By MIKE STOBBE, AP Medical Writer Mon Mar 3, 6:23 PM ET

ATLANTA - Formaldehyde levels in the trailers provided to Gulf Coast hurricane victims vary greatly by brand, and four of the six most commonly used brands are among those with the highest levels, a federal study released Monday found.

Air samples from trailers made by Gulf Stream, Keystone, Pilgrim and Forest River contained more than four times the formaldehyde levels that is found in newer U.S. homes, according to the study by the U.S. Centers for Disease Control and Prevention.

Calls from the Associated Press to the four companies seeking comment were not immediately returned. A number for Pilgrim could not be immediately located.

The study is the first brand-specific information provided about formaldehyde levels in the tens of thousands of travel trailers and other temporary homes hurricane victims have been living in since Katrina and Rita hit in 2005.

Last month the Federal Emergency Management Agency, which provided the trailers, said it was stepping up efforts to get hurricane victims out of some 35,000 trailers after tests found toxic levels of formaldehyde fumes. FEMA provided a variety of brands of small trailer homes, larger park model homes and even larger mobile homes.

Fumes from 519 trailer and mobile homes in Louisiana and Mississippi were — on average — about five times what people are exposed to in most modern homes, the CDC found. In some trailers, the levels were nearly 40 times customary exposure levels, raising fears that residents could contract respiratory problems.

Air samples from 358 travel trailers, 82 park models and 79 mobile homes were taken from Dec. 21 through Jan. 23, said Mike McGeehin, director of a CDC division that focuses on environmental hazards.

The CDC found average levels of 77 parts formaldehyde per billion parts of air, significantly higher than the 10 to 17 parts per billion concentration seen in newer homes. Levels were as high as 590 parts per billion.

In the latest report, the CDC found an average level of 108 parts per billion in Pilgrim-brand travel trailers, 103 in trailers made by Gulf Stream, 102 in those made by Keystone and 85 in those made by Forest River.

Brands Fleetwood and Fleetwood CA had average levels of 39 and 42 parts per billion. All other brands of travel trailers, analyzed collectively by the CDC, averaged 73 parts per billion.

By far the common brand among FEMA's nearly 47,000 trailers, park models and mobile homes is Gulf Stream, with more than 14,600 trailers. Forest River is the second most common with about 3,200 trailers.

The CDC study found that formaldehyde levels in park models and mobile homes were on average lower than those in travel trailers.

The study did not prove people became sick from the fumes, but merely took a snapshot reading of fume levels. They tested only for formaldehyde, a colorless gas with a pungent smell that can cause respiratory problems and has been classified by scientists as a carcinogen.

It's not clear whether the finding in the Gulf Coast trailers can be applied to travel trailers elsewhere in the country, McGeehin said. Scientists have said that heat and other factors may increase formaldehyde levels.

"We have to limit ourselves to the data in front of us," he said. "Right now I don't feel any cause for alarm for people who own these units."