During the first months of the COVID-19 pandemic, many public health authorities promoted hand-washing and maintaining six feet of distance, with the idea that SARS-CoV-2 was spread primarily in relatively large respiratory droplets that soon fell from the air after being exhaled, potentially contaminating surfaces. But epidemiological and animal studies soon pointed to the importance of smaller, airborne droplets known as aerosols in transmission. By now, the evidence is overwhelming, and COVID-19 is acknowledged by most experts as an airborne disease.
The debate, though, does not seem to be settled. There are still a few scientists who, even while they acknowledge that airborne transmission is possible, claim that infection by large droplets or via surfaces could also be responsible for the bulk of cases. Additionally, in some cases health policy has been slow to catch up to the state of knowledge around the ability of aerosols to transmit SARS-CoV-2.
Scientists agree that SARS-CoV-2 spreads through the air, but there is plenty that remains unexplained in terms of how this process fuels the pandemic. Here’s what research shows—and what we still don’t know—about its dynamics.
The virus that causes COVID-19 is airborne
May 2021 marked a turning point for official recognition of COVID-19’s airborne status. That month, a group of researchers summarized ten pieces of scientific evidence supporting the hypothesis of airborne SARS-CoV-2 transmission in a comment published in The Lancet. The list included results from epidemiological studies that pointed to higher indoor transmission of the virus compared to outdoors and nosocomial infections in hospitals where patients were protected against droplet but not aerosol transmission—that is, the use of surgical masks and face shields rather than equipment such as N95 respirators, which effectively filter airborne particles. The team also cited the identification of the coronavirus in places where it could only have reached by aerosols, such as building ducts, as further evidence. Late that month, the US Centers for Disease Control (CDC) acknowledged that SARS-CoV-2 could be transmitted through aerosol particles in the air even at distances greater than six feet from an infectious source.
“It’s pretty clear now that people [with COVID-19] produce aerosols [while] speaking and talking and singing and breathing,” says Kimberly Prather, an aerosol chemist at the University of California, San Diego, who coauthored the comment in The Lancet. These aerosols don’t fall, she adds, but “are like cigarette smoke, and being in a room with someone who is potentially infectious is like being in a room with a cigarette smoker.” Prather says it is “shocking” to her that “there is even a debate at all” about aerosol transmission.
Yet there is. Earlier this month (February 5), The Lancet published a correspondence letter in response to the comment by Prather and her colleagues. The authors of the more recent communiqué, who did not respond to an interview request from The Scientist, question the evidence for claiming that airborne is the predominant mode of SARS-CoV-2 transmission, concluding that, in this respect, “the science is far from settled.” For instance, they argue that for healthcare workers infected despite using surgical masks and face shields, other modes of transmission could be at play: inappropriate technique in removing the masks, and thus contact with contaminated surfaces could also explain these infections, they suggest.
Harvard Medical School infectious disease physician and epidemiologist at Brigham and Women's Hospital Michael Klompas, who was not a coauthor on either the comment or the response letter, says he thinks the arguments developed in the latter are all based on “misunderstandings.”
For instance, in respect to potentially getting infected by incorrectly removing personal protective equipment, Klompas says that the argument dismisses the “differential infection rates” among health care providers wearing surgical masks versus those wearing N95s. Moreover, transmission as a consequence of touching surfaces, as opposed to inhaling viral particles, “doesn’t reflect our current understanding,” which is that infection risk increases with the amount of virus a person is exposed to. Touch, he adds, “is always going to expose you to less virus” compared to breathing in airborne viruses.
Prather and colleagues’ reply to the letter in The Lancet echoes Klompas’s comments. They write that the arguments against airborne transmission reflect a “widely held but fundamentally flawed paradigmatic view” still held by some experts.
Proximity matters, but longer distances aren’t always protective
According to Prather and her coauthors, another common misconception that has served as an argument against airborne being the dominant route of transmission is the claim that the increased risk of infection at close proximity is due to droplets expelled by the infected person, while airborne transmission only occurs over longer distances. However, aerosols do play a role in short-range transmission, and a mathematical model of droplets and aerosol particles suggests that the latter may be the dominant route of SARS-CoV-2 transmission even at distances closer than two meters.
