Is wearing a mask while traveling still helpful if you're the only one?

By Jacqueline Howard, CNN

    (CNN) -- The Biden administration's mask requirements for airplanes and other mass transit are no longer in effect during a review of a ruling by a federal judge in Florida that struck down the order. Some travelers welcomed Monday's ruling, but others have decided to keep their masks on.

Face masks offer the most protection against the spread of virus-carrying particles in the air when everyone wears them. But research also suggests that masks can protect the wearer alone, by acting as a barrier between particles and their nose and mouth.

"I was actually traveling by plane yesterday when the mask requirement on public transit was removed. I definitely kept my mask on throughout my flight," Chris Cappa, a professor of civil and environmental engineering at the University of California, Davis who studies aerosol particles and masks, wrote in an email Tuesday.

While traveling from Sacramento to San Diego, he watched as the number of other passengers who kept their masks on dropped steadily.

"I will be continuing to wear my N95 while traveling for a while still. I personally have more concern when I'm in small, crowded spaces such as on airplanes compared to when I'm in large, relatively open spaces such as airports," Cappa wrote.

When one person is masked and others are not, it's called one-way masking.

"The level of protection with one-way masking depends largely on two factors: how well your mask fits and how effectively the mask material filters out particles that can carry viruses. Masks like N95s and KN95s will generally be more protective than surgical masks or cloth masks because they can make a tighter seal against your face. And surgical masks tend to do a better job at filtering than cloth masks for a similar fit," Cappa wrote.

"However, different masks will fit different faces better or worse, and so it is important to find one that fits you well. For example, you could potentially adjust the earloops so that the mask fits more tightly," he said. "Even the best mask is only as good as the fit. But a well-fit N95 can reduce the amount of potentially infectious particles that you inhale by more than a factor of 20 times."

Even if everyone around you is maskless, Cappa noted that wearing a well-fitting N95 mask can reduce the amount of infectious particles you could breathe in. "If there were theoretically 100 infectious particles that you were about to breathe in with no mask you would only breath in 5 or fewer with the well-fitting N95," he wrote in his email.

Cloth masks -- encouraged earlier in the pandemic while other protective equipment was in short supply -- can filter large droplets. But more effective masks, such as N95s, can filter those in addition to the smaller aerosols or particles that infected people may breathe out, Erin Bromage, an associate professor of biology at the University of Massachusetts Dartmouth, said in December.

N95 respirators that are approved by the US National Institute for Occupational Safety & Health can filter at least 95% of particles in the air when properly fitted, according to the Centers for Disease Control and Prevention. Surgical or disposable masks are about 5% to 10% less effective than N95 respirators, Bromage said.

A CDC study published in February found that people who reported always wearing a mask indoors while in public were less likely to test positive for Covid-19 than people who didn't wear masks between February and December 2021.

Among more than 500 people who reported the type of mask they wear, masking "lowered the odds of testing positive" for Covid-19 by 56% among those who wore cloth masks, 66% among those who wore surgical masks and 83% among those who wore N95 or KN95 respirators, compared with people who wore no mask at all, according to the CDC.

"If everyone else is unmasked those percentages could go down," Cappa told CNN.

Dr. Preeti Malani, chief health officer in the Division of Infectious Diseases at the University of Michigan in Ann Arbor, has been treating people who have Covid-19 for more than two years. Her patients are often not masked, but she is.

"To my knowledge, I haven't had Covid," Malani said. "So the personal protection of masks works very well, especially when layered on vaccination and good ventilation."

Malani added that she feels it's safe to travel, even for those who might be at higher risk for more severe Covid-19, especially if they follow mitigation measures like being vaccinated, testing themselves and wearing well-fitting, high-quality masks -- even if those around them are unmasked.

"It doesn't mean you can't use public transportation. It means that you have to be thoughtful about it," she said, adding that she is more concerned about Covid-19 spread at a crowded bar than on an airplane where the air ventilation is high-quality.

"When you layer wearing a mask on top of vaccination, ventilation, potentially testing, social distancing, you can keep that risk manageable," Malani said. "What I don't want to see is that people are all of a sudden so fearful of Covid that they're not traveling anymore."

She will be traveling to Lisbon, Portugal, this week to attend a meeting of the European Congress of Clinical Microbiology & Infectious Diseases. "I plan to wear my mask on the airplane," she said, not because she worries about getting severely ill from Covid-19 but because she doesn't want to delay travel home if she happens to get infected while overseas.

Cappa wrote in his email that the "riskiest" time for pathogens to spread during travel are when people are "stationary," such as when everyone lines up to disembark a plane.

"This is because the air exchange -- which helps to keep the air clean -- inside planes, buses, and trains tends to be greatest when moving," he said.

"On a plane, you might open the air vents (if your plane has them) and direct them at you as the air coming out has been cleaned through filtration. Also, if you are talking with others it always helps to keep some distance between you," he added. "Of course, that's not always possible on public transit, in which case trying to keep out of your neighbors direct breath plume can help."

Ultimately, Dr. Vivek Cherian, a Chicago-based internal medicine physician and father of three young children, thinks it's too soon to roll back mask mandates for travel.

"In my view we shouldn't lift mask mandates in this country until everyone who wants a vaccine has access and an opportunity to get one including children under the age of five," Cherian wrote in an email Tuesday.

"If you are an individual who is immunocompromised or have immunocompromised or unvaccinated family members, one-way masking can still be effective. The key is to use the best mask available, preferably one with an N95 respirator as they offer a high degree of protection," he said. "I continue to mask up because all three of my children are under the age of five and are not eligible for vaccines at this time."

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