President Donald Trump said Friday the CDC is advising that every person in the US wear a cloth mask or face covering in certain public settings to combat the coronavirus pandemic.

Masks should be worn by all except infants “in public settings where other social distancing measures are difficult to maintain,” such as grocery stores and pharmacies, CDC guidance reads. Masks are especially recommended in areas where Covid-19, the disease caused by the SARS-CoV-2 coronavirus, has shown significant spread from person to person.

The new guidance is a big shift for the federal government, which previously declined, through the Centers for Disease Control and Prevention and surgeon general, to recommend that most of the public wear a mask. Authorities previously only recommended masks for those who are sick and people, such as health care workers, who frequently interact with the sick.

The White House’s recommendation, based on new guidance from the CDC, only calls for people to use cloth masks, including homemade variants. The guidance is voluntary — and Trump immediately added at a press conference Friday that he would not be complying: “You can do it. You don’t have to do it. I’m choosing not to do it.”

The recommendation does not mean you should go out and buy a traditional medical mask. Since there’s a shortage of personal protective equipment (PPE), including masks, for doctors and nurses, experts recommend that people don’t use the limited supply of medical masks, like surgical masks and N95 respirators, for themselves and instead leave those for health care workers.

While the evidence is limited, research suggests that more mask use by the greater public could help stop the spread of Covid-19. Some studies in households and colleges “show a benefit of masks,” Raina MacIntyre, head of the Biosecurity Program at the University of New South Wales in Sydney, Australia, told me, “so it would be plausible that they would also protect in lower-intensity transmission settings such as in the general community.”

Related Everything you need to know about making your own face mask

The PPE shortage makes this evidence a bit harder to act on. If doctors and nurses can’t get medical masks, that’s very bad news for all of us: We need as many health care workers as possible to stay healthy so they can treat and save people who are sick, not just with Covid-19 but with other illnesses too.

“I am worried that telling people to wear masks will strain already weak supplies that are needed by doctors and nurses,” Jennifer Nuzzo, an epidemiologist at Johns Hopkins University, previously told me, before the federal government’s updated guidance. “If we are able to fix that supply chain, I’d feel less worried about this. But some of the shortages initially were due to members of public and medical staff raiding medical offices’ and hospitals’ supplies for home use.”

Experts also worry masks can give people an exaggerated sense of security. Masks don’t make you invincible, and they absolutely can’t replace good hygiene — Wash your hands! Don’t touch your face! — and social distancing. Even in Asian countries where widespread mask use is common, good hygiene and social distancing have been necessary to combat Covid-19. Epidemiological models also suggest coronavirus cases will rise if social distancing measures are relaxed, potentially causing hundreds of thousands, if not millions, of deaths in the US alone.

Still, masks do appear to help. It’s straightforward: Coronavirus appears to mostly spread when germ-containing droplets make it into a person’s mouth, nose, or eyes. If you have a physical barrier in front of your mouth and nose, that’s simply less likely to happen.

But the best protection masks offer may be protection from the wearer. While the evidence is thin on how much masks protect the wearer from coronavirus — since it’s unclear if the virus spreads much through airborne droplets — it’s true that the masks stop people from spreading their own droplets: When you breathe, talk, laugh, sigh, yawn, sneeze, or cough in public, you’re less likely to get droplets on a checkout machine, dining table, or anywhere else if you have a mask on. That could stop people, even those who are asymptomatic, from spreading infection.

That’s especially important for Covid-19, since at least some spread happens when people are asymptomatic, when they have few symptoms, or before they develop symptoms. Universal mask use could stop these asymptomatic carriers — many of whom might not even know they’re sick — from inadvertently infecting other people.

The CDC guidance stated that masks are primarily meant to prevent the spread of disease from the wearer to others.

Some members of the general public would also benefit more. People who know they’re sick or interacting with someone who’s sick were already advised to use a mask. People who frequently interact with others as part of their jobs, like a first responder or a grocery store clerk, are more likely to get good use out of masks too. The logic is similar to why masks are so important for health care workers: Masks are most useful during prolonged, close interactions involving potentially sick people.

With the new guidance, the federal government is trying to walk a line between acknowledging the evidence for public mask use — as well as many people’s desires to take more action to combat the coronavirus — and avoiding further strain on available PPE supplies. So it’s limited its call to just cloth masks (which can be made at home) for now. The CDC also notes that “cloth face coverings should not be placed on young children under age 2, anyone who has trouble breathing, or is unconscious, incapacitated or otherwise unable to remove the mask without assistance.”

But for almost every American, that means the official guidance is to now wear some sort of cloth on the face while in public.

There’s some research in favor of everyone wearing masks

Here’s the gist of the evidence on public mask use: Masks don’t offer full protection — but some protection is better than none.

