“Masks are absolutely essential because you can’t always control when someone gets too close to you. You can’t control the ventilation in every room that you go into. The thing you control the most, that you have one hundred percent control over, is that mask.” – Kimberly Prather
As the death toll from COVID-19 surpassed 300,000 in the United States this week, wearing a mask has never been more important. In this episode, Kimberly Prather and Linsey Marr explain the latest science on how the coronavirus spreads, and what it means for the kind of mask people should wear, and other public health measures.
This podcast was created by Just Human Productions. We’re powered and distributed by Simplecast. We’re supported, in part, by listeners like you.
Kim Prather: You know, you look at the countries that have been successful in this. You know, they’ve done a lot of things, but they’re also the countries that wear masks the most.
Linsey Marr: And if you do choose to wear a mask, um, you know, a good high-performing mask that fits well all the time, you avoid restaurants, bars, masks do work.
Celine Gounder: You’re listening to EPIDEMIC, the podcast about the science, public health, and social impacts of the coronavirus pandemic. I’m your host, Dr. Celine Gounder.
Masks are one of the most important tools we have to fight the pandemic. But a lot of people find them uncomfortable. And I get it. When I’m working at the hospital, I wear one all day. It’s a pain but it keeps me and everyone else safe. But not everyone thinks masks are so bad.
Delphine Wallis: I just need to put out all my masks. I have plenty of good ones.
Celine Gounder: This is my niece, Delphine.
Delphine Wallis: I’ll show you the one that looks like a dog. I’ll show you it. It glows in the dark blue.
Celine Gounder: Delphine loves face masks.
What do you like about wearing masks?
Delphine Wallis: Um, they’re pretty and they have different colors. Let me count how many I have right now.
Celine Gounder: So you’re counting your favorites now.
Delphine Wallis: One, two, three, four, five, six….
Celline Gounder: She has a lot of favorite masks…
Delphine Wallis: This is my favorite because it has flamingoes. And my second favorite is this one…and they’re pink and that’s my favorite color, just like the other ones…and this is my favorite because it’s glittery and I really like glitter. This is my favorite one. Is it like a really cute flower. It’s hidden at the top.
Celine Gounder: My first question for you Delphine, is do you know why you have to wear a mask?
Delphine Wallis: Because of the disease.
Celine Gounder: And then what are you covering on your face with the mask?
Delphine Wallis: My mouth and my nose.
Celine Gounder: So why is it important to cover your nose and mouth?
Delphine Wallis: Because you might breath out and you might have a disease.
Celine Gounder: So is the mask to protect you or is it to protect other people or both?
Delphine Wallis: Everyone.
Celine Gounder: So, where do you wear your masks?
Delphine Wallis: I wear my mask at the store, going biking with my dad. Maybe I should do that after, actually.
Celine Gounder: Go biking?
Delphine Wallis: Yeah, my mom won’t actually let me, because I’m having this conference today, so.
Celine Gounder: [laughs] Because you have this conference call with me?
Delphine Wallis: Yeah.
Celine Gounder: [laughs]…And how does it make you sick?
Delphine Wallis: Um, I don’t know that part.
Celine Gounder: You don’t know that part? Okay.
My niece might be the authority on fun masks, but she’s a little too young to dive into the science of how masks protect us from the coronavirus. Don’t worry, we have some other experts who can fill us in on the rest.
Kim Prather: I’m Kimberly Prather. I’m a professor of atmospheric chemistry at University of California, San Diego
Linsey Marr: My name is Dr. Lindsay Marr. I’m a professor of civil and environmental engineering at Virginia Tech.
Celine Gounder: Linsey Marr and Kim Prather were two of the authors of a recent article in Science about how SARS CoV-2 moves through the air. They’re going to be our guides in this episode on transmission and masks. They’ll help us understand the latest science behind coronavirus transmission…
Kim Prather: Different expertise has come in, uh, and. You know, clarified that it’s in the aerosols, the smaller stuff that flows that goes much further than six feet.
Celine Gounder: what this means for public health recommendations…
Linsey Marr: We need masks. We need good ventilation. We need hygiene–multiple interventions.
Celine Gounder…and how you can protect yourself.
Linsey Marr: You can easily get over a 90% reduction in risk.
Celine Gounder: Today on EPIDEMIC, what airborne transmission means for masks.
An on-going debate since the beginning of the pandemic has been how SARS CoV-2 spreads. Two words you’ve probably heard a lot are droplets and aerosols. These are tiny bits of solids and liquids that get released into the air when you sneeze, sing, or even just talk normally.
Kim Prather: The droplets go like little mini cannonballs is how we’d describe them. And they fall, they’re just falling. Um, whereas aerosols float.
Celine Gounder: Droplets and aerosols can be smaller than the width of spider’s silk, but they’re still bigger than a virus. So when someone infected with SARS CoV-2 sneezes, for example, the virus can catch a ride on these particles. Kind of like a hitchhiker. If someone else breathes in those particles with the virus onboard, they can get sick.
A lot of public health recommendations so far have assumed that the bigger, heavier droplets account for a lot of the spread.