The relationship between proximity to an infected person and transmission risk is well established in airborne pathologies such as measles and tuberculosis. That’s because pathogen-containing aerosols that are generated while speaking, or simply breathing, are more concentrated near the infected person. Additionally, a study on patients with influenza in a hospital setting showed that the concentration of viral particles in the air decreased as distance from the patient’s head increased from less than 0.3 meters to 1.8 meters.
There is “an association between proximity and infection risk that fits with the basic science,” says Klompas. Aerosol emissions are most concentrated “immediately in front of the person’s face, and that dissipates with distance,” he adds, so “the further away you are . . . the lower the probability of infection.”
This has also been observed in epidemiological studies. Early in the pandemic, a team assessed the risk of infection for passengers traveling with an infected person in trains across China. The authors found that SARS-CoV-2 transmission risk decreased with seat distance, and increased with shared travel time near the infected person.
Based on this increasing dilution with distance from the infection source and on various other observations—including the fact that outdoor transmission is rare—Yuguo Li, an indoor-air specialist at the University of Hong Kong, hypothesizes that short-range airborne transmission of SARS-CoV-2 “probably predominates.”
But, Li adds, long-range transmission is also “important.” Even if proximity increases risk, there is plenty of evidence of transmission at longer distances than that typical of one-on-one conversations, such as the so-called superspreading events, and even a few documented cases of transmission across different rooms. Risk of infection in those scenarios may depend on multiple aspects in addition to distance, such as how much aerosol mass the infected person exhales—which in turn depends on factors such as physiology and what activity the infected person is engaged in—the quality of masks used, air ventilation and filtration, and the length of the interaction.
University of Colorado Boulder aerosol scientist Jose-Luis Jimenez, a coauthor of the May 2021 Lancet comment, says that superspreading events where transmission occurs by shared-room air have indeed been a significant contributor to the pandemic. He points, for instance, to the findings of a November 2020 Nature Medicine paper that tracked the first 1,000 confirmed cases of COVID-19 in Hong Kong between January and April 2020. Contact tracing made it clear that many of those cases were organized in clusters, he says, which is consistent with shared-room air transmission. Moreover, 49 percent of all cases could not be traced to any contact. A finding that it’s unknown how half of people became infected after contact tracing is typical of SARS-CoV-2 epidemiological studies, says Jimenez. “I think a lot of those are transmission in the same room.”
In addition to understanding how aerosol particles become more diffuse over distance, another important aspect of transmission is how long aerosolized virus remains infectious after being exhaled.
In an email to the The Scientist, Paul Dabisch, an aerosol scientist at the National Biodefense Analysis and Countermeasures Center in the US, explains that these “exhaled particles originate from fluids in the respiratory tract,” and “contain substantial amounts of water” immediately after exhalation. “But once these particles leave the body, the water quickly evaporates.” One key question is how this evaporation affects survival of the virus.
A preprint posted on medRxiv this past January explores this question. A team at the University of Bristol generated SARS-CoV-2–laden aerosol particles from culture media that, as described in their article, have “many of the same characteristics of real respiratory secretions,” especially a high concentration of inorganic ions. The team trapped these particles within an electric field that kept them suspended in the air, then assessed the virus’ ability to infect cells after different periods of time. They report that, at humidity levels equivalent to those in buildings, SARS-CoV-2 lost half of its initial infectivity within 5 seconds. At 90 percent humidity, the decay was slower, yet the virus still lost about 80 percent of its infectivity after 20 minutes.
“The method [in the new preprint] is very elegant and is very useful,” says Jimenez, who was not involved in the study. “You can learn a lot” from such experiments.
The problem, in his view, is that “it was misinterpreted by an article in The Guardian and then by a lot of other articles.” Jimenez, Prather, and other aerosol scientists tweeted critically about the coverage. Both tell The Scientist that these articles interpreted these lab results as if they accurately represent SARS-CoV-2’s real-world dynamics, which may not be the case. Moreover, they say these stories focused on how close proximity might dominate infection scenarios, as a result of this rapid decay, downplaying transmission by shared-room air. “People have been focused on poorly ventilated spaces and thinking about airborne transmission over metres or across a room,” the Guardian quotes coauthor Jonathan Reid of the University of Bristol’s Aerosol Research Centre as saying. “I’m not saying that doesn’t happen, but I think still the greatest risk of exposure is when you’re close to someone.” (Reid declined to speak with The Scientist for this article.)
