Two Masks Are Better Than One
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Masks have been a big issue throughout the pandemic, from supply shortages to debates about when they should be required to be used.
This week, the Centers for Disease Control and Prevention put out research and guidance on the effectiveness of double masking—wearing one mask over another.
Engineer and aerosol scientist Linsey Marr talks about how a face mask traps a virus, the effectiveness of double masking, and other other questions about face masks.
We know that wearing masks is an important way we can curb the transmission of COVID-19. Do your own experiment to see how droplets small and large travel by making Snotty Plots! Use pretend snot to imitate a sneeze, measure its spread, and then experiment with simulated mask-wearing.
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Linsey Marr is a professor of Civil and Environmental Engineering at Virginia Tech in Blacksburg, Virginia.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Throughout the pandemic, masks have been a big issue, right? First came a mask shortage, then debates about statewide mask mandates, and then, finally, refusal by some people to wear them. Well, this week, the Centers for Disease Control and Prevention put out another wrinkle– guidance on double masking, wearing one mask over another.
So what is the science behind masks and how they work? Like, what does a double mask really do? How does a mask trap a virus? Is there an expiration date to a mask? I think we’ve all wondered these. So here with the answers is Dr. Linsey Marr, Professor of Civil and Environmental Engineering at Virginia Tech in Blacksburg, Virginia. Welcome to Science Friday.
DR. LINSEY MARR: Thank you so much for having me. It’s a pleasure to be here.
IRA FLATOW: You must get asked these questions all the time.
DR. LINSEY MARR: I have been asked these questions hundreds if not thousands of times in the past year.
IRA FLATOW: Good. So you’re well versed to answer ours when we ask them. OK, so let’s talk about the new CDC recommendations for double masking. Does wearing two masks double the filtration? Or how does that work?
DR. LINSEY MARR: The way it works is that the CDC recommendation for double masking is actually getting at two different properties that are really important for a mask’s performance. The first is filtration, the ability of a mask to filter out small particles or we sometimes call them aerosols. And the second part is the fit of the mask, because you can have the best filtration in the world– like an N95– but if it doesn’t fit, if it leaks around the edges, it’s like you have big holes in your mask.
IRA FLATOW: And so the second mask is then sort of helping to push the first one against your face better, making a better seal?
DR. LINSEY MARR: Exactly. The type of double masking that is recommended by CDC now that they found works really well is to use a surgical type of mask– also called a procedure mask, preferably one that’s ASTM certified because those are shown– those have been proven to filter out particles well. So you put that on the bottom, but it’s a rectangle. It doesn’t really fit well to our faces and inevitably has gaps around the cheeks. So if you put a second mask, a tight-fitting cloth mask on top of that, you can greatly improve the fit, reduce leaks, and you add another layer of filtration.
IRA FLATOW: Now, masks can be made of all different types of materials. What are the basics of how a mask traps a virus?
DR. LINSEY MARR: One common misconception is that masks work by sieving out particles that are larger than the holes in the mask, like straining pasta, for example. But, in reality, filtration works in a very different way. The mask itself– the fabric or the polypropylene fibers that are used to make an N95 or surgical mask– are essentially this obstacle course that the air has to flow around. So if you shrink yourself down to the size of one of those individual fibers, you are twisting and turning in the air as you’re trying to navigate through this maze of fibers. As you do that, any particles or aerosols that are in the air may not be able to make all those twists and turns and may end up crashing into one of the fibers and being filtered out.
IRA FLATOW: What makes an N95 mask so much better than another kind of mask?
DR. LINSEY MARR: N95s are really effective for a couple of reasons. First, they’re made out of a special type of material called melt-blown, non-woven polypropylene. And these are very, very small fibers that are then pressed together, and they have very small spaces in between them.
The second thing that an N95 has is an electrostatic charge, kind of like rubbing a balloon against your hair and then your hair sticks to it. They apply a charge to the N95, and then it is better able to capture particles that are naturally charged that are kind of coming towards it and trying to get through there. So this electrostatic charge attracts the particles to the N95 and acts as an additional way to trap the particles in there.
IRA FLATOW: Is it harder, with this double-masking idea, to actually breathe through the mask? And is that a possible problem for people who say, you know, I’m just not going to wear these double masks because I can’t breathe through them?
DR. LINSEY MARR: Absolutely. This is a concern. One of the most important things with your mask is that you can easily breathe through it, because if you can’t, it’s going to be uncomfortable and you won’t want to wear it. And secondly, if it’s too thick or too hard to breathe through, then it’s more likely that air is going to leak in through the sides. It wants to find the easiest way in. And so, certainly, if double masking makes it too hard to breathe, then you need to look for another solution.
IRA FLATOW: And where would you look for that? How do you shop for a mask?
