The threat of a flu pandemic has boosted demand for disposable respirators, and companies or governments stockpiling them face longer waits.

"All manufacturers have gone from shipping instantly to taking a week, a month, 90 days or longer," says Kurt Braun, general manager of distributor Edwards Medical Supply of Illinois. The respirators, also called N95 masks, are harder to get for corporate stockpiles than Tamiflu, the No. 1 antiviral, he adds.

N95s cost 50 cents to $1 each and are used in industries from healthcare to construction. Unlike surgical masks, they fit snugly around the wearer's mouth and nose. When properly fitted, they prevent inhalation of 95 percent of microscopic particles down to a certain size. N95s haven't been tested against flu viruses, but the hope is they'll protect wearers from viruses traveling via cough or sneeze.

The Health Ministry of Quebec wants to stockpile 2.5 million masks. One maker, Inovel of California, has said full delivery will take about eight months. Before stockpiling started, delivery would have taken weeks, says Jim Hornstein, Inovel president.

The top N95 maker, 3M, is rationing popular models to some distributors to ensure everybody gets supplies, says spokeswoman Jacqueline Berry.

Aramsco, a top 3M distributor, has about 10 million N95 masks in stock and estimates there are 100 million nationwide. "By December, it'll be a zero shelf market," says senior vice president Dave Naylor, as more states, governments and companies place orders. Masks that fold and are individually wrapped are in the biggest demand.

Manufacturers are boosting production. In June, 3M announced a new respirator plant in South Korea to serve Asia. The company has also raised capacity at U.S. plants. It won't reveal figures. By the end of the year, Kimberly-Clark Health Care will have increased N95 production at least 55 percent over last year, says David Parks, a general manager.

The United States has ordered 74.6 million N95 masks for the nation's stockpile and has 29.3 million. It expects delivery by October 2007. France, with one-fifth the U.S. population, is stockpiling 685 million masks.

U.S. stockpiles are intended to protect healthcare workers and to help keep hospitals and clinics functioning. The United States needs 1 billion N95 masks, not 100 million, says Bill Borwegen, safety director for the Service Employees International Union. The nation has more than 2.4 million registered nurses.

Bird flu has claimed 139 lives, mainly in Asia, since 2003. The virus has not yet easily spread between humans. If it does, it could spark a pandemic, because humans have no immunity and no vaccine.