As the U.S. moves toward its third COVID-19 spike, mask-wearing remains strangely controversial.

Nonetheless, the experts agree: In the absence of a vaccine, near-universal mask wearing in public — especially in crowded settings — remains the single best tool to slow the spread of the virus.

Despite the mounting evidence, political struggles, social media and still unresolved scientific questions have all continued to prolong a debate that most public health officials settled months ago.

Months of studies and firsthand experience have convinced most epidemiologists that the virus that causes COVID-19 spreads mostly through the air. Mostly, that probably involves larger water droplets produced when people sneeze, cough, sing or even talk. Those droplets generally drop to the ground in a matter of minutes — which means most infections spread through close contacts. Sometimes, the microscopic virus particles float for a much longer time, generally stuck to an equally microscopic droplet of water. These particles can move through a ventilation system or linger on the air for hours.

Doctors don’t know whether people can get infected from droplets that have dropped to the ground and left the virus on surfaces. They probably can — but not easily. Doctors also aren’t sure whether how much difference the dose of the virus makes — either in getting infected at all or the severity of symptoms for those who do get infected.

Both those questions bear on the value of masks. Most fitted cloth masks block most of the larger droplets. A multiple-layered cloth mask with a fine weave can also block most of even the much finer, aerosol droplets. Only a tight-fitting N95 medical mask or a respirator will stop up to 95% of the finest aerosol particles.

The virus itself is only 0.1 micrometers in size. An N95 medical respirator will block 95% of particles that small. Cloth masks block a much smaller percentage. However, the virus normally travels on much larger droplets, many blocked effectively by a cloth mask or surgical mask. Aerosol droplets range from 0.2 micrometers to hundreds of micrometers in size. Most are 1-10 micrometers. A human hair is about 80 micrometers in diameter.

But even if a cloth mask doesn’t stop all the aerosol particles, the evidence now suggests it might reduce the number of particles that get through. That may prevent an infection altogether — or ensure you develop only mild symptoms. This could actually lead to enhanced protection from a serious infection later.

All of those conclusions emerged from a review of the research on mask-wearing published online in the science journal Nature.

“The science supports using masks, with recent studies suggesting that they could save lives in different ways: research shows that they cut down the chances of both transmitting and catching the coronavirus, and some studies hint that masks might reduce the severity of infection if people do contract the disease.”

So here’s some of the evidence from other studies cited in the Nature article.

• Two hairstylists in Missouri who tested positive for COVID-19 wore a double layered cotton mask while working and didn’t infect a single client, although they did pass the infection on to household members.

• A protest drew a large number of participants, most of whom wore masks. Contact tracing did not identify a resulting cluster of infections.

• Children at a summer camp in Georgia in June were not required to wear masks. A large number subsequently tested positive.

• Weekly increases in mortality from COVID were four times higher in places where the government didn’t require masks, according to a preprint study that hasn’t yet been peer-reviewed.

• One study suggested that increasing US mask use to 95% would save 100,000 lives between now and January.

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