The mask itselfвЂ“ the fabric or the polypropylene fibers that are used to make an N95 or surgical maskвЂ“ are essentially this obstacle course that the air has to flow around. But, in reality, filtration works in a very different way. 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.

What are the basics of how a mask traps a virus?ДR. IRA FLATOW: Now, masks can be made of all different types of materials. 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. It doesnвЂ™t really fit well to our faces and inevitably has gaps around the cheeks. So you put that on the bottom, but itвЂ™s a rectangle. The type of double masking that is recommended by CDC now that they found works really well is to use a surgical type of maskвЂ“ also called a procedure mask, preferably one thatвЂ™s ASTM certified because those are shownвЂ“ those have been proven to filter out particles well.

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?ДR. And the second part is the fit of the mask, because you can have the best filtration in the worldвЂ“ like an N95вЂ“ but if it doesnвЂ™t fit, if it leaks around the edges, itвЂ™s like you have big holes in your mask. The first is filtration, the ability of a mask to filter out small particles or we sometimes call them aerosols. 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 maskвЂ™s performance. Does wearing two masks double the filtration? Or how does that work?ДR. OK, so letвЂ™s talk about the new CDC recommendations for double masking. So youвЂ™re well versed to answer ours when we ask them. LINSEY MARR: I have been asked these questions hundreds if not thousands of times in the past year. IRA FLATOW: You must get asked these questions all the time.ДR. LINSEY MARR: Thank you so much for having me. Linsey Marr, Professor of Civil and Environmental Engineering at Virginia Tech in Blacksburg, Virginia. 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 weвЂ™ve all wondered these. Well, this week, the Centers for Disease Control and Prevention put out another wrinkleвЂ“ guidance on double masking, wearing one mask over another. 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.

Learn more about the science of double-masking, via NPR.

Read the new CDC guidance about double-masking.

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. We know that wearing masks is an important way we can curb the transmission of COVID-19. Credit: Center of Disease Control and PreventionД®ngineer 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. 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. Masks have been a big issue throughout the pandemic, from supply shortages to debates about when they should be required to be used. Listen to experts discuss the spread, outbreak response, and treatment. This story is part of Science FridayвЂ™s coverage on the novel coronavirus, the agent of the disease COVID-19.

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