Local scientists help make the case for masks and call for clarity on coronavirus transmission

“The mask is your first line of defense,” Kimberly Prather, a professor of atmospheric chemistry at UC San Diego’s Scripps Institution of Oceanography, said during an Oct. 5 news conference about the need for clearer public health guidelines to combat the coronavirus.

The online event highlighted a letter in which Prather and five other scientists urged researchers to deliver clear information about how SARS-CoV-2 — the coronavirus that causes COVID-19 — is spread in the air.

In guidelines updated Oct. 5, the Centers for Disease Control and Prevention said the virus can sometimes be spread by airborne transmission.

“Some infections can be spread by exposure to virus 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 six feet away from the person who is infected or after that person has left the space,” the guidelines state.

However, the statement adds, “available data indicate that it is much more common for the virus … to spread through close contact with a person who has COVID-19 than through airborne transmission.”

The scientists’ letter, published in the journal Science, also was authored by UCSD School of Medicine infectious-disease specialist Robert Schooley; Linsey Marr, a professor of civil and environmental engineering at Virginia Tech; Melissa McDiarmid, head of occupational and environmental medicine at the University of Maryland School of Medicine; Mary Wilson, a clinical professor of epidemiology and biostatistics for the UC San Francisco School of Medicine and an adjunct professor of global health and population for the Harvard T.H. Chan School of Public Health; and Donald Milton, a University of Maryland School of Public Health physician specializing in how respiratory viruses spread.

“One is far more likely to inhale aerosols than be sprayed by a droplet, and so the balance of attention must be shifted to protecting against airborne transmission,” the letter states.

“In this particular virus,” Prather said, “a lot of the spread is from asymptomatic individuals … it’s coming out in their speech. Speech produces mostly aerosols. … There’s very few droplets that are even there.”

The letter states that for researchers to be able to converge on clear guidelines, “we must clarify the terminology to distinguish between aerosols and droplets using a size threshold of 100 [microns], not the historical 5 [microns].”

Viruses in droplets, which are larger than 100 microns, fall to the ground in seconds within about six feet of the source or spray onto people nearby, the letter states. Viruses in aerosols, which are smaller than 100 microns, can “remain suspended in air for many seconds to hours, like smoke, and be inhaled.”

Prather said she is “very involved in the aerosol side of things,” with a background in aerosol chemistry and physics.

She said the group was motivated to write the letter because droplet and aerosol terminology is “confusing the public.”

“I’m very interested in seeing the public understand the issue of what aerosol transmission means,” Schooley said. “We spend a lot of time talking about social distancing, as if six feet is a magic number. And the advice that people should wear a mask only if they can’t social distance is really poorly thought-out advice in the context of aerosol spread.”

Prather, who said she is advising local school districts on reopening standards, said, “The six-foot rule is problematic because some school districts are requiring all the kids to wear masks [and] some are not if they can stay six feet away. That’s obviously not good.”

“Wearing masks is critical,” Schooley said.

The best masks not only prevent transmission to others but also protect against acquiring the virus, according to Marr, whose lab evaluates various masks for efficacy in laboratory settings. Those made with fabrics containing “a tighter weave, a high-quality thread count” are better, she said.

Filters between two layers, along with material that conforms to your face, also are beneficial, she said.

“There’s a lot of evidence in this disease that the severity of your illness is a function of how much virus you’re exposed to,” Schooley said. “Wearing a mask decreases the amount of virus that you’re exposed to and will decrease the likelihood that you’re going to end up with a very serious case of the disease. There are a lot of variables in this disease, some things we can’t control. But we can control how much virus we are exposed to.”

Prather said plexiglass barriers around desks or between people are good at stopping larger droplets, but aerosols will “bend right around those plexiglass barriers. It’s a false sense of security.”

Aerosols also will bend around the edge of face shields, she said, and she encouraged those in face shields to also wear masks. “There’s got to be better public guidance on these things that to us, those who study aerosols, is so obvious,” she added.

A “perfect world” of measures to prevent virus spread also would include better ventilation and filtration, Prather said.
“Ventilation is just so important. In some places, that’s just a matter of opening the door or the windows. Having clean air is the best thing you can do.”

“The riskiest places are indoor locations with poor ventilation where people are talking, yelling and no masks.” she said. “That’s where we’re seeing a huge amount of the spread.”

Prather and Marr likened the spread of aerosols to that of cigarette smoke. Virus particles “behave almost exactly the same [as cigarette smoke],” Prather said. “If you’re in the room with just one smoker, you can watch that haze — and that’s particles — build up over time with poor ventilation.”

Moving activities outdoors also is a reliable suggestion to stop the spread of the virus, the letter states.

For indoor activities, Prather said, “aerosols will go beyond six feet and they can build up in a room. If you’re indoors, you’re all sharing the air in that room. … These are the super-spreader events … so masks are absolutely essential when you’re indoors.”

“There’s all kinds of ways” to control transmission, Prather said as she encouraged people to read a list of frequently asked questions the group prepared. ◆