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On April 3rd, the U.S. Centers for Disease Control and Prevention started recommending that people wear a mask over their face when they go out in public. Except, for many of us, this isn’t the message we’ve gotten for the last couple of months. So... what changed?

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This episode was filmed on April 7th, 2020. If we have a more recent episode about COVID-19, we will include it in the description.

On April 3rd, the U. S. Centers for Disease Control and Prevention started recommending that people wear a mask over their face when they go out in public.

These guidelines don’t replace social distancing — it is still really important to stay away from other people as much as possible right now. Which I am doing by filming this at my home. But masks are designed to keep the virus from spreading in more crowded places that we still need to go to, like grocery stores.

Except, for many of us, this isn’t the message we’ve gotten for the last couple of months. Before this, many health organizations recommended not wearing a mask unless you are sick or caring for someone who is. So... what changed?

Well, it’s not that scientists learned something new about masks. It’s that they uncovered more about how the virus seems to be spreading. In most cases, a mask won’t keep you from getting sick if there are viruses in the air around you.

We’ve talked about this before, but when you breathe while wearing a basic mask, a lot of the air you inhale comes in from around the mask’s edges. And since that air doesn’t pass through the mask, the virus gets carried in along with it. Respirators and more sophisticated devices can offer better protection if they’re fitted and used properly, but — and I’m sure you’ve heard this — those should be saved for healthcare workers, who are getting up close and personal with COVID-19 patients on a regular basis.

Still, while basic masks can’t really keep viruses from getting in, they are much better at keeping them from getting out. See, when you cough or sneeze, or even talk, it’s not just air that comes out, but little particles of moisture that scientists call respiratory droplets. And if you’re sick, those droplets can be teeming with viruses.

But a mask will trap many of those droplets inside. That’s why the CDC and World Health Organization have recommended for months that anyone who’s coughing, sneezing, or sick wear a mask. But now, things are changing — at least, for the CDC.

And that’s because new research suggests it’s not just people who know they are sick that can infect others. We now know that some cases of COVID-19 can be pretty mild. And while most people with a mild case develop a fever, it's possible they might not even know they’re sick.

But beyond that, there seem to be cases where people with the virus never show symptoms at all but can still transmit it. And for the rest, evidence is mounting that it’s possible to spread the virus when you’re presymptomatic — in other words, before you start showing symptoms. For instance, a study published on April 1st found that around 6% of the cases in Singapore could be linked to a person who was presymptomatic.

And in Chinese cases outside the epicenter of Hubei province, the figure was more like 13%. By definition, people who are presymptomatic aren’t yet coughing or sneezing — and may not be for several days. But even normal activities like talking, or ones you can’t avoid, like breathing, can create respiratory droplets.

And while we currently don’t have evidence that those particles are big enough to transmit the virus, some scientists — and a few preliminary studies — suggest it is a possibility. In any case, asymptomatic and presymptomatic transmission does seem to be happening. And that’s why the official guidance in the U.S. and elsewhere is shifting to recommend we all start wearing masks. Because it is possible for someone to be transmitting COVID-19 without even knowing they are sick. So, by wearing a mask, we are helping protect everyone else.

In response to this, you’ve probably seen people make their own masks, using things like scrap fabric or bandanas. And although that might seem like a kind of trivial precaution, the limited evidence we have suggests they do seem to be helpful. Like, look at one study that came out in 2013.

Because it was done seven years ago, it wasn’t on the COVID-19 virus, but it did find that a cotton T-shirt blocked about half of the viruses in a cough, while a tea towel stopped 72%. Of course, at this point, there is a lot that we don’t know about this coronavirus, and there haven’t been many studies about homemade masks. That’s why some experts disagree with the new CDC recommendations.

But other researchers think that doing something is better than nothing. If you want to make a mask, the CDC website has several patterns, including ones that don’t require any sewing, and we’ll include a link to that in the description. But there is an important thing to know here:

Even though just wearing a mask can feel kind of reassuring, for them to work, you have to use them right. I happen to have a mask right here, so I can do a demo for you. You should wear them tight over your nose and mouth and avoid touching the sides and straps as much as possible.

And you shouldn’t touch the front of the mask at all. It sounds silly, but you should basically treat it like it’s covered in raw chicken or poop. So, when it’s time to remove your mask, do it using the straps or strings.

Then, after every use, store your mask in a bag until you can wash it in a washing machine or with soap and hot water. And of course, wash your hands before and after touching the mask. As our understanding of COVID-19 continues to improve, there’s a good chance these guidelines could change again.

But, look - that’s honestly a good thing. Guidelines change when we understand more about this virus and this disease. And that continues to happen.

At the end of the day, this is going to be information that helps us keep each other safe and figure out how to get through this. Thanks for watching this episode of SciShow! We hope you’re staying safe and well out there.

If you’re at home with the kids these days,. I wanted to recommend SciShow Kids! That’s our channel that is designed for young elementary schoolers.

My son loves it! We are watching a lot of it right now. We have more than 300 episodes over there right now, and we hope they’ll be helpful to you.

You can find them at [♪ OUTRO].