A study published last year in the journal PloS One found that respirators are more efficient than surgical masks at reducing the transmission of respiratory infections to healthy people, but that surgical masks are more efficient than homemade masks.

Another study, published online last week in the journal Risk Analysis, concluded that N95 respirators combined with the use of humidifiers and ventilation in a household improves infection control by 20% to 70%, according to a mathematical model. The authors, from Stanford University, said wearing surgical masks with nylon hosiery or another device to “reduce face-seal leakage” is an alternative if N95 respirators are not available.

But even for people willing to wear such a contraption, there is no single action that will provide complete protection in areas with confirmed swine flu cases, health officials said. It isn’t practical to wear a mask all the time, even a quality mask, and the devices aren’t foolproof.

“Once they get moist, they are no longer useful,” Mascola said. “Your saliva is going to be pooling in that mask. That will make is not useful because germs will be able to permeate.”

Taking a mask on and off contaminates it and makes it less useful, as well. It is effective “only for a 20-minute to a half-hour period,” she said. “Even in those places during the SARS epidemic, they found hand-washing as effective as wearing masks.”