According to the U.S. Centers for Disease Control and Prevention (CDC), the masks actually don't do much good. In fact, the U.S. has made a point of not recommending their usage.
In fact, some doctors warn they might even be harmful, causing people to take risks — like venturing into crowds or neglecting to wash hands — in the mistaken belief that the mask protects them.
Experts say people who come in close contact with known swine flu patients should wear high filtration masks like those used by health professionals, which are more effective but also more expensive (about 12 pesos or a dollar) and generally unavailable in Mexico City. But even these masks, which filter out fine particles carried in the air, must be used properly to give real protection.
However, some say one of the benefits of the masks is to help people feel more safe during a situation that is largely out of their control. And in a country that is tightly in the grip of a scary epidemic, that solace may be no small thing.