Kim Prather: The traditional medical view is that it’s in these big droplets. And so with big droplets you’re safe as long as you keep this magical six feet apart.
Celine Gounder: But research has started to point to aerosols… not droplets… as the more important mode of transmission.
Kim Prather: All of the data we have when virus has been detected, SARS CoV-2, this virus has been detected it has been detected in the micron and sub-micron sizes and the very smallest sizes. We don’t have yet any evidence that it’s in the sort of more traditional droplets that you hear about.
Celine Gounder: The growing evidence that aerosols are major carriers of the coronavirus has big implications for public health. Think about social distancing. Recommendations, like the six-feet rule, are based on the assumption that droplets are responsible for the spread.
Kim Prather: Those aerosols can move across the room. Six feet is nothing to an aerosol. And as you go further away, that ratio of aerosol particles becomes even higher. So just based on what’s the chance of you inhaling something versus having it hit you in the eye, nose or mouth. It’s just logical that it’s much more likely to inhale aerosols than it is to get splattered by the random ballistic missile that’s coming at you.
Celine Gounder: And someone can produce a lot more aerosols than droplets.
Kim Prather: There’s about a thousand more aerosols produced while you’re speaking sometimes more if you’re yelling or singing.
Celine Gounder: This changes how people should assess their risk for exposure, especially indoors.
Kim Prather: You know, think of cigarette smoke. You say that to a little kid. You said that to anyone. And they’re like, Oh yeah, everybody knows how smoke builds up in a room. Same thing with aerosols.
Celine Gounder: That ability to build up in a room is why Kim says aerosols — and not droplets — are the likely culprit behind many superspreader events.
Kim Prather: And so in that case, just staying 6 feet apart is definitely not enough. Um, this is where masks become hugely important.
Celine Gounder: This shift in focus from droplets to aerosols could impact everything from the kind of mask you should wear to your day-to-day behavior. But it also means we need to rethink how we design our buildings, especially when it comes to ventilation. Many existing buildings re-circulate the same air over and over again because it saves on energy costs.
Kim Prather: You know, we went into climate change, we develop buildings that are certified as low energy, and by definition, we sealed them up. We got rid of ventilation.By going towards, you know, saving energy, we actually made buildings that have much less healthy indoor air.
Celine Gounder: Improving the ventilation in a building could be as simple as opening windows. But there are other options too. Improved filters in HVAC system, even using UV irradiation to kill viruses inside the ductwork. Hospitals generally have pretty good ventilation. But many of the places we spend the most time don’t.
Kim Prather: The big one: homes. Homes by definition are poorly ventilated. They’re sealed up.
Celine Gounder: Another big one is schools. Kim says investments in better ventilation in schools should be a top priority. It would help keep schools running safely during the pandemic. But it would have other benefits, too. It would make schools more resilient if there were another epidemic. It would also improve air quality for people with asthma or other respiratory issues.
Kim Prather: I’s a no brainer to pick schools as being an investment that will save, um, in the medical community for, for years to come.
Celine Gounder: But in the meantime, one of the best options is simple and inexpensive. Wearing a mask. Next, we’ll hear about an experiment Linsey ran at her lab to figure out the safest mask to wear. That’s after the break.
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To study how masks can limit the spread of coronavirus, Linsey built a pretty interesting experiment in her lab.
Linsey Marr: We set up a special chamber, which is really just a large plastic bin. And we put two mannequin heads in it. And those mannequin heads are on opposite sides of the chamber. So like they’re talking to each other at close distance.
Celine Gounder: Both mannequins have holes where their mouths would be. The mannequin standing in for a sick person has a nebulizer attached to its mouth.
Linsey Marr: If we want to test how well the mask protects others, we put a mask on the mannequin that’s attached to the medical nebulizer. So that’s our kind of sick person and we would run the nebulizer and generate this spray of droplets into the chamber. And we would see how many end up in the other opposite mannequin’s mouth.
Celine Gounder: The more droplets and aerosols they find in the other mannequin’s mouth, the more likely the virus would be to infect someone. Linsey’s experiment showed that wearing a good cloth mask can block a lot of the droplets and aerosols that would carry the virus.
Linsey Marr: For the really small ones, the cloth masks don’t do nearly as well as an N 95. But we think those are less important for transmission. So for the larger aerosols, one micron and larger, the cloth masks block even up to 80% of those. And once you get up to two microns, just a little bit bigger, now we’re looking at kind of 80% or better for a good cloth mask. And so that reduction is going to make a big difference in how many viruses are out there in the air that other people could be exposed to.
Celine Gounder: How well do masks protect the wearer?
Linsey Marr: Masks also protect the wearer. The same mechanism or the same way that they filter out particles and aerosols on the way out. They also work for air flowing into the mask that you might be breathing in.
Celine Gounder: Linsey says they’re not quite as effective at protecting the wearer of the mask from someone else who’s sick. But they still provide a lot of protection.
Linsey Marr: Even a thin single layer of tightly woven cotton can block out half of those aerosols or one to two microns. And once you get up to slightly larger sizes, that can filter out 80% of what’s coming in.
