The Centers for Disease Control and Prevention on Monday updated its covid-19 guidance to say that the coronavirus can spread through aerosols, not just large droplets. This is an important distinction because it means that maintaining 6 feet of distance indoors is not always enough to prevent transmission. Virus-containing aerosols can linger in the air and spread throughout a room, especially poorly ventilated spaces where people are talking or singing, which is why wearing masks is so crucial.
Previously, CDC guidance did not include warnings about so-called airborne transmission, which involves tiny aerosols emitted by an infected person that can float and travel throughout a space (as opposed to transmission via larger droplets that don’t travel far before falling to the ground). Today, the CDC’s official guidance page says: “COVID-19 can sometimes be spread by airborne transmission. Some infections can be spread by exposure to viruses in small droplets and particles that can linger in the air for minutes to hours. These viruses may be able to infect people who are further than 6 feet away from the person who is infected or after that person has left the space.”
Last month, the CDC briefly altered its guidance page to note the risk of aerosol transmission, but it later removed that update. CDC officials said the language had been added by mistake and had not yet undergone scientific review.
Gizmodo reporter Ed Cara has written about the difficulty in classifying viruses as airborne versus not. Unfortunately, the science is complex, and the same virus can sometimes be airborne and sometimes not, depending on the circumstances. For example, an air conditioning unit can spread exhalations from an infected person far across a room, as happened in a Korean Starbucks where 27 people were infected at once.
Some viruses, like measles, are most definitely airborne, which is part of why they are so contagious. Others, like influenza, seem to be mostly droplet-spread but sometimes airborne. The virus behind covid-19, known as SARS-CoV-2, appears to be somewhere in the middle. Regardless of exactly how airborne it is, though, people should take the necessary precautions: wearing masks around others and avoiding crowded indoor spaces. Airborne transmission is much less of a risk outdoors, since aerosols emitted by an infected person are quickly dispersed by the large volume of air. Improving indoor ventilation is likely to help reduce the risk of transmission, as Atlantic writer Zeynep Tufecki has reported. Washing hands and avoiding touching your face continues to be wise, but as the CDC’s guidance page notes, “Spread from touching surfaces is not thought to be a common way that COVID-19 spreads.”
As the U.S. heads into winter and the holiday season, the temptation to spend time indoors with friends and family is going to be high for many of us. But given what we know about how the virus spreads, the best place to see other people continues to be outdoors. If you’re indoors with people outside your household or social bubble, wear masks and keep the windows open, because 6 feet of distance and hand sanitizer are not enough to stay safe.