Small aerosol particles spewed while people breathe, talk and sing may contain more coronavirus than larger moisture droplets do. And the coronavirus may be evolving to spread more easily through the air, a new study suggests. But there is also good news: Masks can help.
About 85 percent of coronavirus RNA detected in COVID-19 patients’ breath was found in fine aerosol particles less than five micrometers in size, researchers in Singapore report August 6 in Clinical Infectious Diseases. The finding is the latest evidence to suggest that COVID-19 is spread mainly through the air in fine droplets that may stay suspended for hours rather than in larger droplets that quickly fall to the ground and contaminate surfaces.
Similar to that result, Donald Milton at the University of Maryland in College Park and colleagues found that people who carried the alpha variant had 18 times as much viral RNA in aerosols than people infected with less-contagious versions of the virus. That study, posted August 13 at medRxiv.org, has not been yet been peer-reviewed. It also found that loose-fitting masks could cut the amount of virus-carrying aerosols by nearly half.
In one experiment, the Maryland team grew the virus from the air samples in the lab. That could be evidence that may convince some reluctant experts to embrace the idea that the virus spreads mainly through the air.
The debate over aerosol transmission has been ongoing since nearly the beginning of the COVID-19 pandemic. Last year, 200 scientists wrote a letter to the World Health Organization asking for the organization to acknowledge aerosol spread of the virus (SN: 7/7/20). In April, the WHO upgraded its information on transmission to include aerosols (SN: 5/18/21). The U.S. Centers for Disease Control and Prevention had acknowledged aerosols as the most likely source of spread just a few weeks before.
Previous studies in monkeys have also suggested that more virus ends up in aerosols than in large droplets. But some experts say that direct evidence that the virus spreads mainly through the air is still lacking.
“There’s lots of indirect evidence that the airborne route — breathing it in — is dominant,” says Linsey Marr, a civil and environmental engineer at Virginia Tech in Blacksburg, who studies viruses in the air. She was one of the 200 scientists who wrote to the WHO last year. “‘Airborne’ is a loaded word in infection control circles,” she says, requiring health care workers to isolate patients in special rooms, wear protective equipment and take other costly and resource-intensive measures to stop the spread of the disease. For those reasons, infection control experts have been reluctant to call the coronavirus airborne without especially strong proof.
Most COVID-19 cases have been among close household contacts — typically within the 6-feet splash zone of large droplets. It can be hard to tease out whether such infections were passed on by large droplet contamination or by breathing the same air. But for other situations, such as when patrons get infected while sitting across a restaurant from someone with COVID-19, aerosols are really the only explanation, Marr says.
Mechanical engineer Kwok Wai Tham of the National University of Singapore set out to sample how much virus COVID-19 patients produce when they breathe, talk or sing, in part, to address skeptics’ concerns. “I’m doing this to convince some very close friends,” he says. He and colleagues rolled a mobile lab into 22 patients’ rooms and had volunteers stick their heads into a large metal cone.