“The virus in the real world may behave like in the preprint or it may not,” says Jimenez. “There are many studies that show that what happens to the virus in the air depends very strongly on the other material,” he says, given that the virus represents only a very small fraction of the aerosol composition, and is “surrounded by saliva or by mucus, sodium chloride, respiratory fluids.”
Prather agrees with this point, saying that the method described in the preprint “is a great way to study things. But despite the authors’ attempt to model real aerosols, she says that “to extrapolate from what they get in [their system] to the real world is not ok.”
Jimenez, Prather, and other scientists joined Reid and some of other authors of the preprint to write a consensus document where they clarify two main points: first, that the study is a model “approximating the real world” and may not represent exactly what happens when an infected person exhales airborne particles; and second, that even if those results do indeed reflect SARS-CoV-2 behavior in reality, transmission within a shared room—not only at close proximity—could still be explained.
Assuming that in the real world, the virus indeed decreases in infectivity quickly, it will still retain some potency for longer periods of time, explains Jimenez, which would explain superspreading events and other long-range airborne transmissions. The fast decline in infectiousness reported in the preprint is not enough to make the virus innocuous, he adds, especially in light of how much aerosol mass is exhaled by some individuals. There are those “who don’t emit any virus,” and those “who emit so much virus” that this decay “doesn’t make a huge difference.” Also, “even if the virus loses some infectivity in 5 minutes, in 5 minutes it can travel 30 meters, so it can go anywhere in a room,” Jimenez concludes.
The infectivity that remains after the potentially fast decay observed by the Bristol team may be lost at a slower rate “once a particle has reached an equilibrium following evaporation,” explains Dabisch. Studies by Dabisch and others have shown that in this later phase, the infectivity of SARS-CoV-2 drops by half after one to two hours. The aerosols in these experiments are of similar composition to the ones used by the team at Bristol University. However, instead of using an electrodynamic field, the aerosols are placed into rotating drums to keep them suspended.
“The preprint from University of Bristol and our previous studies are actually measuring two different, but complementary, things,” concludes Dabisch, who was not involved in the preprint but has collaborated with the authors to develop methods to assess infectivity of the virus in the air. He notes that, in this new study, the team in Bristol is “measuring the losses during the initial rapid evaporation that occurs immediately after a particle is generated,” while Dabisch and colleagues had previously measured “the longer-term survival of the virus in the particle after this evaporation has occurred.”
Dabisch’s team has shown that, during this second phase, the period of infectiousness may depend on environmental conditions. For instance, they found that higher intensity of simulated sunlight, higher temperatures, and higher humidity levels accelerate the decay of the virus’ infectiousness. These findings are consistent with epidemiological studies suggesting that SARS-CoV-2 thrives better in winterlike conditions.
Matching knowledge with health policies
Many unanswered questions remain about how SARS-CoV-2 spreads from person to person. “The aerosol transmission of disease is a complex, multi-faceted process,” says Dabisch. In order to better understand it, we need to know more than the effects of distance and time in the air. Other important factors include the amount of virus that is exhaled by infected individuals, and how much viral load needs to be taken in by an uninfected person to contract COVID-19, he says.
Dabisch and colleagues recently found that, in macaques, the probability of infection was dependent on the viral dose they inhaled. Moreover, a higher dose was needed to give the monkeys a fever than the dose required to simply infect them. According to Dabisch, “this suggests that the amount of virus that someone is exposed to may influence the symptoms they experience, and reinforces the importance of public health measures that minimize exposure to virus, such as social distancing, masking, and increased ventilation.”
According to some experts, however, decisions on public health policies have not entirely matched the data. “If you believe that short-range airborne [transmission] predominates” and that long-range is also significant, then the most important thing to do is to dilute SARS-CoV-2–contaminated air with “virus-free air,” says Li. This can be, for example, outdoor air, filtered air, or air treated by ultraviolet germicidal irradiation.
Klompas notes that, while some experts have gradually admitted the importance of the airborne infection route, there is a “reluctance to modify policies to match that insight.” For him, “the most obvious example” is that surgical masks instead of N95s are still widely used “even for clinical care in many hospitals.” It was not until last month that the CDC acknowledged that masks such as N95s offer greater protection for the public than do other options.
That SARS-CoV-2 is airborne is mostly acknowledged today, says Prather. What is needed now, she adds, is to connect the dots, using this knowledge to better protect everyone.
Clarification (February 22): This article has been updated to provide additional detail about Michael Klompas’s affiliations.