Masks can’t replace all the other approaches needed to fight the coronavirus, like washing your hands, not touching your face, and social distancing. But when paired with all these other tactics — and when used correctly, which may not be as easy as you think — masks offer an extra layer of protection.

The quality of the research on this topic is weak, with a lot of small, underpowered studies. “There’s not this overwhelming body of evidence that says that’s exactly what we should be doing right now,” Saskia Popescu, an epidemiologist focused on hospital preparedness, told me. “That’s why there hasn’t been an earlier push from public health agencies.”

But the studies that do exist generally favor more people wearing masks.

A 2008 systematic review, published in BMJ, found medical masks halted the spread of respiratory viruses from likely infected patients. In particular, studies on the 2003 outbreak of SARS — a cousin to the coronavirus that causes Covid-19 — found that masks alone were 68 percent effective at preventing the virus. By comparison, washing hands more than 10 times a day was 55 percent effective. A combination of measures — hand-washing, masks, gloves, and gowns — was 91 percent effective.

A 2015 review, also published in BMJ, looked at mask use among people in community settings, specifically households and colleges. Some studies produced unclear results, but the findings overall indicated that wearing a mask protected people from infections compared to not wearing a mask, especially when paired with hand-washing. A big issue was adherence; people were often bad at actually wearing masks, which, unsurprisingly, diminished their effectiveness. But if masks were used early and consistently, the authors concluded, they seemed to work.

Other studies have produced similar results, typically finding at least some protective value from masks as long as they’re used consistently and properly.

There are some risks. If people start feeling like masks make them invulnerable and begin acting recklessly — ignoring social distancing or failing to wash their hands — that could actually make wearing masks worse than not wearing them. But if people take all the other precautions and add masks to their repertoire, as other countries’ experiences suggest people can do, then masks seem to help.

There’s no good research on how masks affect people’s behaviors. “It could be good. It could be bad,” Popescu said. “But either way, we need to have that knowledge.”

There’s also a risk of improper use actually exposing people to more illness. If people don’t put on the masks correctly, they won’t be as protective (though some barrier is likely better than none). If people touch the front of their masks and then touch other parts of their face, they can infect themselves with droplets their mask caught. If people reuse masks, they can breathe in virus-containing droplets from the masks while putting them on or taking them off.

The results also vary depending on the type of mask. One kind of mask that’s been talked about a lot lately is the N95 respirator, which is a more complex, expensive mask meant to fit more tightly on the face. N95 respirators in theory outperform surgical masks (which are the more traditional, looser-fitting medical masks), but they’re genuinely difficult to fit and use properly — to the point that a 2016 review in CMAJ couldn’t find a difference among health care workers using N95 respirators versus surgical masks for respiratory infection, likely due to poor fitting. Given how difficult these are to use, and the extra layer of protection they can provide, experts argue these masks should, above all, be saved for health care workers.

Cloth masks, meanwhile, are much less effective than the modern alternatives, as a 2015 study in BMJ found. And they can be extra risky, since they can trap and hold virus-containing droplets that wearers can then breathe in. But they still, in general, offer more protection than no mask at all, several studies concluded.

Experts at the CDC also offered some advice for proper mask use, which differs somewhat depending on the type of mask: Wash your hands before and after taking off a mask — before to avoid getting anything on your face and mask, and after to get rid of anything that was on your mask. Don’t fidget with your mask while it’s on. If possible, throw away disposable masks after using them. And if you can’t throw a mask away, make sure to thoroughly disinfect it with ultraviolet light sterilizers — not something most people have around. Or, if using a cloth product, soap and water or laundering works.

Based on the evidence, masks appear to help both the wearer and other people. The latter is particularly important for the coronavirus, since the disease can spread from those with few to no symptoms. So whether it’s for selfish or altruistic reasons, there could be a benefit to everyone, even the asymptomatic, wearing masks — as is standard and recommended by public officials in many Asian countries (including Taiwan and South Korea, both of which seem to have done a better job containing Covid-19 than the US).

George Gao, director general of the Chinese Center for Disease Control and Prevention, said as much in an interview with Science magazine:

The big mistake in the US and Europe, in my opinion, is that people aren’t wearing masks. This virus is transmitted by droplets and close contact. Droplets play a very important role — you’ve got to wear a mask, because when you speak, there are always droplets coming out of your mouth. Many people have asymptomatic or presymptomatic infections. If they are wearing face masks, it can prevent droplets that carry the virus from escaping and infecting others.

But part of persuading people to wear masks in these countries was simply more people wearing masks in public, removing the stigma that only sick people wear masks and making masks more socially acceptable. That’s an argument for everyone wearing a cloth mask now — and medical masks once supply shortages are fixed — when we go out. Not only could that help protect ourselves and those around us, it might help instill a healthier norm for the rest of society too.

