WASHINGTON, Dec 13 (Reuters) - Federal health experts will begin testing for formaldehyde in trailers provided to people displaced by Hurricane Katrina after complaints of health problems, U.S. officials said on Thursday.

The U.S. Centers for Disease Control and Prevention will conduct indoor air-quality tests for formaldehyde starting Dec. 21 on 500 of the roughly 46,000 government-provided trailers and mobile homes in Mississippi and Louisiana, officials said. Tens of thousands of people lost homes in the hurricane's onslaught in 2005.

Some trailer residents have attributed health problems to exposure to formaldehyde.

The CDC is doing the job at the request of the U.S. Federal Emergency Management Agency, whose response to the disaster has been criticized as slow and ineffective. Tests had been due to begin last month, but officials postponed them.

Jim Stark, director of FEMA's office in New Orleans coordinating recovery efforts, said about 3,700 people already have asked to move from the trailers due to formaldehyde fears, and have been offered housing alternatives.

"In most cases, this will mean a trip to a motel or a hotel until appropriate rental units can be found," Stark said.

FEMA will provide immediate alternative housing to any trailer residents with such health concerns, Stark said.

Formaldehyde is a chemical used widely in the manufacture of building materials. It also is used in embalming fluid. It can irritate the skin, eyes, nose and throat, and high levels of exposure may cause some types of cancers, the CDC said.

FEMA bought the trailers to provide temporary housing for displaced Gulf Coast residents. Stark estimated about 90,000 people are living in the trailers, mostly in Louisiana and Mississippi with smaller numbers in Alabama and Texas.

The CDC said the testing will take about five weeks. Dr. Henry Falk, head of the CDC's Coordinating Center for Environmental Health and Injury Prevention, said officials hope disclose the results by mid-February.

During the testing, officials also will look for other "obvious problems such as heavy mold," Falk said.

"It is important not to be in an environment with high formaldehyde exposure for a very long term. We really want to try to head off the concern about those long-term effects," Falk said at a news conference in New Orleans, which was carried on a telephone link.

"The tests may all be high, they may all be low, they may vary by particular type of trailer," Falk added. "I think depending on the findings, that will help us in anticipating what we might need to do in the future."

There are no federal guidelines or scientific standards on formaldehyde levels in such trailers, officials said. (Editing by Maggie Fox and Jackie Frank)