By Faye Flam

SCIENCE has a lot to say about the effectiveness of wearing a mask to stop the spread of the coronavirus, but the communication of that science has been corrupted by a combination of partisan divides, sensationalist media stories, distrust, false dichotomies, and letting the perfect be the enemy of the good.

The studies on masks aren’t perfectly definitive, but that’s typical of many issues involving health risks — from mercury contamination to cancer screenings. It’s still better to make decisions based on an incomplete body of evidence than to ignore evidence altogether. In this case, it helps to add a dose of situational awareness and common sense.

The public health community got off on the wrong foot with masks by advising against wearing them and then making a sudden flip-flop and telling us not to leave home without them. Further complicating the picture is a mix of people’s individual attitudes. Some members of the public still fear the virus and want to remain safe, and others are at peace with their risk but want some guidance on how to be a good citizen — or at least be perceived as one.

There have been reasonably convincing studies showing that masks stop some of the particles that might carry the virus out of people’s mouths. That suggests masks’ potential to protect others. Then there are observational studies, which look at mask use in the real world.

On June 1, The Lancet published an analysis of 172 such studies, many of them done in health care settings. The authors concluded that mask-wearing combined with eye protection and social distancing could cut down on the spread of the virus, though the authors admitted to a high degree of uncertainty.

Another study came down on the side of mandatory mask-wearing by watching disease trends in Wuhan and New York City. But some other researchers noted flaws in that study, published in the Proceeding of the National Academy of Sciences. The one-to-two-week delay between infection and test results would suggest infections in New York City dropped well before masks were made mandatory. Some experts wanted the study retracted.

When there are multiple changes in behavior going on at the same time, it can be impossible to connect any one of those changes to rising or falling case numbers.

That doesn’t mean the information in those studies can’t be useful. Physician and infectious disease specialist Muge Cevik, who has been a prescient guide to relative risks, pointed out to me that mask-wearing should be informed by other studies on how the virus spreads. A consensus is finally starting to form that there’s a negligible risk outdoors away from other people, and that very brief encounters pose very little risk, such as people walking, running, or cycling past you.

Common sense would suggest that if an activity poses negligible risk, then wearing a mask offers only a negligible benefit, and should be optional.

On the other extreme are potential super-spreading events — anywhere many people are confined indoors, especially if there’s close contact. Trump’s planned Oklahoma rally is a good example. There, common sense would dictate that such events should not take place at all.

Then there’s the middle ground. Mask wearing is likely to do the most good in settings where people have little choice but to interact in enclosed spaces — grocery shopping, riding public transportation, ride-sharing, getting a haircut, or seeing a doctor.

Also in this middle category is gathering outdoors in large groups — such as at a protest. If most protestors wear a mask at all times, this will likely reduce transmissions.

Cevik, who works at the University of St. Andrews in the UK, pointed out that the six-foot rule applies best outdoors, while in badly ventilated indoor settings, aerosol particles might accumulate and put people at risk even if they never get that close to others. And length of exposure matters a lot, so bus drivers, haircutters, and store clerks face a much higher risk than their customers. Their risk very likely goes down if customers wear masks.

Then there’s a problematic category of activities, such as eating in restaurants, where masks can’t be worn constantly. Would diners be stuck trying to pull masks on and off with every bite? Some experts say such “fiddling” with masks is only going to spread any viruses the mask has captured. As a compromise, many restaurants are seating people outdoors and allowing them to keep masks off while eating. Gyms and yoga studios pose a similar challenge.

The risks associated with close contact and crowds seem obvious and intuitive. And yet Americans have been fixated on the unlikely possibility that infectious doses of virus would fly off cyclists or creep in on packages. In response, some have adopted irrational mask-wearing practices, such as keeping one on while riding or driving, but pulling it down to congregate and chat with groups of people.

And it’s no surprise that politics would infuse the issue, given the moral tone of the mask debate and different messages on mainstream and conservative media. In the US, we have some fraction of people wearing a mask all the time, and some fraction never wearing one. It would be better if everyone wore one when it was likely to help.