How masks stop the spread of COVID-19: what science tells us

Even before the COVID-19 pandemic broke out, people in many Asian countries regularly wore masks to protect themselves from air pollution, colds, the flu and other airborne illnesses. Now, in the midst of the pandemic, the University of California at San Francisco notes, “In countries where mask wearing was already commonplace, such as Japan, Taiwan, Thailand, South Korea and Singapore...rates of severe illness and death have remained comparatively low.”

By contrast, the West has been slow in adopting the habit — and now it’s more important than ever. It might help to understand why and how masks help prevent the spread of the coronavirus. What does science tell us?

Wearing a mask saves lives

Although masks are not 100% effective in preventing spread of the disease, studies in several countries have shown that they significantly reduce the risk of becoming infected with COVID-19, by 40% and up to as much as 79%. Yet in late September 2020, less than half the people surveyed in the U.S. reported that they wore a mask. If the percentage of people wearing masks increased to 95% from late September 2020 through February 2021, nearly 130,000 lives could be saved, health experts estimate.

And there would be other benefits, according to an economic analysis cited by the Centers for Disease Control and Prevention (CDC): “Increasing universal masking by 15% could prevent the need for lockdowns and reduce associated losses of up to $1 trillion.”

Does a mask protect you or me?

Wearing a mask protects both of us. It helps protect those around you by capturing any virus-containing droplets and aerosols you would otherwise breathe out into the air. It can also protect you, "by blocking viral particles from entering the nose and mouth." 

Research suggests that a mask can help you in another way as well — by reducing the viral load if you do get infected. Viral load is a way of describing the amount of virus particles in your body. The lower the viral load, the less likely it is that you’ll become seriously ill with COVID-19.

You can’t tell who’s infected

COVID-19 travels from one person to another mainly through the air. When you breathe, talk, shout, sing, sneeze or cough, some type of moisture — saliva, mucus or water — is expelled from your nose or mouth. If you’re infected with COVID-19, the virus is present in the mucous membranes in the back of your throat, and it rides along on that moisture.

Moisture particles called droplets are large enough that some can be seen with the naked eye — or even felt when they land on you. They’re heavy enough that they fall out of the air quickly and end up on the ground, nearby objects and other people.

Aerosols are much smaller than droplets and can’t be seen. They’re so light that they can hang in the air for hours, making it easy for people to breathe them in, especially inside buildings and other enclosed spaces. This makes them even more dangerous than droplets.

As you think about that, keep in mind that people who are infected with COVID-19 do not always show symptoms. Once you’re infected, it usually takes between five days and two weeks before symptoms appear. This stage is called pre-symptomatic. When people are pre-symptomatic, the amount of virus they shed from the nose and mouth is extremely high, putting others around them in danger.

Many people who are infected with COVID-19 are asymptomatic — they never develop symptoms and may not even know they’re sick, but they can still pass the virus on to others. Although more testing is needed, it appears that around 40% of people who are infected are asymptomatic.

Current evidence suggests that people who have no symptoms are the main transmitters of the coronavirus, especially in crowds and family gatherings. That’s why you should wear a mask whenever you leave your home and will be around people who are not part of your household. No mask can provide 100% protection, so even if you are wearing a mask, you should still stay at least six feet away from others for an extra measure of safety.

Which masks are most effective?

To create the best barrier against droplets and aerosols carrying the virus, the CDC recommends wearing a mask that has at least two layers of a washable fabric that’s easy to breathe through. It’s even better if the layers are made of different fabrics with different weaves. Research shows that “multi-layer cloth masks can both block up to 50-70% of fine droplets and particles” and also prevent those that do get through the mask from traveling very far.

Sometimes you’ll see people wearing masks that have built-in valves or vents to let air flow out when they exhale. This design is not recommended by the CDC, because the valve allows the virus to escape into the air and infect others. These types of masks have been banned by major U.S. airlines for that reason.

And of course masks are effective only if they’re worn correctly. Your mask must cover both your nose and mouth. Avoid touching the front of the mask, which may be contaminated with the virus and other germs, and always wash your hands thoroughly before putting it on and after removing it. Learn more about the CDC guidelines for mask-wearing.

Bringing the pandemic to an end

Because COVID-19 is so new, we’re still learning about how it works. Even after a coronavirus vaccine becomes available, we won’t know for some time how long vaccination will protect us. But the data make one thing clear: For the foreseeable future, wearing a mask, washing our hands carefully and often, and maintaining social distance will remain the best tools we have to bring an end to the COVID-19 pandemic. The more of us who practice those habits, the faster that will happen.