The Centers for Disease Control and Prevention revised its coronavirus guidance Monday, acknowledging that it can sometimes spread through airborne particles that can "linger in the air for minutes to hours" and among people who are more than 6 feet apart.
The CDC cited published reports that demonstrated "limited, uncommon circumstances where people with COVID-19 infected others who were more than 6 feet away or shortly after the COVID-19-positive person left an area."
"In these instances, transmission occurred in poorly ventilated and enclosed spaces that often involved activities that caused heavier breathing, like singing or exercise," the CDC said in a statement. "Such environments and activities may contribute to the buildup of virus-carrying particles."
The agency added that it is "much more common" for the virus to spread through larger respiratory droplets that are produced when somebody coughs, sneezes, sings, talks, or breathes. People are infected through such droplets mostly when they are in close contact with an infected person, the CDC said.
"CDC's recommendations remain the same based on existing science and after a thorough technical review of the guidance," the agency said. "People can protect themselves from the virus that causes COVID-19 by staying at least 6 feet away from others, wearing a mask that covers their nose and mouth, washing their hands frequently, cleaning touched surfaces often and staying home when sick."
The updated guidance comes after the agency mistakenly posted a revision last month that said the virus could spread through aerosols, small droplets that can linger in the air. The guidance was quickly removed from the CDC's website because it was just "a draft version of proposed changes," the agency said.
To what degree the coronavirus can spread through airborne particles has been a contentious debate among scientists for months. Some epidemiologists have charged that the World Health Organization as well as federal regulatory agencies in many countries have been slow to accept that the virus can spread by air. It's a debate that could have implications for the importance of air filtration in reopening businesses and schools.
Dr. Bill Schaffner, an infectious disease specialist at Vanderbilt University, said the new guidance is largely in line with what he says the science indicates about the coronavirus spreading through the air. He said in a phone interview after reviewing the new guidance that airborne transmission is something of a "side street" for spread.
"Some cars do get through on the side street," he said. "But the highways of transmission are close in, usually within enclosed spaces and for periods of time longer than 15 minutes with people standing within 3 to 6 feet of each other."
Schaffner added that the new guidance doesn't necessarily change how he thinks about reducing the risk of infection for most people. Wearing a mask, socially distancing and avoiding large indoor gatherings remain the most important steps people can take, he said.
But places of business, where many people come in and out everyday, might want to reexamine their ventilation systems, he said.
"Have your air handling system reviewed and see how efficient it is and whether you're getting sufficient air exchanges per hour, and where the stuffy corners of the building are," he said. "See if you can do something to enhance the air handling."
Saskia Popescu, an epidemiologist and biodefense expert at the University of Arizona, said the new CDC guidance is "quite good." She added that "the term airborne means so much to so many people," and the guidance does a good job of emphasizing that the virus appears to only spread by air in certain environments, such as crowded indoor and poorly ventilated spaces.
"We know that these events are occurring, but they're not the primary driver," she said in a phone interview. "This is a good reminder that there are environments that are higher risk for airborne transmission and we just need to communicate that."
She added that hopefully people take away from this guidance that "that hard and fast rule of 6 feet isn't some invisible force field that respiratory droplets and secretions and aerosols just hit and fall down dead." Instead, she said, people need to understand that risk factors change in different environments.
"There are a lot of factors that go into transmission and you can't cherry-pick which ones," she said. "This is saying, 'hey, some environments are higher risk, so you need to take all of these safety measures,' that risk reduction is additive, and not just to get so focused on one."