Western, central and northern Minnesota under windy, dry weather conditions 

MPR News helps you turn down the noise and build shared understanding. Turn up your support for this public resource and keep trusted journalism accessible to all. 

Turn Up Your Support

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, 

, calls for attention and guidance around a third route of transmission: tiny respiratory particles that float in the air and are called "aerosols." They could be responsible for infecting someone who comes along and breathes them in.

Researchers are actively debating the importance of aerosol transmission, and whether precautionary approaches should be taken even without clear answers about its role in the spread of the coronavirus.

"The ratio is something we need to figure out," says 

, an atmospheric chemist at Scripps Institution of Oceanography, who signed the letter to WHO.

Here's what we know about the different modes of transmission: 

A microscopic virus-filled particle of breath or spittle, around 5-10 microns — about the size of a red blood cell — 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. 

Through what researchers call "the respiratory route." Virus-laden droplets are expelled from the nose or mouth of an infected person and find their way into nearby eyes, noses and mouths, "like if you're standing next to your kid and they cough in your face," says 

, a flu researcher at the University of Pittsburgh.

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

A virus-packed particle, smaller than 5 microns, that's also expelled from an infected person's mouth when they breathe, speak, cough or sneeze. 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 Prather.

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.

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 

, a physician at Harvard Medical School who treats coronavirus patients. Viral spread via air currents has been documented in certain indoor environments: at a 

 in Guangzhou, China where people sitting downstream from an air conditioner caught COVID-19, and at a March 

 in Washington, where many choir members got sick with coronavirus after likely breathing in tiny airborne particles generated by individuals in the choir 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. 

 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.

 Through indirect transmission, if a person touches the surface of a virus-covered object, picks up the virus on their hands, then introduces the virus to their eyes, nose or mouth. 

 The Centers for Disease Control and Prevention describes this as a "

" route of coronavirus transmission but maintains that close contact between people is thought to be responsible for most new infections.  

Health

COVID-19

Pien Huang

Author

A letter from over 200 scientists to the World Health Organization asks for further investigation into how the virus spreads.

Aerosols, droplets, fomites: What we know about transmission of COVID-19

Go Deeper.

Like this?

News you can use in your inbox