DR. LINSEY MARR: There’s a couple of things to look for in a mask. There’s the basic cloth masks, but– and those work OK. They might filter out 50% of particles on average. But you should actually look for a mask that has a dedicated layer of filtration material. And that could be a surgical-type mask or, say, a polypropylene filter. I’ve seen things that have HEPA filters, which stands for High Efficiency Particulate Air filtration. Those are very effective, too.
We have found that vacuum cleaner bags work well and certain types of HVAC filters. So you definitely should look for some kind of filtering layer in your mask. You also want something that fits well, and people’s faces are such different shapes that this is somewhat of a personal preference thing.
But, also, it’s really important to have one of those metal nose bridges that you can bend to fit closely around your nose because that’s a place where there’s a lot of– it’s easy to have a lot of leakage. And also, the ear loops are OK. But if you want an even tighter fit, if you get something that secures, fastens around your head, that’s even better.
IRA FLATOW: Let me get back to the vacuum cleaner bag for a second because I don’t think people are going to put that over their head or their mouth, are they?
DR. LINSEY MARR: Absolutely not. We don’t recommend that you put a vacuum cleaner bag or these other filter materials directly against your face. Rather, you would want to sandwich them in between two layers of some other fabric that’s more comfortable. And this is actually how an N95 is made. The filtration layer is kind of sandwiched in between some other layers.
IRA FLATOW: I’ve had– yeah. I’ve had masks that are double layers, and then they also have a pocket in them that you can slip a coffee filter or, now, I guess, I could cut up a vacuum cleaner filter and put it in there. Is that a good technique?
DR. LINSEY MARR: Yes, absolutely. If you have a cloth mask that has a pocket in it and you insert some kind of filter material– I think, months ago, a coffee filter was certainly OK, better than nothing. But now that we have these new, more easily transmissible variants, we need to focus on improving the quality and performance of that filter that we’re putting in there, so something like a HEPA filter. I, for example, went to my local big box store and bought a HEPA filter that’s made for air purifiers and I kind of cut it out of the frame and pulled it apart and have cut pieces to put into my mask. I think you can buy these already pre-cut in some places online, I’ve seen. So certainly this is the time to be thinking about upgrading our filters and our masks.
IRA FLATOW: Now, a couple of things about the viruses and the interaction with the masks. A mask, as you say, traps all of these virus particles but doesn’t kill the viruses. So are we wearing, basically, a biohazard on our face?
DR. LINSEY MARR: That’s one extreme way of putting it. Yes, viruses can, of course, be trapped on– they will be trapped on your mask. They can survive for some time, although they tend not to survive very long on porous, fabricky types of materials. On the more plasticky type of materials– like surgical masks and N95s– they might survive longer. Sunlight kills them off fairly quickly, within an hour. But that said, you should always handle your mask carefully as if it is contaminated because it might be. And so you can put it on and take it off just using the straps and avoid touching the front of it.
IRA FLATOW: If you just let it hang out for a while, I mean, would a day or two kill the viruses on the mask?
DR. LINSEY MARR: Yes, absolutely. Letting it just sit aside for a day undisturbed will– those viruses will decay away or die off.
IRA FLATOW: When you’re wearing a mask for a while, there is this moisture that can build up in the mask from your breath. Does a wet mask affect the efficacy, the efficiency of the mask, how well it filters?
DR. LINSEY MARR: That’s a great question. You know, there haven’t been a lot of studies on that. I think it will still work. Whether it works as well or perhaps better is going to depend on the type of material it’s made out of, if it absorbs water and swells up, and how much water there is. That said, generally, we recommend that, if your mask does get wet, it’s time to switch to a new one.
IRA FLATOW: So you should not be throwing your mask into the washing machine.
DR. LINSEY MARR: Cloth masks can go into the washing machine, and you probably want to do that, just like your dirty socks. But the filtering layers are sometimes– because some of them have this electrostatic charge, you don’t want to get those wet. And so that’s why the cloth masks sometimes have those pockets, so you can take that filtering layer out, wash the mask, and then put the filtering layer back in.
IRA FLATOW: Does a mask have an expiration to it? I mean, how many times can you wear it before you need to throw it out?
DR. LINSEY MARR: That’s a good question. Again, with N95s and surgical masks that are made out of this melt-blown, non-woven polypropylene, that material is really tough, and it’s going to outlast any other part of a mask, like the straps. The straps will go first. That said, if you do see dirt or soiling or any kind of wear and tear on those types of masks, it’s time to dispose of them and get a new one. We’ve tested masks– N95s– for reuse in health care settings for really up to 10 sterilization cycles, meaning that the health care workers are wearing them for 10 days over, they assume, eight hours per day.
IRA FLATOW: So you can overload a mask after a number of uses.
DR. LINSEY MARR: It’s possible, certainly, that the mask could get– if you’re in a really dusty environment that the mask could get kind of soiled and clogged and won’t work as well. So then, certainly, it’s time to switch to a new mask.
