Scientists underscore need for mask-wearing, warn of aerosol spread beyond 6 feet

New York Governor Andrew Cuomo was joined by Rosie Perez and Chris Rock at a press conference Thursday in New York, where the two performers helped to promote coronavirus testing, social distancing, and mask-wearing.Spencer Platt/Getty

Experts from the University of California San Diego are emphasizing the need for everyone to wear a mask to prevent the spread of the coronavirus, while warning that the virus may float through the air farther than the well-known 6-foot spacing officials have asked people to maintain.

The researchers, in a Perspective article published Wednesday in the journal Science, warned of the dangers of aerosols emitted by people who are infected by the virus and who don’t even know it.

“Evidence suggests that SARS-CoV-2 is silently spreading in aerosols exhaled by highly contagious infected individuals with no symptoms,” said the article, which defined aerosols as particles less than or equal to 5 micrometers in size. A micrometer, or micron, is one-millionth of a meter.

“For society to resume, measures designed to reduce aerosol transmission must be implemented, including universal masking,” said the researchers, who also called for “regular, widespread testing to identify and isolate infected asymptomatic individuals."

The CDC already recommends face coverings in public due to research showing disease spread by asymptomatic people doing things as seemingly innocuous as talking. On May 1, Governor Charlie Baker issued an order requiring Massachusetts residents to cover their noses and mouths if they’re unable to keep a 6-foot distance from others in any “place open to the public.”

The UC San Diego researchers raised questions about that 6-foot distance, particularly when people are indoors.

Aerosol particles are so small they tend to linger in the air, rather than fall to the ground like larger droplets, and that means they can spread farther, according to the the article, whose lead author was Kimberly Prather, a professor at UC San Diego’s Scripps Institution of Oceanography and in the university’s department of chemistry and biochemistry. Prather is an expert on aerosol chemistry.

How far can those aerosol particles go? The researchers said people should consider the case of cigarette smoke and how far it can go.

“A good comparison is exhaled cigarette smoke, which also contains submicron [less than 1 micron] particles and will likely follow comparable flows and dilution patterns,” the article said.

The researchers noted that the recommendation from the World Health Organization that people stay at least 6 feet away from each other was developed based on studies of larger respiratory droplets carried out in the 1930s, a time when the technology did not exist for detecting submicron aerosols.

“Increasing evidence for SARS-CoV-2 suggests the 6 ft WHO recommendation is likely not enough under many indoor conditions where aerosols can remain airborne for hours, accumulate over time, and follow air flows over distances further than 6 feet,” the article said.

“It is important to wear properly fitted masks indoors even when 6 ft apart,” the researchers said.

The article said the minimum dose of virus that would cause an infection is unknown, but other diseases have been shown to be able to infect people through aerosols, including measles, SARS, and chickenpox.

The researchers said masks “provide a critical barrier, reducing the number of infectious viruses in exhaled breath, especially of asymptomatic people and those with mild symptoms" — and even homemade masks can provide filtration that can protect uninfected people.

“It is particularly important to wear masks in locations with conditions that can accumulate high concentrations of viruses, such as health care settings, airplanes, restaurants, and other crowded places with reduced ventilation,” the researchers said.

The study comes at a time of growing emphasis from experts and public officials on the importance of mask-wearing, particularly as a way to prevent the virus from spreading.

“It used to be the dogma of infection control that wearing masks in public didn’t really change disease transmission. But that was for diseases pre-COVID and now the phrase that is being used is universal source control, which means that we all wear masks to protect each other,” Dr. Paul Biddinger, medical director for emergency preparedness at Partners HealthCare, said in a media briefing on Tuesday.

“I think there’s still a lot of popular misconception that masks are primarily to protect ourselves. And they probably do offer a degree of protection, but it’s mostly that we wear masks so that if any one of us is infected, we don’t infect others are around us," he said.