They're the hottest accessories on the streets of Mexico City: surgical masks, in shades of baby blue and ivory, adorning the noses and mouths of city residents.

To anyone accustomed to watching pandemic coverage, the masks have become unsettlingly familiar, particularly in footage from Asian countries. Think SARS and avian flu.

But do they work? Who should wear them? And should the average Pittsburgher start raiding Rite Aid?

"Outside of health care settings and outside of settings where you are coming ... face-to-face with someone who has an infectious disease, the evidence [for wearing masks] is not very strong," said Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, on a conference call with reporters yesterday.

In Mexico, the armed forces and police have distributed millions of the masks in Mexico City. Many pharmacies and department stores have sold out of them.

During the avian flu outbreak two years ago, the CDC said that face masks likely would not stop people from breathing in tiny particles, such as viruses, but might protect against splashes or sprays from coughs or sneezes.

Rather than wearing masks, the CDC recommended simply avoiding crowds and sick people.

A study released in January from Australia's University of New South Wales found evidence for the masks' effectiveness, reporting that adults who wore masks in their homes were four times more likely than non-mask-wearers to be protected against respiratory viruses.

Given that the closest confirmed case of swine flu is in Elyria, Ohio, about 150 miles away from Pittsburgh, however, there's definitely no need to wear masks anywhere around here, said Jim Lando, public health physician with the Allegheny County Health Department.

"There is no need for people to be wearing masks in the general public here," he said. "We don't have swine flu circulating. It's not even a consideration here."

In fact, overuse of masks has caused problems in previous disease outbreaks, Dr. Lando said, causing a shortage for health care workers in some parts of the world during the SARS scare.

If anyone should be wearing masks, he said, it would be health care workers in close contact with sick patients.

Instead of masks, he said, most Pittsburghers should take the same precautions against swine flu that they would against an ordinary seasonal flu. Those measures include frequent hand washing, drinking plenty of fluids and avoiding touching their hands to their eyes, noses and mouths.

And there's no need to give up bacon and sausages. Despite the name "swine flu," the CDC says that people cannot get the flu by eating pork or pork products.

"We tell people to really practice good hygiene," said Dave Zazac, spokesman for the Allegheny County Health Department. "We're going back to Public Health 101."