Is the science of masks changing?
Recommendations about wearing face masks are changing. Why?
Welcome to Not a Doctor, the only (free!) newsletter about health and science that talks about the filtration quality of vacuum cleaner bags. (What a world!)
I’m Melody Schreiber, a journalist and the editor of What We Didn’t Expect. I’m not a doctor, or a scientist, or really an expert of any kind. I just like to ask questions and try to find the answers to them.
Today, we’re getting down and nerdy about the science of face masks.
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New recommendations
You’ve probably already seen the news: The CDC now recommends wearing a cloth face mask in public in order to keep asymptomatic people from spreading the virus.
Heck, you may have already designed a mask from a bandanna or some old socks.
But why did this advice change? And if you’re going to start wearing a cloth mask, what’s the best way to do it?
This is going to be a two-parter: today, I talk about why the advice changed, and tomorrow, I’ll discuss the best ways to protect yourself.
I’ve written before about the differences between medical and cloth masks and why it’s important to reserve medical masks for health workers, so I won’t go into all of those details again. If you haven’t already read that update, though, I recommend checking it out!
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What scientists say
There’s still no strong, compelling evidence that cloth masks protect you from getting what other people have, experts said last week. And there’s no solid evidence that cloth masks can protect other people from getting what you have, they said.
But others have argued that there’s a big difference between “absence of evidence and evidence of absence” — meaning, just because there hasn’t been a lot of research on it, doesn’t mean it’s not worth looking into.
Education about proper mask use will be essential. One reason why studies haven’t really show mask effectiveness for the general population is because many people don’t know how to wear them properly or take them off and on correctly.
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What the research says
There’s an image going around social media of different fabrics that protect against extremely small particles like an aerosolized virus:
Image source: Delthia Ricks/Twitter. Original source: an air filter company (hmm)
This graph seems to be based on a 2013 study looking at cloth versus surgical masks for protecting against the flu. (It’s very important to keep in mind that the coronavirus and the flu are quite different viruses, though they may be spread in similar ways.)
The researchers used two microorganisms similar in size to influenza (a bacterium and a bacteriophage, to be precise), and they tested several different materials, including vacuum cleaner bags, tea towels, and pillowcases in a lab setting.
In the lab, surgical masks were the best at filtering out the particles — meaning, protecting you from breathing in what other people might be expelling.
Vacuum cleaner bags and tea towels were next, but they made it extremely difficult to breathe, the researchers pointed out. The best materials for cloth face masks, in terms of both filtration and ability to breathe, were pillowcases and 100 percent cotton t-shirts, they said.
(Note: this is different than the graph above, which was put together by an air filter company. The actual researchers, though, said this: “The pillowcase and the 100% cotton t-shirt were found to be the most suitable household materials for an improvised face mask.”)
The researchers also had volunteers put on masks made from t-shirts. Both homemade t-shirt masks and surgical masks helped keep some droplets from going into the air when people coughed; however, the cloth masks were one-third as effective at catching these droplets as surgical masks were.
That makes homemade masks a “last resort” for flu protection, “but it would be better than no protection,” the authors concluded.
They also pointed out that, although the cloth masks reduced the risk of transmission, some transmission still occurs — including through ways other than coughing.
“Thus any mask, no matter how efficient at filtration or how good the seal, will have minimal effect if it is not used in conjunction with other preventative measures, such as isolation of infected cases, immunization, good respiratory etiquette, and regular hand hygiene,” they wrote.
Yang Wang, a professor at Missouri S & T, says they’ve also tested different materials for filtration ability, but I was not able to find the study source for this graph, so I don’t know the conditions or disclaimers around it:
Image source: Yang Wang/Twitter
In another analysis, a preprint* review, researchers looked at 31 studies around face masks, and found that there wasn’t enough strong evidence that they worked in a community setting, like a well person running essential errands.
However, they said, “there is enough evidence to support the use of facemasks for short periods of time by particularly vulnerable individuals” in situations where they might encounter the virus — for instance, at doctor’s visits.
