First off, remember that face masks sold in drug stores are not very effective. The only type that the US Government would recommend are N95 masks, which are called "respirators." They filter out 95% of anything which is .3 microns and larger, which means that they will, if fitted properly, help with "close quarters" situations where the flu virus is lurking about.
As happened in the SARS scare a few years ago, face masks and respirators became very hard to find. People were hoarding them. Right now, doing a search for N95 Face Mask on eBay, you can find plenty of people selling them inexpensively and they have them in stock. Expect these prices to go up dramatically if a pandemic does take off -- that is, if you are lucky to find them. It wouldn't hurt to spend $10 or $15 on a box of N95's, so I'd suggest you go to eBay and grab a box or two.
Here are the USG comments on reducing the risk of infection during a pandemic:
• Avoid close contact and crowded conditions whenever possible, rather than relying on the use of masks or respirators;
• In crowded settings, consider using face masks to protect noses and mouths from other people's coughs and reduce the likelihood of coughing on others; and
• Use an N95 respirator when contact with an infectious person cannot be avoided, such as when caring for a sick person at home.
According to the guidance, people should consider wearing a face mask during an influenza pandemic if they are sick with the flu and think they might have close contact with other people, live with someone who has flu symptoms or will spend time in a crowded public place.
People should consider wearing a respirator during an influenza pandemic if they are well and expect to be in close contact with people who are sick with pandemic flu.
More information about masks and respirators and about community preventive measures is available at the U.S. government’s pandemic flu Web site.
More detail is available by clicking on the blog headline.