Here's the reason for it: Most scientists believe the main route of transmission of the coronavirus is through respiratory droplets emitted by people when we speak, cough, sneeze, sing, or simply breathe. These droplets have weight, and are believed to fall to the ground within six feet of the person emitting them. So keeping a safe distance from others creates a buffer against viral transmission.
Now, the idea that the virus could linger in the air for some time, and travel around poorly ventilated, enclosed spaces is putting this one truth on shaky ground.
Recently, more than 200 scientists penned an open letter to the World Health Organization, asking its leaders to adopt precautions in recognition of the "significant potential" for the virus to be spread via smaller respiratory particles that linger in the air. Because they can linger and circulate, this kind of of transmission is called "aerosolized." And, frankly, it's a more frightening prospect.
Shortly after that letter, the WHO issued a new scientific brief agreeing that transmission from much smaller particles than droplets can't be ruled out, and more research is needed about outbreaks that have occurred indoors in poorly ventilated spaces.
Let's break it down below.
What is the difference between droplets and aerosol particles?
The particles we expel from our mouths are various sizes.
Respiratory droplets are on the larger side (although they're still very small). You can sometimes see them, and definitely feel them, when a child sneezes, uncovered, right into your face.
Aerosol particles, also called microdroplets or aerosols, are a very small version of these droplets. You would need 10 or more of them to span the width of a human hair. You can see these microdroplets build up when you breathe on a mirror.
Aerosols are so small that they can linger in the air for a while, instead of falling quickly to the ground like respiratory droplets. Scientists don't know, however, how long aerosols stay in the air.
Do scientists really know that this kind of transmission is happening?
No. Scientists say more research is needed.
But many, like UC Irvine chemistry professor Ann Marie Carlton, who signed the letter to the WHO, say transmission through aerosols seems incredibly likely.
"A certain kind of contortionist thinking is required to explain transmission in the absence of aerosol," she said.
A danger Carlton and others point to is that airborne virus particles can build up in an enclosed room with poor air circulation, especially if the air is recirculated over and over. Aerosols act like smoke: they can accumulate and they can drift from one end of a room to another.
Scientists concerned about aerosol transmission want health officials to issue specific precautions to be taken indoors.
If the virus is airborne, what can we do to reduce our risk of getting it this way?
A lot of what we are doing to protect ourselves from the spread of the virus through respiratory droplets will protect us from aerosols too, like moving gatherings outside, for example, and wearing a mask. When everyone wears a mask, it can reduce how many droplets — of any size — can get out into the environment.
Given that indoor environments with poor air circulation are a major concern for potential airborne transmission, scientists have some recommendations to make these spaces safer:
- Open windows and doors to improve ventilation.
- Use air filters that pull in air from outdoors rather than recirculate air.
- Install ultraviolet lights, which may potentially zap the virus.
- Avoid overcrowded public buildings and public transit.
If the jury is still out on airborne transmission, why bother?
The jury is still out on a lot of what we know about the coronavirus. But scientists are raising awareness about this possibility to urge public health officials to adopt what is known as the precautionary principle. This is the notion, common in Europe, that when science hasn't yet shown whether something is a health risk, officials issue precautions to protect people until they do know.
"We don't want to scare people," said Manabu Shiraiwa, also a chemistry professor at UC Irvine who signed the letter. "It's important to acknowledge it," he said, referring to airborne transmission, because then "we can do something about it."