How Well Do Masks Work? (Schlieren Imaging In Slow Motion!)


[coughing sounds] This is how COVID-19 is spread.

Through air currents, potentially carrying microscopic droplets full of coronavirus.

When we talk.


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


Hey smart people, Joe here.

If youve been feeling confused about masks, what they do, what they don’t do, and whether

or not you should wear oneI hope this video will help.

And since I’m all by myself at the moment, I don’t need to wear this.

Or this.

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

invisible to our eyes.

If youve 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


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 in the space between, it’s bent in ever-so-slightly different

ways, creating shadows and bright spots in the final image.

As you can see in this Veritasium video from my friend Derek, it lets us see everything

from shockwaves to heat rising off a candle.

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 are 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?

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 2 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 microdroplets

are extremely hard for any mask to filter out.

But with a mask on, in the warm, humid 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 masks work, is that any droplets that do get through now have less


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 will know more months from now, than they do today.

Which is exactly how science is supposed to work.

What we do know is whether youre 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.

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 a candle out through your mask from about 1 foot away.


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 a person talking through a sheet of laser light, illuminating

all of the microdroplets released as we speak.

With a mask, those droplets are all caught.

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.

Links down in the description!

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 thinkWell, I’m not

sick, I’ve got a strong immune systemso 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 healthcare workers

who work all day in masks.

And masks won't stop the pandemic on their own.

Theyre best used in combination with other safety measures, like handwashing and physical


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.

Or maybe your nose itches, and your glasses fog up, your face is sweaty.

No fun.

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 to shade the sun.

Until one day some masculine men decided they were tired of getting rained on and suddenly

enough people were using umbrellas that they became like the most boring, socially acceptable

thing ever.

It's true, look it up.

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 you want to protect others and have them protect you.

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: Were all in this together.

And that’s something we all need to hear right now and every day.

Stay curious.

Thanks for watching this video and I hope that you'll send it to somebody that you know

that needs to hear this message.

But remember, what we know from social psychology is that shaming people into wearing masks,

that's not going to work.

SO come at it with an open mind and an open heart, and we can all do this together.

And like always, I'd like to thank everyone who supports the show on Patreon.

We couldn't do it without you, this video or any other video.

And we're so glad especially now to have your support.

If you'd like to join our community, there's a link down in the description you can learn

more about that, and if not, we're just glad you're here this week.

See you next time.