IRA FLATOW: I’ve seen some masks that come with a carbon filter inside of them. Is this legitimate?
DR. LINSEY MARR: Those filters– you know, I’ve been meaning to test those in the laboratory because I haven’t seen any results on those yet. So, in theory, if they are designed to filter out PM2.5, which is 2.5 microns and smaller, they should work well for the virus. I don’t know how well. My concern about those is that the ones I’ve seen are small– so they’re not fitting, entirely filling up the entire area of your mask, so air is going to leak in around the sides– and that they’re a little stiff, which is going to degrade the fit of your mask,
IRA FLATOW: If you want to start over and start making the best mask that you can now, how should you do that?
DR. LINSEY MARR: There’s a couple of different options. One is to look for a good cloth mask that fits your face well and has a pocket where you can put a filter into. That could be just the surgical type of material. The double-masking recommendation from the CDC will work well. The kind of a single layer cloth mask, if that’s what you’re still using, then it’s time to think about upgrading that.
IRA FLATOW: Now, I know that you study ventilation, which is one of my favorite topics, believe it or not. I mean, is it possible to really make a space equivalent to outdoors in terms of ventilation and movement of air? And would that solve a lot of problems we’re having with mask wearing, also?
DR. LINSEY MARR: Good ventilation is essential for reducing the risk of transmission of COVID-19 because the virus is in the air and it builds up just like– it behaves and can build up just like cigarette smoke. And it’s surprisingly easy to get good ventilation in a room if you have windows and doors, really just to open those even just partway. And it’s surprising, actually, how much of a difference that can make.
IRA FLATOW: This is Science Friday from WNYC Studios. What are– I’m trying to figure out what a mask laboratory looks like. Give me an idea of how you go about testing masks. I’m thinking of, like, Consumer Reports with all kinds of different materials in them.
DR. LINSEY MARR: There are a couple of different ways we test masks. One is to look at just the material itself. And so we cut out little circles of the material a few inches in diameter, and we put that into a holder, kind of a cylindrical holder. And then we put that into a chamber, which is really just like a big bag that’s sitting on top of our counter. And we fill that bag with sodium chloride particles. That’s salt solution particles from a medical nebulizer, the type that people might use at home to administer medication.
And then what we do is we set up our equipment to measure kind of what will pull air through that material, through the filter or through the mask material, and measure how many particles come through compared to how many are actually inside that chamber. That’s one way we can measure the efficacy of a mask.
The other way is to do it in a more realistic way where we put the mask on to some mannequins. So we have these foam heads in our lab. And we cored out the mouth of these heads, and we put either a nebulizer into one of them to generate these particles coming out of one. And the other one, we run a tube through, and we connect that to our instruments to measure how many particles get in.
IRA FLATOW: Is there such a thing as an ideal mask? When you study things in science or engineering, you have the ideal fluid, the ideal whatever. Is there an ideal mask that you’re shooting for? And is it possible to make that?
DR. LINSEY MARR: I think the ideal mask would be 100% effective at filtering out particles. N95s are 95% effective. It would be easy to breathe through, and it would be very comfortable to wear for long periods of time. That’s a challenge, because usually if it’s better at filtering out particles, it becomes harder to breathe through. So we have to strike a balance.
IRA FLATOW: So the bottom line, again, on this is, if you’re going to make your own mask, buy your own mask, or shape your own mask, you start out with what and end up with what?
DR. LINSEY MARR: Start out with a tightly woven cotton material for the outer layers of your mask. Have a pocket in the mask into which you can insert a good filter material. The mask should have a metal nose bridge that you bend carefully to fit around your nose and eliminate leaks. If you have straps that can tighten around your head, you’ll get a tighter fit compared to ones that go around your ears. And it should be fashionable.
IRA FLATOW: Fashionable. You know, that really makes sense, because then if we make fashions out of the mask, more people will wear them than won’t wear them.
DR. LINSEY MARR: Absolutely. We should make it cool to wear masks and get creative with them.
IRA FLATOW: And how young a kid should start wearing a mask?
DR. LINSEY MARR: CDC recommends kids as young as two, but not younger than that because it can be a choking hazard.
IRA FLATOW: All right, I think you have done it. You have answered everything there is to know about masks for us.
DR. LINSEY MARR: Well, I’ve certainly tried. Masks, though, are not 100% effective, so it’s important to also practice distancing, ensuring good ventilation, avoiding crowds, and washing our hands.
IRA FLATOW: There you go. Thank you very much, Dr. Marr.
DR. LINSEY MARR: Thank you. It’s been a pleasure.
IRA FLATOW: Linsey Marr, Professor of Civil and Environmental Engineering at Virginia Tech in Blacksburg, Virginia.