This story was updated on July 7 at 1:54 p.m. to include WHO's response to the letter.
By now, it's common knowledge that the coronavirus can be spread by being in close contact with someone who's infected and then breathing in their respiratory droplets. Or by touching a contaminated surface and rubbing your eyes, nose or mouth.
An open letter signed by 239 researchers addressed to the World Health Organization, published Monday in Clinical Infectious Diseases, calls for attention and guidance around a third route of transmission: tiny respiratory particles that float in the air and are called aerosols and that, the researchers state, could be responsible for infecting someone who comes along and breathes them in.
"Most public health organizations, including the WHO, do not recognize airborne transmission except for aerosol-generating procedures performed in healthcare settings," the letter states, "Hand-washing and social distancing are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people."
WHO's response, at a press conference Tuesday, was measured. "There is some evidence emerging but is not definitive," said Dr. Benedetta Allegranzi, WHO's technical lead for infection prevention and control. "The possibility of airborne transmission in public settings, especially in very specific conditions — crowded, closed, poorly ventilated settings — cannot be ruled out. However, the evidence needs to be gathered and interpreted."
WHO officials say they've working for weeks on a brief that summarizes evidence on the various modes of transmission and will publish it in the coming days.
In the meantime, here's what we know about the different modes of transmission:
Droplet
What it is: A virus-filled particle of breath or spittle that comes out of the nose or mouth of an infected individual when they breathe, speak, cough or sneeze. Droplets generally fall to the ground within a few feet of the person who expels them.
How a virus could spread this way: "A large droplet flies through the air and lands on your body," says Linsey Marr, a professor of civil and environmental engineering who researches airborne transmission at Virginia Tech. "And that could be your eyes, nostrils or mouth." For instance, "if you're standing next to your kid and they cough in your face," says Seema Lakdawala, a flu researcher at the University of Pittsburgh.
How much does it contribute to the spread? Directly coming into contact with respiratory droplets is currently considered the most frequent mode of transmission, according to the U.S. Centers for Disease Control and Prevention and WHO.
Aerosol
What it is: A microscopic virus-packed particle that's also expelled from an infected person's mouth when breathing, speaking, coughing or sneezing. Unlike a droplet, smaller aerosol particles can remain suspended in the air.
"They'll continue to float and follow the air streams in a room," says Kimberly Prather, an atmospheric chemist at Scripps Institution of Oceanography, who signed the letter to WHO.
How a virus could spread this way: Through the respiratory route in which a person breathes in clouds of tiny virus particles that have accumulated and may be traveling on air currents.
How much does it contribute to the spread? It's an open question that begs more research. "Aerosols probably have some contribution toward spread, but we don't know to what extent and what situations are most relevant," says Abraar Karan, a physician at Harvard Medical School who treats coronavirus patients. Viral spread via air currents has been documented in certain indoor environments: at a restaurant in Guangzhou, China, where people sitting downstream from an air conditioner caught COVID-19, and at a March choir practice in Washington state, where many choir members got sick with coronavirus after likely breathing in tiny airborne particles generated by individuals who were infected but symptom-free at the time. What's still unclear is how long the virus lingers in the air, how far it travels (possibly through a room but not down the street) and how commonly it spreads this way.
Fomite
What it is: An object covered with virus particles, possibly because someone recently sneezed or coughed respiratory droplets onto it, or swiped a germ-covered hand on it. A countertop or a phone could become a fomite in that same manner. The particles could survive from several hours to several days.
How a virus could spread this way: Through indirect transmission if people touch the surface of a virus-covered object, pick up the virus on their hands and then introduce the virus to their eyes, nose or mouth.
How much does it contribute to the spread?: The CDC describes this as a "possible" route of coronavirus transmission but maintains that close contact between people is thought to be responsible for most new infections.
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