Masks are a flashpoint amid the coronavirus pandemic. Here's what science says about them.

Masks have become a symbol of the coronavirus pandemic -- at first largely foreign to Americans, then treated skeptically by officials looking to preserve protection for health workers and then embraced by a public desperate to stem the overwhelming tide of the virus in northeastern cities.

Since then, they have become a political flashpoint, a source of defiance and confusion, all while a debate raged about what type to wear and their effectiveness.

Early in the COVID-19 pandemic, the Centers for Disease Control and Prevention (CDC) and other government officials discouraged Americans from wearing masks, saying they were unlikely to help stop the spread of the virus, according to the best evidence at the time.

But as evidence accumulated, the CDC made an abrupt about-face on April 3, encouraging all Americans to don face coverings to slow transmission of the virus, which was largely believed to be spread through respiratory droplets.

Meanwhile, across the world, scientists were frantically marshaling resources to set up a wide range of studies to examine the issue. Some set up dummies in their laboratories, mimicking coughing or sneezing using spray canisters and capturing the distance these droplets were able to fly.

Others embarked on massive population-scale studies, trying desperately to discern whether masking policies were likely to stop the spread of the virus within communities.

By June, the World Health Organization (WHO) weighed in, agreeing with the CDC that people should wear masks -- especially when other preventative measures, like standing 6 feet away from other people -- are not possible.

Now, more than six months into this global pandemic, experts say enough evidence has been amassed to conclude that masks are critical in mitigating COVID-19 spread.

“It is a simple, inexpensive measure that can have a significant impact in reducing the spread of the virus, ” said Dr. Simone Wildes, an infectious disease physician at South Shore Health in Massachusetts. “We have to remember if we don't take these measures there will be more cases and more deaths.”

But what kind of mask is best? Experts agree that most people should not be using N-95 masks, which protect both the wearer and others from viral particles, are still in short supply and should be reserved for medical professionals. And they acknowledge that masks are not the only solution. Staying at least 6 feet away from other people when possible and washing your hands frequently are other important ways of stopping the spread of COVID-19, they said.

“The masks are not a cure for COVID-19,” said Dr. Amesh Adalja, senior scholar at Johns Hopkins Center for Health Security. “It is a simple intervention that can be used to slow the spread of the coronavirus.

Why masks probably help others

Accumulating scientific evidence points to two potential benefits of wearing a mask or homemade face covering. First, it might prevent you from spreading respiratory droplets to others if you are the one who is infected. Second, it might prevent some of the viral particles from getting into your mouth and nose -- a point that has been much murkier from scientists and public health officials.

This first potential benefit has become impossible to ignore as we have come to understand the phenomenon of “silent spreaders,” with some studies estimating that more than 40% of people may be carriers of the virus without knowing it or feeling any symptoms, according to a review published in Annals of Internal Medicine this June.

“There are many people that are out there that have mild symptoms that may not even know they are infected that may be spreading the virus. A facial covering can help prevent that from happening,” said Wildes.

The CDC has weighed in on this topic, saying that existing science supports the idea that masks can help protect the people around you, should you be infected. Right now, the agency stops short of saying that the mask itself can definitely protect you from getting infected, although research is ongoing.

Most of the good data we have on masks has looked at their effectiveness in health care settings, like hospitals and clinics. One of the strongest studies comes from the scientific journal The Lancet this June, which compiled multiple studies from both healthcare and community settings to find that wearing a mask may bring the risk of transmission of coronaviruses from 17% to 3%.

A study in Nature Medicine in April showed that masks reduce the amount of seasonal coronavirus as well as other respiratory viral particles in droplets or aerosols from exhaled breaths, however it did not study patients with COVID-19 directly. Another study in the Journal of Health Affairs in June, found that infection was 2% lower in places where mask mandates were enforced, though other factors could have contributed to this difference.

And multiple studies have found that the thicker the mask, the better. A thin, loose-fitting mask is less likely to protect the people around you compared to a multilayer mask (ideally 12-16 layers) that covers your nose and your mouth.

And why they may help you

While less conclusive, growing evidence suggests masks might also protect you, the wearer. This theory relies on the idea that while a mask might not protect you from inhaling small particles of virus through the gaps in the mask, it is likely to prevent you from inhaling massive quantities of viral particles, compared to if you were wearing no mask at all.

“Though not conferring absolute protection, wearing a mask is does provide the wearer some level of protection which should provide reinforcement for those that aren’t willing to take an altruistic public health approach” said John Brownstein, a Harvard Medical School professor and ABC News contributor.

And experts think that inhaling a little bit of virus is better than inhaling a lot of virus, because a lower viral load could mean you’re less likely to get seriously sick.

“This theory is really based on evidence that we have from almost every virus, whether it’s spread in a respirator way, gastrointestinally, sexually, that if you get a large innocuous of virus, you get more ill,” said Dr. Monica Gandhi, director of UCSF-Gladstone Center for AIDS Research (CFAR).

“If you get less of a dose, you get less sick. And if you use masks you get less of a dose,” Gandhi said. “I really do believe that population masking will lead to more mild disease. It’s the biggest argument to [wear a] mask.”

Even as evidence supporting mask use continues to accumulate, many Americans have become weary of wearing them as the pandemic drags on. Others remain skeptical of mask policies because of the government’s early messaging that masks wouldn’t help.

“We have to be humble about our assumptions,” said Dr. Anne Schuchat, principal deputy director of the CDC, during an online Q&A on Monday. “It’s not that we were wrong and then we changed our mind, it’s that we keep learning.”

Existing scientific data tells us it’s likely masks could save lives. And doctors say at the very least, they don’t hurt.

“I can understand the frustration some individuals are experiencing about the need to wear the mask at all times,” said Wildes. “But in the midst of a pandemic with more than 2.5 million cases in the US and more than 127,000 deaths with no cure or vaccine yet, we have to take all measures we can to slow the spread of the virus.”

Ayodola Adigun, M.D., M.S., is an attending in pediatric and adult psychiatry and a former fellow in child and adolescent psychiatry at Yale University. Alexis E. Carrington, M.D. is currently completing her internal medicine preliminary year at Elmhurst Hospital in New York City. Stephanie Farber, M.D. is a plastic surgeon from Pittsburgh, PA. Jessica Johnson, M.D. is a senior resident in emergency medicine at Stanford University. Sony Salzman is coordinating producer of the ABC News Medical Unit.