(Describer) A man exhales deeply.

HOST: This is how COVID-19 is spread--

(Describer) Clouds stream out.

through air currents potentially carrying microscopic droplets full of coronavirus when we talk, breathe, or cough. We can see this, thanks to an imaging technique that lets us peer into the invisible world of airflow. And using this same technique, we can see if masks really do help to stop the spread of coronavirus.

[MUSIC PLAYING]

(Describer) Title: It's Okay to Be Smart. The host wears a cloth mask.

Hey, smart people. Joe here. If you've been feeling confused about masks-- what they do, what they don't do, whether or not you should wear one-- I hope this video will help. And since I'm all by myself at the moment,

(Describer) He takes it off.

I don't need to wear this, or this.

(Describer) He wears a fake mustache.

This is Matthew Staymates, a fluid dynamicist and mechanical engineer at the National Institute of Standards and Technology. And while stuck at home, he turned his garage into a lab. Using a technique called Schlieren imaging, he captured some incredible images of a world that's normally invisible to our eyes. If you've ever seen wavering distortions above a hot road, that fluttery mirage happens because different temperatures and densities of air bend light in different ways, like a fluid lens. And when we talk, warm air streams out of our mouths, meeting the sea of colder air around us. A Schlieren setup lets us see these tiny differences. A narrow source of light is bounced off a concave mirror towards a camera, with a thin metal blade blocking some of the focused light. As that light interacts with air and the space between, it's bent in ever-so- slightly different ways, creating shadows and bright spots in the final image. And as you can see in this video from my friend, Derek, at Veritasium, it lets us see everything from the helium coming out of a balloon to heat rising off of a candle.

(Describer) Staymates breathes out through a mask and air doesn't go far.

And at 250 frames per second, it shows us exactly why masks work to slow airborne infection. Now, it's important to remember that we can't see viral particles in these images. They're too small. But the viruses are carried on that breath just like a river carries fish. In fact, an individual viral particle is smaller than the weave of most fabrics. So why does this work?

(Describer) The mask.

Well, masks help in two interesting ways. First, it's important to understand that viruses don't float around alone. They fly out in droplets of moisture. Most droplets are heavy enough that they fall within two meters or so, but any person or object within that distance can get bathed in virus. And without a mask, some of those droplets can evaporate into super tiny particles of infection that can float on air currents far from your mouth or nose, and these micro droplets are extremely hard for any mask to filter out. But with a mask on, in the warm human space between your mouth and the mask, those larger droplets don't have time to evaporate, and they can be captured by common fabrics. If it feels hot and humid in here, that means it's working. And the second way that masks work is that any droplets that do get through now have less momentum. The air currents disperse in eddies and swirls instead of flowing away in streams, which means droplets won't travel as far. So which mask is the best mask? If it seems like recommendations change all the time, that's because doctors and scientists know more now than they did months ago. And they'll know more months from now than they do today, because that's exactly how science is supposed to work. What we do know is, whether you're using a bandana, an N95 mask, or a cloth mask you sewed at home, they all still have a lot of droplet-blocking power, but that mask has to fit. This is almost like not wearing a mask at all.

(Describer) The nose is uncovered.

A mask that's too tight or has too many thick layers can actually force more air out the sides. A good test is to see if you can blow out a candle through your mask from about one foot away.

(Describer) Joe does.

[BLOWING]

These Schlieren images are clear evidence that masks are effective, but the best science relies on multiple lines of evidence. Here's an experiment that we did with someone talking through a sheet of laser light, illuminating all of the micro droplets released as we speak.

(Describer) Blue dots fly out.

With a mask, those droplets are all caught.

(Describer) They glow blue behind it.

And this microbiologist sneezed, sang, talked, and coughed over Petri dishes. A simple mask blocked nearly all of the germs. And even though this experiment detected bacteria and not viruses, they both leave our airway in the same respiratory droplets. It should be pretty clear to you now why masks work, and the body of research that proves that is getting stronger all the time. But really, the most important question is why should you wear a mask? You know, it's often said that masks aren't to keep you from getting sick but to keep you from getting others sick. Wearing a mask can dramatically reduce the chance of spreading COVID, but that's a strange thing to think about because you might think, "Well, I'm not sick. I've got a strong immune system, so I don't need to wear this." But we know that COVID-19 can go undetected, and almost half of infections may come from people who don't show symptoms. That could be you, or it could be the person next to you. And if it is, wouldn't you want them wearing a mask? They probably feel the same way about you. Can masks harm your health, like lowering your body's oxygen levels? No, that's a myth, and it's been tested over and over again by health care workers who work all day in masks. And masks won't stop the pandemic on their own. They're best used in combination with other safety measures, like hand washing and physical distancing. Stereotypically macho people have actually been shown to resist masks more. I get it, dudes, you don't feel cool, even though lots of cool dudes wear masks.

(Describer) ...like Bane.

Or maybe your nose itches, or your glasses fog up, your face is sweaty-- that's no fun. I mean, I get it. For most people, wearing masks is new and weird and different, but there are lots of things that used to be weird and different until everyone just started doing them. I mean, umbrellas used to be viewed as completely feminine accessories for rich people and only used to shade the sun, until one day, some masculine men decided that they were tired of getting rained on. And suddenly, enough people were using umbrellas that they became the most boring socially acceptable thing ever. It's true. Look it up. And most people didn't use condoms in sexual relationships until the risks of HIV and other infections pushed people to change their behavior. And now it's not only normal, it's a way to say, "I respect you and I want to protect both of us." This is also a sign that you want to protect others and have them protect you.

(Describer) A mask.

And the sooner that this becomes part of everyone's normal everyday life, the sooner we can all get back to normal everyday life. In every pandemic in history, it's the actions and choices of individual people that have made the difference. A mask can stop a virus, but it also sends a message-- we're all in this together. And that's something we all need to hear right now and every day. Stay curious.

(Describer) He puts his mask back on. Title: PBS Accessibility provided by the US Department of Education.