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August 24, 2020 9:00 AM EDT

Five months after coronavirus took root in the U.S., face masks remain one of the most controversial and confusing parts of the pandemic.

Changing public-health messaging hasn’t helped. In March, when personal protective equipment (PPE) was running short, top U.S. public-health officials told Americans that the general public did not need masks because they don’t fully block respiratory particles that spread COVID-19, such as those in a sick person’s cough or sneeze. Most masks are best at preventing particles from getting into the air where others might inhale them, so, at first, the U.S. Centers for Disease Control and Prevention (CDC) recommended that only sick people wear them.

But as research progressed, studies showed compelling evidence that even asymptomatic people could infect others with coronavirus—which meant anyone could be unknowingly contagious, and everyone should be trying not to breathe on other people. As science evolved—and a dire PPE shortage eased—the CDC revised its guidance to suggest that everyone wear fabric masks in public.

President Donald Trump has also sent mixed messages on masks. He has called wearing masks “patriotic,” but has also opted not to wear them during public appearances or require them at his rallies. Other politicians have also resisted mandates on mask wearing—Georgia Governor Brian Kemp even sued Atlanta Mayor Keisha Lance Bottoms after she required them in public. (He later dropped the lawsuit.)

As a result, many people are understandably confused about how masks work and when they should be worn. The science continues to evolve, but here are experts’ latest recommendations about masks and coronavirus.

How can I make homemade masks more effective?

“The more layers, the better,” says Raina MacIntyre, an infectious disease expert from the University of New South Wales who has studied mask efficacy. In a recent study, MacIntyre and her colleagues recommend a minimum of three layers of fabric. (That said, any face covering is better than none. If all you’ve got is a bandana or t-shirt, wear that.)

While the layers closest to your face can be something soft, like cotton, the outer layer should be a water-resistant fabric, like polyester, that can repel droplets. “There’s an easy way to test any fabric,” MacIntyre says. “Take a piece of fabric and drop one drop of water on it and see what happens.” If it soaks in, it’s not repellent enough for the outer layer. If it beads up, you’re in business.

Fit is also important. You can easily breathe in germs if there are gaps between the edges of your mask and your face, MacIntyre says. For that reason, she recommends a face mask over a face shield, which is typically open at the bottom.

Finally, your nose should be inside your mask at all times, says Ben Abbott, an environmental sciences professor at Utah’s Brigham Young University who recently compiled a guide to masks. “The droplets are produced as you breathe out of your nose just as easily as when you breathe out of your mouth,” he says.

But aren’t medical masks better?

N95 masks, the fitted respirators doctors wear when caring for contagious patients, are better than cloth masks at blocking viral particles from coming in. Cloth masks offer some protection via a physical barrier—which gets more effective with more layers—but N95s offer better filtration. However, the general public is urged not to buy N95 masks so there are enough for health care workers and the highest-risk individuals.

Studies do show that multi-layered cloth masks are about as good as surgical masks at containing the spray of respiratory particles when the wearer sneezes, coughs or talks. That means if everyone wore a cloth mask in public, there would be far less virus circulating and a much lower chance of anyone getting sick. “My mask protects you, your mask protects me,” Abbott says.

Do I need a filter?

You’ve probably seen advertisements for disposable filters that fit between the layers of a face mask to block viral particles. They may add a little extra protection, but MacIntyre says they’re probably not necessary as long as you wear a multi-layer mask and keep your distance from others. (Using one every day would also add up to a lot of waste and cost, she adds.)

If you decide to buy one, “You really need to make sure the filter goes across the whole area of the mask,” MacIntyre says. “The air will flow down the path of least resistance,” so if you’re only using a small filter, air will just move around it. Experts recommend filters made of polypropylene, a plastic-derived material that carries an electrostatic charge that can help it trap incoming and outgoing particles. In a pinch, a few layers of tissue or paper towel could even add a little extra filtration, according to a recent study.

When should I wear a mask?

Research increasingly suggests coronavirus can spread via tiny respiratory particles that hang suspended in the air for minutes and even hours, in addition to direct contact with droplets from a sick person’s cough or sneeze. Airborne spread is particularly likely in indoor environments that aren’t well-ventilated.

“It’s pretty clear now that the indoor [transmission] risk is much higher than outdoor risk,” since viral particles dissipate better in fresh air, Abbott says. That means you should be wearing a mask any time you’re inside with other people—an increasingly likely situation as people go back to school and workplaces.

Even if you’re in your own section of a shared office, respiratory particles may circulate throughout the whole space depending on how well it’s ventilated, so it’s safest for everyone to wear a mask all day, Abbott says. (For proof that masks work inside, look to a Missouri hair salon where two stylists with coronavirus didn’t pass it to any of their 140 customers thanks to face masks.)

Even if you’re outside, Abbott says you should be masked any time you can’t maintain at least a six-foot distance from others.

And yes, you need a mask even if you think you’re not sick. “You’re actually most contagious in the days prior to showing any symptoms,” Abbott says. “That’s why it’s so important for a large portion of the population to be wearing masks. If you wait until you have symptoms, it’s too late.”

What about when I’m exercising?

If you’re working out in an indoor gym, you absolutely need to wear a mask, Abbott says.

If you’re exercising outdoors (and alone) in an uncrowded area, it’s probably safe to take the mask off. You’re unlikely to spread or catch the virus if you quickly pass someone else on a walk, run or bike ride, he says.

Should I wear it at home?

Realistically, most people aren’t going to wear masks at home. But if you live with someone at high risk of getting severe coronavirus (like an elderly or chronically ill person) or who works in a high-risk environment (like a hospital), it may not be a bad idea, MacIntyre says.

And remember, coronavirus spreads easily within households. Housemates have probably been exposed to each other’s germs, so it doesn’t do much good when, for example, one partner wears a mask in public and the other doesn’t, Abbott says.

How should I wash my mask?

You can throw cloth masks in the washing machine with your regular laundry, MacIntyre says. Just make sure to use high heat and wash your mask after every use. MacIntyre recommends keeping at least two on hand so one is always clean.

Are there valid medical reasons not to wear a mask?

For the vast majority of people, wearing a fabric mask is safe and beneficial. The worst side effects most people can expect are a little discomfort, acne and “a false sense of security” that causes them to ease up on social distancing and other precautions, Abbott says.

Masks may exacerbate breathing problems for those who have issues with it under normal circumstances, such as patients with advanced emphysema or severe asthma, MacIntyre says. But that’s not a free pass to walk around bare-faced. “If you’ve got a condition that makes it more difficult for you to wear a mask, you just need to use other techniques to minimize the amount of time you need to wear a mask,” such as ordering food and supplies online to cut down on time spent in public, MacIntyre says.

And though face shields aren’t as effective as face masks, MacIntyre says they are still a decent backup for people who truly cannot wear a mask.

Write to Jamie Ducharme at jamie.ducharme@time.com.

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