Democratic leaders of a House science subcommittee alleged yesterday that the Federal Emergency Management Agency manipulated scientific research into the potential danger posed by a toxic gas emitted in trailers still housing tens of thousands of survivors of hurricanes Katrina and Rita.

FEMA "ignored, hid and manipulated government research on the potential impact of long-term exposure to formaldehyde" on Katrina and Rita victims now living in the FEMA trailers, the congressmen wrote in a letter to Homeland Security Secretary Michael Chertoff, whose department includes FEMA.

Reps. Brad Miller (N.C.) and Nick Lampson (Tex.) cited agency documents given to Congress in alleging that the federal Centers for Disease Control and Prevention -- generally considered a repository of nonpartisan scientific expertise -- was "complicit in giving FEMA precisely what they wanted" to suppress the adverse health effects.

The lawmakers said the CDC's Agency for Toxic Substances and Disease Registry ignored one of its experts, Christopher T. De Rosa, after he informed FEMA there was no "safe level" of long-term exposure. They said FEMA bypassed that opinion and "shopped" the agency for its desired recommendation to study only short-term exposure.

"Any level of exposure to formaldehyde may pose a cancer risk, regardless of duration," De Rosa wrote in a Feb. 27, 2007, letter to a FEMA lawyer, recently obtained by a House Science and Technology investigative subcommittee that Miller chairs. "Failure to communicate this issue is possibly misleading and a threat to public health."

De Rosa wrote the letter after learning that the CDC bypassed his office to produce a Feb. 1, 2007, report for FEMA that did not consider long-term exposure risks, contradicting his recommendation to the agency in June 2006.

"Honest scientific studies don't start with the conclusion, and then work backwards from there," Miller said.

FEMA said the health agency's report last February did not address long-term health effects but rather concerned ways to avoid toxic exposure to formaldehyde. "FEMA did not suppress or inappropriately influence any report," agency spokesman James McIntyre said.

More than 40,000 trailers are still being used by families displaced by Katrina in August 2005 and Rita weeks later. FEMA announced last July that it would test occupied trailers, after congressional investigators disclosed that the agency had suppressed warnings for more than a year from its field workers about health problems experienced by Katrina survivors.

Tests on 500 trailers, finally begun last month, are being performed by CDC, the lawmakers noted. "The Committee is concerned about the independence and scientific integrity of any indoor air testing for formaldehyde levels in these trailers done under the auspices of FEMA," Miller and Samson wrote.

"For those who are too poor to live elsewhere, FEMA's position remains as it was in 2006: there are no possible adverse health effects that can't be cured by opening the windows," they added.