Across Asia, hundreds of millions of motorcyclists, bicyclists and pedestrians wear face masks to protect themselves against air pollution. The masks come in many sizes and colors: Car-exhaust black is the most common, but Hello Kitty pink, Nike swooshes and Burberry tan plaid have been spotted. What has been lacking, until now, is data on whether they actually work.

The consequences are serious. Air pollution is rapidly increasing in Asian cities; it contributes to many deaths from lung disease, and studies have shown that heavy smog quickly erodes lung function.

Environmental health scientists from the University of Massachusetts, above, recently began testing masks they bought in street markets in Kathmandu, Nepal. Using a Styrofoam head, they found that the type of mask that is by far the most popular — inexpensive, washable cloth rectangles held in place by ear loops — provided little protection against the smallest particles, of less than 2.5 micrometers, that penetrate deepest into the lungs.

They were “better than nothing,” said Kabindra M. Shakya, an author of the study, which appeared in The Journal of Exposure Science and Environmental Epidemiology. But “wearers who stand next to a diesel truck and think they’re protected” were clearly at risk, said Richard E. Peltier, the lead author.