Celine Gounder: And there’s a multiplier effect if everyone is wearing a mask. Imagine two people talking to each other. Let’s assume they’re both wearing masks that are 50% effective at blocking aerosols and droplets.
Linsey Marr: So the sick person releases a hundred viruses into the air. The mask blocks 50 of those. So 50 get out into the air. The other person who’s exposed, um, you know, is going to, let’s say they breathe in all 50 or pull in all 50 of those towards their mouth. But if their mask is 50% effective, then it only 25 are going to get through. So you’ve taken a mask that’s only 50% effective. And now both people are wearing them. All of a sudden you’ve gotten 75% effectiveness. You can achieve really high levels of reduction and exposure if everyone’s wearing a mask.
Celine Gounder: But not everyone is on the same page when it comes to what makes for a good mask.
Linsey Marr: There are some masks out there that are really good. There are also masks out there that are barely better than anything. So there’s huge variability in cloth masks. People don’t wear them perfectly all the time. We see a lot of people with large gaps or the mask isn’t covering their noses.
Celine Gounder: Fit is another issue Kim says a lot of people don’t think about.
Kim Prather: That’s why an N 95 is actually not as perfect as you might think. Right. Everything’s in 95 is the gold standard. But if those aren’t fitting your face right, they can drop from being 95% — which is what the 95 stands for — they can drop to 30%, where they don’t form fit to your face properly, which is probably pretty common for people in the public. So personally, I’d rather give masks out to the public, and this is what they’ve done in other countries. They’ve given masks out to the public that are easier to wear correctly.
Celine Gounder: So what are you supposed to do if the materials and fit aren’t always as good as they should be? Linsey says the solution is to mask with several layers at once.
Linsey Marr: Yeah, based on our study and other studies, we’re now recommending a three layer mask where it’s a sandwich and the middle layer is some kind of filter material.
Celine Gounder: Something like a HEPA filter or a surgical mask. Linsey says you could even cut up a vacuum bag or an HVAC filter, as long as it’s rated MERV 14 or higher.
Linsey Marr: The bread of the sandwich, the outer layers should be a tightly woven material that is flexible so that it can help the filter, help the whole mask fit well to your face and be comfortable all day.
Celine Gounder: So maybe layering a cloth mask over a surgical mask, say.
Linsey Marr: Yes, absolutely. If you have a surgical mask underneath and wear a cloth mask on top to help hold the surgical mask closer to your face and also that cloth mask will provide an extra layer of filtration. You’ll get very good performance that way.
Celine Gounder: Being vigilant about wearing a mask is even more important now as some people are getting together for Christmas and other holidays in the coming weeks.
Kim Prather: I say, uh, you can eat outside. You should be at least six feet apart. When you’re eating, when you’re physically shoving food in your mouth or drinking, you can pull the mask away, but as much as you can, I know this is terrible, but as much as you can eat it, this is ter… I’m not very fun at events right now. Um, you should have that mask on. Even outside just to be safe because think about it this way. I’m an atmospheric chemist. I I, know how wind patterns go. And then, so like, let’s say you told someone that, and they were downwind. If someone’s down wind of someone that’s sick, even if they’re six feet away, that air is going to be blowing from that person straight to them for what, an hour or two hours of sitting outside. That will build up over time. It’s almost like people are looking for permission to drop the mask. Having that mask is just so important. I, especially right now is we are just in a terrible, terrible state. I think we have to be over overcautious right now.
Celine Gounder: Kim says she usually wears a blue surgical mask. But that isn’t enough anymore.
Kim Prather: The numbers are going up where I live. And so I am like, I now am wearing a medical mask or a surgical mask and an N95. I’m wearing both. [laughs] So I’m being extra, extra cautious at this point just because the spread in our community is skyrocketing.
Celine GOunder: The week we were working on this episode, the number of Americans killed by COVID surpassed 300,000. This week also saw a nearly 30% increase in cases nationally over the previous 14 days. And with Christmas and New Years around the corner, there’s never been a more important time to wear a mask and avoid large gatherings.
Kim Prather: Masks are absolutely essential because you can’t always control when someone gets too close to you. Right. You can’t control the ventilation in every room that you go into. And I always just say the thing you control the most, you have a hundred percent control over is that mask.
Celine Gounder: Hey, everyone, quick programming note. This is the last episode of EPIDEMIC for the year. We’ll be back with new episodes every Thursday starting January 7th. See you in 2021!
OK, the credits.
EPIDEMIC is brought to you by Just Human Productions. We’re funded in part by listeners like you. We’re powered and distributed by Simplecast.
Today’s episode was produced by Zach Dyer and me. Our music is by the Blue Dot Sessions. Our interns are Tabata Gordillo, Annabel Chen, and Bryan Chen.
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And check out our sister podcast AMERICAN DIAGNOSIS. You can find it wherever you listen to podcasts or at AMERICANDIAGNOSIS.fm. On AMERICAN DIAGNOSIS, we cover some of the biggest public health challenges affecting the nation today. In Season 1, we covered youth and mental health; in season 2, the opioid overdose crisis; and in season 3, gun violence in America.
I’m Dr. Celine Gounder. Thanks for listening to EPIDEMIC.