In Beijing, where levels of pollution have spiked above 750 micrograms per cubic meter this week, wearing a mask that actually reduced the concentration of inhaled particles by half would still expose people to 10 times more than exposure levels deemed safe by the U.S. Environmental Protection Agency.
Scientists are also still debating whether masks actually protect patients from doctors' germs. Despite being used by surgeons for decades, masks have been tested in only a few clinical trials, Brosseau said. And results showed that rates of wound infection in patients were the same, whether their doctors wore masks or not.
For a step up in protection, consumers can buy a category of mask known technically as N95 respirators, which are generally available at hardware stores. These facemasks are often used in industrial workplace situations to protect against things like lead dust and welding fumes, and they are certified by the National Institute for Occupational Safety and Health to trap 95 percent of particles sent through them in testing situations. N99 and N100 masks are also available.
In order to perform to those standards, though, respirators need to be professionally fitted to each person's individual face to make sure there is a tight seal with no leaks. They don't work for men with beards. And if they truly fit right, they are uncomfortable to wear.
"If it's going to work, it has to fit your face," Milton said. "If you buy a box of these things at the hardware store, it's not clear you're getting anything that's going to work for you."
Even when masks are fitted correctly, Milton added, studies have shown that they're very good at trapping relatively large and extremely small particles, but midsize fine particles are the most likely to slip through. Both viruses and components of air pollution fit into that size category. And it's that size of particle that seems to get stuck in human lungs and cause health problems.
Overall, experts said, studies suggest that the best way to protect people against pollution and epidemics is not to encourage mask wearing but to address underlying problems, like excessive coal burning or poor health habits.
"We don't want people to put on these masks and think they don't need to get vaccinated or wash their hands or do other routine things like cough into their sleeves," said Pritish Tosh, an infectious diseases physician and researcher at the Mayo Clinic in Rochester, Minn. "If someone chooses to put on a mask, it's up to them. But it's important to do other things first."