Since early reports revealed that a new coronavirus was spreading rapidly between people, researchers have been trying to pin down whether it can travel through the air. Health officials say the virus is transported only through droplets that are coughed or sneezed out — either directly, or on objects. Lockdowns are already easing in some places, and people are preparing to return to a version of work and social life. But a crucial question stubbornly remains: Can the pathogen behind COVID-19 be “airborne”?
Reducing our risk for COVID-19 depends on how well we understand the way SARS-CoV-2, the virus that causes COVID-19, is transmitted. Experts believe SARS-CoV-2 mainly spreads from person to person through respiratory droplets released when a person with the virus coughs, sneezes, or talks. It’s also possible that you can contract the virus if your hands come in contact with your mouth, nose, or eyes after touching a contaminated surface.
Airborne and everywhere
An aerosol is a clump of small liquid or solid particles floating in the air. They are everywhere in the environment and can be made of anything small enough to float, like smoke, water or coronavirus-carrying saliva.
When a person coughs, talks or breathes, they throw anywhere between 900 to 300,000 liquid particles from their mouth. These particles range in size from microscopic – a thousandth the width of a hair – up to the size of a grain of fine beach sand. A cough can send them traveling at speeds up to 60 mph.
Size of the particle and air currents affect how long they will stay in the air. In a still room, tiny particles like smoke can stay airborne for up to eight hours. Larger particles fall out of the air more quickly and land on surfaces after a few minutes.
By simply being near other people, you are coming into constant contact with aerosols from their mouth. During a pandemic this a little more concerning than normal. But the important question is not do exhaled aerosols exist, rather, how infectious are they?
Aerosols as virus delivery systems
The new coronavirus, SARS-CoV-2, is tiny, about 0.1 microns - roughly 4 millionths of an inch - in diameter. Aerosols produced by people when they breathe, talk and cough are generally between about 0.7 microns to around 10 microns – completely invisible to the naked eye and easily able to float in air. These particles are mostly biological fluids from people’s mouths and lungs and can contain bits of virus genetic material.
Researchers don’t yet know how many individual pieces of SARS-CoV-2 an aerosol produced by an infected person’s cough might hold. But in one preprint study, meaning it is currently under peer review, researchers used a model to estimate that a person standing and speaking in a room could release up to 114 infectious doses per hour. The researchers predict that these aerosolized bits of saliva would easily infect other people if this happened in public indoor spaces like a bank, restaurant or pharmacy.
Another thing to consider is how easy these particles are to inhale. In a recent computer model study, researchers found that people would most likely inhale aerosols from another person that is talking and coughing while sitting less than 6 feet away.
Evidence of aerosol transmission
It is almost impossible to study viral transmission in real time, so researchers have turned to environmental sampling and contact tracing to try to study the spread of the coronavirus in aerosols. This research is happening extremely fast and most of it is still under peer review, but these studies offer extremely interesting, if preliminary, information.
To test the environment, researchers simply sample the air. In Nebraska, scientists found airborne SARS-CoV-2 in a hospital. In China, scientists also found the virus in the air of a number of hospitals as well as a department store.
But environmental sampling alone cannot prove aerosol transmission. That requires contact tracing.
One restaurant in Guangzhou, China, was the site of a small outbreak on Jan. 23 and offers direct evidence of aerosol transmission. Researchers believe that there was one infected but asymptomatic person sitting at a table in the restaurant. Because of the air currents circulating in the room due to air conditioning, people sitting at two other tables became infected, likely because of aerosols.
Overall, the evidence suggests that it is much more risky to be inside than outside. The reason is the lack of airflow. It takes between 15 minutes and three hours for an aerosol to be sucked outside by a ventilation system or float out an open window.
Another preprint study of outbreaks in Japan suggests that the chances of direct transmission are almost 20 times higher indoors compared to outdoors. In Singapore, researchers traced the first three outbreaks directly to a few shops, a banquet dinner and a church.
Once outside, these potentially infectious aerosols disappear in the expanse of the atmosphere and are much less of a worry. It is of course possible to catch the virus outside if you are in close contact with a sick person, but this seems very rare. Researchers in China found that only one of 314 outbreaks they examined could be traced back to outdoor contact.
There has been recent concern over aerosol transmission during running and biking. While the science is still developing on this, it is probably wise to give other bikers or runners a little more room than normal.
WHO Note: According to current evidence, COVID-19 virus is primarily transmitted between people through respiratory droplets and contact routes. In an analysis of 75,465 COVID-19 cases in China, airborne transmission was not reported.