WASHINGTON, April 4 (Reuters) - More than three-quarters of people given protective face masks put them on incorrectly, U.S. researchers said on Wednesday in a study showing confusion about using the breathing aids in an emergency or pandemic.

Experts from the U.S. Centers for Disease Control and Prevention and the National Institute for Occupational Safety and Health tested 538 people in New Orleans who used a cup-shaped face mask called the N95 respirator after Hurricane Katrina destroyed much of the city in 2005.

People were advised to use the masks -- designed to filter out fine particles and germs -- when cleaning up flooded homes because of mold, which can cause respiratory irritation.

Only 24 percent put the masks on right, the researchers reported in the journal Emerging Infectious Diseases.

"Improper donning would promote the entry of unfiltered air through leaks or gaps between the respirator and the skin, compromising the protection offered," said the researchers, led by the CDC's Kristin Cummings.

N95 masks are recommended for use in dusty places but also to avoid infection from tuberculosis and viruses such as the H5N1 avian influenza virus. But they must be carefully fitted using a metal bridge over the nose.

"Errors included nose clip not tightened (71 percent) and straps incorrectly placed (52 percent); 22 percent put on the respirator upside down," the researchers wrote.

Many experts are investigating the use of N95 masks to protect people during an epidemic or pandemic of influenza. The H5N1 bird flu virus is considered a likely culprit if it acquires the ability to pass easily among humans.

H5N1 mainly infects birds but has killed 170 of the 288 people known to have been infected in 12 countries.

"While uncertainty remains about the level of protection needed against influenza and that offered by an N95," the researchers wrote, an improperly donned mask "will provide less protection than a properly donned one."

Other studies have suggested that people have trouble using the masks properly, so training might be an important part of distributing masks to fight disease, the researchers said.

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