In addition, researchers wrote in 2006 that a mask made out of eight layers of t-shirt fabric, if fitted properly, could filter out some (artificially created) droplets for workers handling poultry who were at risk of getting bird flu.
Their mask (which is pictured in that piece — check it out, it’s something else!) could be uncomfortable, they wrote, and was not recommended for anyone with existing breathing issues.
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A change in approach
In a crisis, research happens quickly and scientific knowledge evolves faster than perhaps any other time.
However, based on what I’ve been able to find, the science around masks hasn’t really changed. Medical masks are great and health workers need plenty of them. Cloth masks can probably help you from getting other people sick, to a much lesser degree.
So, the advice we’re hearing isn’t changing because of huge scientific discoveries in the mask realm. Instead, it’s changing because of the urgency of our situation and our interpretation of what’s happening.
And part of the about-face on face masks has to do with asymptomatic transmission — an area where our scientific knowledge is evolving.
The idea behind wearing a mask out in public is this: Even if you don’t show symptoms, you could be spreading the virus. We still don’t know how many people are likely asymptomatic carriers, and how contagious they are. But we’re starting to get a better idea.
In February, a WHO report said that the “proportion of truly asymptomatic infections is unclear but appears to be relatively rare and does not appear to be a major driver of transmission.”
The CDC also said, at first, that people who have the virus without symptoms probably play a small role in transmitting this virus.
But last week, they revised that statement; as many as 25 percent of people who have the virus may not ever show symptoms, they said.
At a briefing on Sunday, Anthony Fauci said that anywhere from 25 to 50 percent of cases may have no symptoms at all.
Those rising figures could go even higher.
One Italian village helped stop the virus in its tracks by testing all residents. Sergio Romagnani, a professor of clinical immunology at the University of Florence, estimated that 50 to 75 percent of the people who tested positive for the virus never displayed any symptoms.
I’ve talked before about the study in China showing that 86 percent of cases were either asymptomatic or mild (which are two different things, ugh!). The people in that study were about half as contagious as people with more severe symptoms — yet because they never realized they were sick, they might have contributed to 79 percent of confirmed COVID-19 cases.
And according to a newer report, about four in five cases (78 percent, according to their rough calculation) being detected in China right now are asymptomatic.
Some experts disagree with that, though.
Image source: Caitlin Rivers/Twitter
It’s not clear in the recent China data whether people are truly asymptomatic or just pre-symptomatic (when symptoms haven’t developed yet).
It’s very possible that the initially high numbers could drop over time as previously asymptomatic people begin to develop symptoms.
On the Diamond Princess cruise ship, for instance, 41 percent of people first seemed to be asymptomatic — but that figure fell to 32 percent over time, as some people ended up developing symptoms.
But one thing is clear: There are significant numbers of people who are asymptomatic carriers of this virus. That means any of us could have it, and unwittingly give it to others.
That’s where masks come in. They’re not perfect, they come with A LOT of caveats, but they’re one small thing we can do in a time when we often feel helpless.
However, it’s important to remember, at the same time, that some of the best solutions we have for solving this crisis have been in front of us this whole time — from physical distancing to handwashing, which are both extremely effective at protecting you; to electing (and holding accountable) leaders who can prepare and guide us through crises like these. But that’s an update for another day.
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Glossary
preprint study or analysis: research that hasn’t been peer-reviewed or accepted for publication yet, so all results and conclusions need to be taken with a hefty grain of salt
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Tomorrow I’ll be tackling some of the more practical aspects of protecting yourself — including the best designs and practices around masks, as well as other protective gear and actions you might consider.
If you’ve seen a particularly good tutorial on mask-making, or if a favorite Etsy shop has switched to sewing masks, please comment below or email me at melodyaschreiber@gmail.com. I’m hoping to include some good suggestions in tomorrow’s update.
In the meantime, if you know someone who might appreciate this newsletter, please feel free to forward it to them.
Thank you! Super helpful