After some toing and froing, the US Centers for Disease Control and Prevention (CDC) updated its Covid-19 advice on Monday to acknowledge that the virus can be airborne, lingering in the air for “minutes or even hours" and even spreading between people who are more than 1.82 meters (6 feet) apart.
The change comes after a heated debate between public health experts and US authorities over the risk of airborne transmission and Covid-19. Last month, the CDC revised its website information to say that aerosols, minuscule airborne particles (smaller than 100 micrometers) expelled from a person's mouth when they talk, cough, sneeze, or breath, are among the most common ways the coronavirus spreads. While this revision was met with praise from health experts, it was swiftly taken down.
Now, similar advice has returned. On Monday, October 5, the CDC issued the update to its How COVID-19 Spreads guide, saying scientific evidence now clearly shows aerosols and airborne particles are one of the most prominent forms of Covid-19 transmission. This is important because airborne particles linger in the air for longer than larger droplets, which means in poorly-ventilated places like indoors, maintaining a 6-feet distance may not be enough, and masks are crucial.
“Today’s update acknowledges the existence of some published reports showing 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,” the CDC said in a statement. “In these instances, transmission occurred in poorly ventilated and enclosed spaces that often involved activities that caused heavier breathing, like singing or exercise. Such environments and activities may contribute to the buildup of virus-carrying particles.”
The new guidance states that the highest risk of transmission occurs between people who are within 1.82 meters (6 feet) of a person with Covid-19 by exposure to the virus via small airborne droplets that can lurk in the air “for minutes to hours”. However, it goes on to say “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.”
Importantly, the revamped advice highlights the difference between the risk of transmitting the virus via large droplets (larger than 100 micrometers) like saliva that typically fall to the ground in seconds, usually within 2 meters (over 6 feet) of the source, and via aerosols (smaller than 100 micrometers) that can “float” in the air for minutes and even hours.
In practice, this means that social distancing measures that rely on keeping a minimum distance of 2 meters (> 6 feet) between people might not be enough to stop the spread of the virus, although it certainly will reduce the risk of transmission. Keeping the space well ventilated and wearing a face mask will also help to further reduce this risk significantly.
All of this supports what most scientists and public health experts have been saying for months. But it isn’t just the CDC that has dragged its feet over updating its advice. Back in July, hundreds of scientists wrote to the World Health Organization (WHO) urging it to revisit its information on whether Covid-19 can be spread through airborne aerosol transmission, suggesting the hesitancy to do so is putting people in danger. The WHO has since accepted this and updated its advice, but many other health authorities across the world continue to hesitate about changing their stance on airborne transmission, which many scientists fear could be hampering efforts to curb the virus and provide the public with clear advice.