WASHINGTON - If a super-flu strikes, face masks might not protect you. Even so, the government says people should consider wearing them. In certain situations. Just in case. But there's no need to stockpile.

The consumer advice issued yesterday reflects an underlying lack of scientific clarity. Whether widespread use of masks will help - or harm - during the next worldwide flu outbreak is a question researchers are studying furiously.

But the public keeps asking. So the Centers for Disease Control and Prevention came up with preliminary guidelines.

"We don't want people wearing them willy-nilly," said Michael Bell, a CDC infection-control expert. "The overall recommendation really is to avoid exposure."

When that's not possible, the guidelines say to consider wearing a simple surgical mask if:

You're healthy and can't avoid going to a crowded place.

You're sick and think you may have close contact with healthy people, such as a family member checking on you.

You live with someone who is sick and thus might be in the early stages of infection - but you still need to go out.

Consider wearing a more expensive, better-filtering mask called an N95 respirator if you're well but must take care of a sick person, they conclude.

But remember that neither mask takes the place of basic precautions such as hand-washing and avoiding contact with people who have respiratory infections, said CDC director Julie Gerberding.

A mask "can have a role in personal protection but they are not the only thing," she told a news conference in Atlanta.

Influenza pandemics can strike when the easy-to-mutate flu virus shifts to a strain that people have never experienced. Scientists cannot predict when the next will arrive, although concern is rising that the Asian bird flu might trigger one.

Brewing a vaccine would take months, so the hunt is on for simple control measures that communities could use.

Topping that list: Avoid crowds, and avoid close contact with the sick unless you must care for someone.

Flu spreads most easily to those within 6 feet of an infected person. And it tends to be most contagious before people realize they're truly sick - when the very first coughing and sneezing begins and they're still likely to be out and about.

Why aren't masks a no-brainer?

They can help trap virus-laden droplets flying through the air with a cough or sneeze. But it's unclear whether large droplets or far tinier, so-called aerosolized particles are a bigger risk from flu.

Simple surgical masks only filter the larger droplets. Better-filtering respirators, however, are not for everyone. They must be fitted to the user's face and don't go over beards. Many people find them hard to breathe in. And most don't use them properly, Gerberding said.

Plus, the CDC has wrestled with whether masks would create a false sense of security.

See complete guidelines

on masks and the flu via http://go.philly.com/health