The recent back-and-forth debate—and policy reversal—over the use of face masks to prevent the spread of Covid-19 reveals a glaring double standard. For some reason, we’ve been treating this one particular matter of public health differently. We don’t see op-eds that ask whether people really need to keep 6 feet away from each other on the street, as opposed to 3 feet, or that cast doubt on whether it’s such a good idea to promote bouts of handwashing that are 20 seconds long. But when it comes to covering our faces, a scholarly hyper-rigor has been applied. In recent weeks, experts have counseled caution—or rejected the use of masks by the general public outright—as they pleaded for better, more decisive evidence. Why?

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They’re right, of course, that the research literature on mask usage doesn’t provide definitive answers. There are no large-scale clinical trials proving that personal use of masks can prevent pandemic spread; and the ones that look at masks and influenza have produced equivocal results. But this smattering of evidence doesn’t tell us much, either way: The trials neither prove that masks are useful, nor that they’re dangerous or a waste of time. That’s because the studies have been both few in number and beset with methodological problems.

Take, for example, a large randomized trial of mask use among US college students in the 2006–07 influenza season. The reduction in illness among those wearing face masks in that study was not statistically significant. But because the research was carried out during what turned out to be a mild season for the flu, the trial lacked statistical power for that question; there weren’t enough sick people for researchers to figure out whether wearing masks improved on hand hygiene alone. They also couldn’t rule out the possibility that students were already infected before the trial began.

Or take another study of the same influenza season, this time in Australia, which found no definitive effect. That one looked at adults living with children who had influenza. Less than half the people randomized into the group of mask wearers reported using them “most or all of the time.” In fact, they were often sleeping next to their sick children without them. This bears little resemblance to the question of whether you should wear a mask among strangers at the grocery store in the midst of a pandemic.

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But here’s the thing: One could make the same complaints about the evidence supporting mask use by health care workers too. While everyone agrees this practice is absolutely critical in hospitals and clinics, that’s not because we have convincing proof from randomized trials. The few clinical trials we have of using masks for health care workers to prevent influenza don’t show a clear effect; nor can they even demonstrate that the more substantial N95 respirators work better than surgical masks. Those trials are also far from ideal. For example, one tested the efficacy of cloth masks by comparing health care workers who wore them to those wearing surgical masks or respirators, and also to a control group that followed “standard practice” in the hospital. It turned out that the majority of workers in the control group were wearing surgical masks anyway, so the study couldn’t really show whether the cloth masks were better (or worse) than wearing no masks at all.

Indeed, the scientific basis for health care workers using masks doesn’t come from clinical trials of influenza outbreaks or pandemics. It comes from laboratory simulations showing that masks can prevent viral particles from getting through—there are at least a couple dozen of those—and from case-control studies during the 2003 coronavirus epidemic that caused SARS. Those SARS studies weren’t limited to health care workers.

It’s true that health care workers or other people looking after people sick with Covid-19 are exposed to far higher levels of coronavirus than anyone else. In the context of a mask shortage, they obviously have priority claim to access. But that’s not a reason to say there isn’t support for the use of masks by everyone else. After all, there aren’t any clinical trials proving that a 6-foot social distance prevents infection, as far as we know. (The World Health Organization only recommends a 3-foot separation.) Nor do clinical trials prove that washing our hands for 20 seconds is superior to doing so for 10 seconds when it comes to limiting the spread of disease in a respiratory disease pandemic. The scientific basis for that 20-second handwashing advice from the US Centers for Disease Control and Prevention derives from laboratory studies measuring virus on the hands after different washing times.