coronavirus behind covid-19 can be airborne, meaning it can spread through the air (and linger there) via tiny aerosols emitted by the breath of infected people, not just large droplets. But there’s no easy answer to this ongoing debate, for several reasons: It’s still difficult to know how often aerosol transmission of covid-19 occurs, and the idea that a virus is either airborne or not may be an outdated concept.
Since the start of the pandemic earlier this year, health agencies like the World Health Organization have maintained that close contact with someone’s large respiratory droplets, which don’t travel very far and quickly fall to the ground, is the primary method of transmission for the virus. They have stated that aerosol transmission is possible, but only during certain medical procedures. Fomite transmission (touching surfaces contaminated with the virus then touching your mouth, nose, or eyes) is similarly considered possible but rare.
“Studies by the signatories and other scientists have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond 1 to 2 meters from an infected individual,” the letter states.
There have been numerous studies demonstrating the possibility of aerosol transmission. One study in March, for instance, found that viral particles could be suspended in the air for up to three hours under certain conditions. Other studies have suggested that poor indoor ventilation may have played a part in causing clusters of covid-19, possibly through spreading aerosol particles elsewhere in the environment.
But much of the evidence for aerosol transmission is circumstantial. Soon after the March study, other researchers criticized the surrounding media coverage for being too conclusive. For instance, the study wasn’t conducted under real-world conditions, and there still appears to be no smoking gun piece of research proving that aerosols regularly cause new cases of covid-19.
The WHO, for its part, told the New York Times that there have been internal discussions about the role of aerosol transmission in spreading the viral disease, but the evidence its experts have seen so far isn’t solid enough to state definitively that it’s happening. On Monday, the agency confirmed to Reuters that its technical experts would review the letter and its recommendations.
Part of the problem here seems to be a matter of semantics. Germs that are classically considered airborne, like measles or tuberculosis, regularly infect new people through aerosol particles that can survive in the air for hours, long after the infected person has left the room. That doesn’t seem to happen with covid-19, based on what we’ve learned from case studies. But many scientists, including those who signed the letter, contend that aerosols or droplets can still infect people under conditions that make the disease practically airborne for brief periods of time, such as in places with poor ventilation or in situations where you’re spending lots of time in close contact around a group of people indoors, like at choir practice.
The argument made in the open letter is that, while covid-19 may not spread through the air as easily or often as the highly infectious measles does, it’s airborne enough of the time that agencies like the WHO should be loudly warning about the risk.
But this distinction may not matter for the general public. Public health agencies, scientists, and media outlets have been vocal about the higher risk of infection that crowded indoor environments like bars pose for months now; avoiding those scenarios would be smart, regardless of whether the virus spreads via larger droplets or tiny aerosols.
Some scientists have argued that this debate over aerosols versus droplets should be retired or at least redefined. Rather than think of airborne transmission as a simple yes or no, it should be considered a spectrum. On one end, you have the highly airborne measles, and on the other, respiratory diseases that aren’t very airborne at all, such as the flu. (Though a 2018 study actually found that the flu may sometimes be airborne—another illustration of why this distinction is so fuzzy.) Somewhere in the middle of this spectrum, you likely have covid-19, a disease that’s mostly droplet-spread but can be airborne sometimes.
Trying to figure out where exactly in the spectrum covid-19 lies is definitely important for scientists to study, and new findings may prompt changes in restrictions and regulations going forward (such as mandating that buildings prove that their ventilation can quickly circulate fresh air). For the average person, though, it may not change the precautions that we should already be taking to stay safe: Avoid close contact with people outside your household as much as possible, especially indoors; wear masks when around others; and wash your hands frequently.
Studies around the world have found that precautions such as the widespread use of surgical masks (which block some aerosols and droplets but aren’t perfect shields) in hospitals have been able to protect high-risk groups like health care workers from catching and spreading the virus. If aerosol transmission were playing a leading part in the pandemic, then that wouldn’t necessarily be true, since only medical-grade respirators like the N95 are thought to effectively prevent infectious aerosols from being inhaled.
Importantly, many countries appear to have contained their outbreaks under the assumption that the virus usually spreads through close contact with droplets, without treating the virus as airborne.
We don’t know whether added measures, such as the widespread donning of N95 masks by health care workers and the public, would meaningfully help more than surgical and cloth masks. Other measures, such as making sure that indoor spaces like restaurants and airplanes have good ventilation, will be beneficial whether the virus can spread through aerosols or droplets.
Maybe it is worth being overly cautious, as the scientists in the letter argue. This certainly wouldn’t be the first time that the WHO could be accused of not moving fast enough. Recently, for example, the agency once again came under fire for downplaying the role of asymptomatic transmission in spreading covid-19. But it’s also worth emphasizing that there really are a lot of unknowns when it comes to airborne transmission of the virus.