NEW ORLEANS - (AP) Particleboard appears to be one of the main sources of potentially harmful fumes in government-issued travel trailers that have housed thousands of Gulf Coast storm victims, according to a study released Wednesday.

The report issued by the U.S. Centers for Disease Control and Prevention in Atlanta recommends using different building materials to produce emergency housing for the Federal Emergency Management Agency.

Better ventilation in the units also could make them safer, the CDC said. The tests looked at formaldehyde emissions in the walls, floors, ceilings, tables and cabinets in four FEMA trailers that weren't occupied by hurricane victims.

Formaldehyde, a preservative commonly used in construction materials, can cause breathing problems and also is believed to cause cancer. FEMA no longer offers travel trailers for use by disaster victims, but still uses mobile homes.

Scientists speculate that formaldehyde levels in FEMA travel trailers were higher than in mobile homes because the former contain more composite wood products, such as particleboard, in a smaller space with poorer ventilation.

In a previous study, CDC scientists tested air quality inside hundreds of FEMA trailers and mobile homes occupied by victims of Hurricane Katrina in 2005, and detected potentially dangerous levels of formaldehyde in many units.

The latest tests were performed by California's Lawrence Berkeley National Laboratories to narrow down which components were emitting fumes.

Michael McGeehin, director of the CDC's division of environmental health hazards, said the report's findings only apply to FEMA trailers that sheltered Gulf Coast storm victims.

"They do not apply to other trailers in use elsewhere in the country," he said in a statement.

The amount of formaldehyde emitted by each trailer part didn't exceed limits set by the U.S. Department of Housing and Urban and Development, the CDC said. However, McGeehin said those HUD standards were meant to be applied to larger mobile homes.

The CDC studied four trailers made by different companies: Pilgrim International, Inc.; Gulf Stream Coach, Inc.; Thor Industries, Inc.; and Coachmen Industries, Inc.

Becky Gillette, formaldehyde campaign director for the Sierra Club, said the test results illustrate the "terrible inadequacies" of the HUD standards, which she said date back to 1984.

"It's just long past time for the formaldehyde standards to be strengthened," she said.