The news: The US Centers for Disease Control and Prevention has updated its guidelines to acknowledge that the coronavirus can be spread by tiny particles that linger in the air. The agency said it made the decision because of the mounting evidence that people with covid-19 can infect people even if they are more than six feet away, or shortly after the infected person left the area. These cases all occurred in poorly ventilated and enclosed spaces, and often involved activities that cause heavier breathing, like singing or exercise. However, “the CDC continues to believe, based on current science, that people are more likely to become infected the longer and closer they are to a person with COVID-19,” it said in a statement. The long-coming update could help to finally clarify the situation after the CDC published guidance acknowledging airborne transmission and then suddenly retracted it last month.
The significance: Evidence that airborne transmission is occurring has been mounting for months; 239 experts wrote an open letter to the World Health Organization in July calling for them to acknowledge it. The WHO still has not recognized airborne transmission as a significant factor in the pandemic, and the CDC’s slowness to do so has caused frustration among aerosol researchers, some of whom say it is the main route for infections. The CDC maintains it occurs only in “limited, uncommon” circumstances. Airborne transmission has become a topic of fierce contention, partly because it makes it far riskier to reopen spaces like restaurants, gyms, bars, schools, and offices.
What do we do now? The CDC advises that people stay at least six feet away from others, wear a mask that covers their nose and mouth, frequently wash their hands, clean high-touch surfaces often, and stay home when they are feeling sick. However, the implications of airborne transmission mean the CDC perhaps ought to shift its emphasis and go further, advising people to properly ventilate buildings, limit the number of people indoors at any given time while encouraging them to stay farther apart and masked, and try to socialize outdoors where possible. “The thing people need to understand is aerosol transmission is like everyone breathing out cigarette smoke, and you want to breathe in as little of others’ as possible. Everyone you are around, imagine they are breathing smoke, and try to avoid it,” Jose-Luis Jimenez, a chemistry professor at the University of Colorado, Boulder, who has studied aerosols for 20 years, told MIT Technology Review in an interview.
Pushback: Jimenez criticized this new CDC update for being written confusingly, using the phrase "small droplets" instead of the widely accepted word "aerosols." Crucially, Jimenez said the document downplays the importance of airborne transmission. "We know that superspreading events are a major component of transmission. And every single superspreading event that has been studied appears to be dominated by aerosol transmission," he said. Jimenez also said that the CDC update appears to suggest airborne transmission from sharing a room together is rare, when it is not. "For example, it is the most likely explanation for the outbreak at the White House," he said.
This story was updated after publishing to include comments from Jose-Luis Jimenez.
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