When people with the COVID-19 infection breathe out, clear their throats, cough, sneeze, speak, or otherwise move air out through their nose or mouth, droplets of all different sizes, which can contain the virus, are ejected into the air. A substantial portion of people infected with SARS-CoV-2 - around 40 percent - wouldn't even know they are ejecting virus-laden droplets, as they may not exhibit symptoms.
Droplets suspended in the air are called an aerosol. Droplets that are large can remain in the air for seconds to minutes before falling to the ground. Smaller droplets stay in the air longer – minutes to even hours.
Outside, the circulation of fresh air disperses the drifting droplets quickly, and so the combination of wearing cloth face coverings and maintaining physical distance (6 feet or more) is very effective at impeding the spread of COVID-19 in outdoor spaces where air moves. However, in indoor spaces, especially those with poor ventilation, coronavirus-laden droplets can build up to a level that is risky.
To the extent possible, we should all be minimizing the amount of time we spend in any one place, especially if others are nearby, or if you are indoors with other people. This limits the time a person is potentially exposed to the virus.
It is also very important to stay as far away from one another as possible, minimum 6 feet. The farther, the better. Some experts recommend staying at least 25 feet away from others, even when outdoors. And it's a good idea to avoid crowded indoor spaces.
The traditional definition of airborne transmission is that small droplets containing a pathogen remaining viable over long time periods travel long distances in the air and infect other people when the pathogen is breathed in. Measles and tuberculosis are examples of respiratory diseases that remain infectious in the air for long time periods. The measles virus can live for up to two hours in the air where an infected person coughs or sneezes. Tuberculosis can live in the air for up to six hours.
Under experimental conditions, researchers found that the COVID-19 virus stayed viable in the air for three hours. The researchers estimate that in most real-world situations, the virus would remain suspended in the air for about 30 minutes, before settling onto surfaces. This is similar to what was found for SARS and MERS, which some researchers consider likely to be spread via airborne transmission. One study estimates that a person infected with the COVID-19 virus who speaks loudly for one minute produces at least 1,000 virus-containing droplets that remain airborne for more than 8 minutes. Furthermore, the Centers for Disease Control and Prevention recommend airborne precautions for the care of COVID-19 suspected or confirmed patients.
Many unknowns remain about SARS-CoV-2, such as how many virus particles need to be breathed in for an infection to begin. However, active COVID-19 virus travels through the air when ejected by infected people, and can infect cells in the petri dish. By staying as far away from one another as possible, keeping on the move, avoiding touching our faces, frequently washing our hands well with soap and water, coughing or sneezing into the crook of our elbows, wearing cloth face coverings (masks), and staying home when sick, we can protect ourselves and others.
Drafted 14 September 2020