The circle in the background is a kind of mirror,

that, combined with a special photography technique,

allows you to see the flow of air.

In this case, it is showing how far air particles travel when someone breathes.

And when they cough — which makes the respiratory particles travel further, and faster.

This is what it looks like when someone coughs into their hand.

And into their elbow.

And this shows the travel of air particles when coughing while wearing two types of masks:

a dust mask, and a surgical mask.

This isn't a scientific study. It simply shows something face masks do very well:

They limit how far away from you respiratory particles can travel.

But whether you've been told to wear a face mask to prevent the spread of Covid-19,

probably depends on where you live.

In some East Asian countries, it's already common practice.

Some European countries have also started mandating face masks in public spaces.

But in the US, the message from the federal government has been inconsistent.

“There's no reason to be walking around with a mask...

it's not providing the perfect protection that people think that it is.”

“The CDC recommends wearing cloth face coverings in public settings

where other social distancing measures are difficult to maintain."

“This is voluntary. I don't think I'm going to be doing it.”

Unless you live somewhere that mandates it, whether you wear a face mask is probably up to you.

But making that decision for yourself is a lot easier

if you understand what face masks can actually do.

And what they can't.

Here's where the experts agree: If you are sick, and you leave the house,

you should wear a face mask, because it keeps your germs in.

Most of the uncertainty around masks is related to a separate question:

whether healthy people should wear them too.

Right away, though, that distinction, between sick with covid-19, and healthy,

is more complicated than it sounds.

People who catch covid-19 don't feel symptoms for 5-6 days on average,

and even up to 14 days,

but are contagious before that.

And based on what we know right now, about 25% of people who catch it never show symptoms at all,

but are also contagious.

In other words, especially with covid-19,

feeling healthy doesn't necessarily mean you don't have it.

"I think we should all be acting, and also all assume, that we're infected."

Shan Soe-Lin is an expert on global public health,

and Robert Hecht is a professor of clinical epidemiology.

And they say the possibility of being asymptomatic is one of the best cases for universal mask wearing.

"Those who are actually infected and don't know it, or showing almost no symptoms,

it's not even primarily for their own health.

It's for the health of their family and neighbors."

But let's say you definitely, 100%, do not have the virus.

The first thing you need to know is that no mask can actually guarantee that you won't get it.

Instead, the best way to think about face masks is as part of a larger set of protective measures,

along with social distancing and hand-washing.

Used together, they don't protect you completely,

but they dramatically lower your odds of infection.

"I think of it as a protective triad, with the virus trying to get in.

And if you don't have one of the branches in it,

then you're not protecting the whole thing.

And if you didn't have a mask, could you do better with social distancing and hand-washing? Sure.

But having all three is really, really important."

To understand how that works, let's look at the two ways covid-19 is transmitted.

One way is by touching an infected surface and then touching your face.

This is called fomite transmission.

And frequent hand-washing is the best defense against this.

But wearing face masks correctly can play a role too:

"Masks do a really good job of keeping you from touching your face."

The second way someone's respiratory particles can reach you is directly. Like this:

This is a slowed-down recording of a sneeze.

It shows the kinds of respiratory particles a sneeze emits,

and how far they travel.

From a sick person, these droplets are packed with millions of viral particles

that fall in close range, and infect whatever they land on.

Social distancing protects us from droplets, by keeping you away from the particles emitted

through sneezing, coughing, or even talking.

But face masks could help there, too:

"If someone sneezes a couple of feet away from you, it'll protect some of the bigger splashes."

That also means face masks are especially important

for people who don't have the privilege of being able to social distance:

"That would be cops. That would be grocery clerks."

"Taxis, Uber, FedEx, UPS..."

"Anyone who's coming into contact with a lot of people,

and maybe can't always maintain that six-foot distance."

Look at this video again, though,

and you'll see that we also produce these longer-range, smaller particles,

which evaporate, and can leave the virus hanging in the air.

These are called aerosols.

And we still don't really know how infectious they are for the average person.

But experts think they mostly pose a threat to medical workers,

who work up close to patients with Covid-19.

"If you're a foot away from the patient, who's coughing as you're trying to intubate them,

aerosols are going everywhere.

For you and me, just walking around, walking your dog, going to a grocery store,

your risk is way higher from fomites."

A common mask that protects against aerosols is called an N95 respirator.

Its name comes from its material's ability to filter out 95% of aerosol particles.

It's also fitted to form a seal around your mouth and nose, so there's no leakage.

In lots of places, N95s are in short supply, because of diminished government stockpiles

and disruptions in the supply chains.

And that's true for all disposable masks, not just N95s.

That's why the US Center for Disease Control says that unless you're a health care worker,

you shouldn't be using either N95s or regular surgical masks.

They're needed in hospitals, and the level of protection most people need can come from

a simple cloth mask.

The CDC recommends any kind of tightly woven cotton for the mask, or even a T-shirt.

You can make them by consulting the countless internet tutorials out there,

or buy them from the many vendors who manufacture them.

But they only work if you wear them correctly.

"So what kind of masks are you wearing? Do you have them handy? Could you show us?"

"We thought you might ask."

"This is all it takes to put it on:

Just grab it by the loops. Keep your hands off the central part.

Put it over your nose and mouth. Loop it around the ears.

Be careful not to touch it when you're outside. Keep your hands away from your face.

When you come back in, after walking the dog,

or doing the groceries, do the same thing in reverse.

Put it off to the side. Don't start playing with it.

And then at the end of the day, pop it in the washer."

"You wash it every day?"

"I wash them every night. And, you know, if you don't have a washer,

you can hand wash them every night. They're very small and they'll be dry by the next morning.

So if people can have a couple, it would be better.

But if you only have one, then just wash it at night."

Face masks highlight something important in the fight against covid-19:

That it isn't always about protecting yourself.

It's about protecting the people around you.

And if everyone is doing it, we all protect each other.

"There's a visual messaging that comes from wearing a mask:

A reminder to everybody, that we're not in normal times now.

Everyone needs to be doing everything they can to protect themselves, and other people."