Masks aren’t a cure-all — and doctors and nurses need them more

The federal government, particularly through the CDC, had for months rejected calls for the general public to wear masks. Before the government announced its updated guidance, I tried to find out why — and the CDC didn’t seem like it was budging on the issue anytime soon.

Why shouldn’t the public use masks if they provide some protection? “CDC does not recommend that people who are well wear a face mask to protect themselves from respiratory illnesses, including Covid-19,” CDC spokesperson Arleen Porcell responded. Okay, but why? “The science says that surgical masks won’t stop the wearer from inhaling small airborne particles, which can cause infection. Nor do these masks form a snug seal around the face.” Sure, masks don’t stop everything, but isn’t some protection better than none? I got no response after that.

Since then, however, the federal government has shifted course — now recommending at least a cloth mask for every American in public when social distancing measures are difficult to maintain.

That comes after the federal government’s messaging backfired: As health care workers have clamored for masks, it’s become harder to tell the public that masks wouldn’t benefit everyone else. By obfuscating and failing to fully explain the issue, experts told me, officials have likely sown distrust toward their guidance — and the public has rushed to buy masks anyway.

I can’t explain the motives behind the federal government’s original stance and shift. But based on my conversations with other experts and officials, it seems many people are afraid of saying anything that could exacerbate the PPE shortage for health care workers or get members of the general public to think — incorrectly — that they could ease on social distancing if they just wear a mask.

“I fear that if we tell everyone they should go out and buy masks, it will not only contribute to the PPE shortage,” Jaimie Meyer, an infectious disease expert at Yale University, told me, “but it will give a false sense of a ‘quick fix’ for protection, whereas people still need to be practicing social distancing strategies that are much more effective, though perhaps socially, psychologically, [and] logistically challenging.”

So as tough as social distancing can feel, it’s, unfortunately, a requirement for now that just won’t be supplanted by masks.

The PPE shortage is a real problem too. There are reports of doctors, nurses, and other health care workers using bandanas and scarves for masks and trash bags for gowns. Hospitals are considering do-not-resuscitate orders for dying Covid-19 patients out of fear that such intensive, close-up procedures could get doctors and nurses without PPE infected with the virus. The CDC, acknowledging the shortage, previously recommended homemade masks for health care workers when no other options are available.

Different levels of government are racing to fix the PPE shortage, which is driven by both high demand as the coronavirus spreads and a lack of supply as countries, hospitals, and individuals hoard what they can find. Experts say the shortage reflects poor government preparedness for a pandemic, given that disease outbreak simulations repeatedly found PPE problems in the past.

Whatever the cause, the shortage for health care workers is bad news for all of us.

As coronavirus has spread, experts have talked up “flattening the curve.” The idea is to spread out the number of coronavirus cases — through social distancing, testing, contact tracing, and other protective measures — to avoid overwhelming the health care system. Here’s what that looks like in chart form:

The PPE shortage could make it harder to flatten the curve of new cases if doctors and nurses get sick. But the line representing health care system capacity also isn’t a constant. If we develop more capacity, it can handle more cases at once. If capacity falls — if doctors and nurses get sick because of a lack of protective equipment, or refuse to work without conditions that can ensure their safety — even a flatter curve will be hard for the system to handle.

That’s why experts, even those who acknowledge that the public would benefit from using masks, say that doctors and nurses should get priority: This isn’t just about keeping people on the front lines safe; it’s about keeping all of us safe.

So until the shortage is fixed, experts say that people should not compete with our health care workers for mask supplies. If you do have masks, consider donating them to hospitals and clinics (though most will only accept unopened packages). There’s also a new group, Project N95, trying to connect PPE suppliers and health care workers.

Policymakers could also take steps to fix the shortage: sending out strategic reserves, funding more production, and, in President Trump’s case, using the Defense Production Act to better prioritize PPE for hard-hit states in the short term and mandating more production in the medium and long term. And individual companies could shift their production lines to produce more PPE, like some clothing and pillow companies are doing.

For now, the federal government recommends cloth masks for the general public, echoing what other organizations had previously suggested. A “road map” on dealing with coronavirus by the conservative-leaning American Enterprise Institute, for instance, made a similar recommendation: “Personal protective equipment should continue to be reserved for health care workers until supplies are sufficient for them and abundant. For this reason, right now members of the general public should opt to wear nonmedical fabric face masks when going out in public.”

Again, the research suggests cloth masks aren’t as effective as medical masks, but they offer some protection. If you want to make your own, people are increasingly offering tips for how to do so on social media.