The Federal Emergency Management Agency is setting strict new limits on formaldehyde levels in the mobile homes it buys for disaster victims.

After insisting that existing trailers are safe, the agency said Friday it will take "extraordinary precautions" by buying trailers with formaldehyde emissions comparable to that of conventional housing. The requirement will cover a three-year contract to purchase up to 3,300 units and a smaller contract for units intended for disabled residents.

Some will be available for this year's hurricane season.

An agency spokeswoman said FEMA will continue to use previously purchased trailers — except for the smallest "travel trailers." But each existing unit will be tested and the results will be provided to states and residents so they can decide whether to accept them.

Formaldehyde is a preservative commonly used in building materials. Prolonged exposure can lead to breathing problems and is also believed to cause cancer.

After hurricanes Katrina and Rita in 2005, residents of FEMA-issued trailers complained of headaches, nosebleeds and other ailments. Critics say FEMA was slow to respond, particularly as families remained in trailers for extended periods, and lawsuits claim the trailers contained dangerous levels of the chemical.

FEMA insisted that the trailers were safe, but after coming under increasing pressure, the agency enlisted the help of the U.S. Centers for Disease Control and Prevention to test them.

The CDC initially said in February 2007 that, with proper ventilation, formaldehyde levels were safe in the short term. A year later, however, the CDC released preliminary results from additional testing showing that FEMA trailers and mobile homes had formaldehyde levels that were, on average, about five times higher than in most modern homes.

Both agencies then began urging residents to move out of the trailers — some 34,000 of which remain occupied from Katrina and Rita.

In a news release, FEMA Administrator David Paulison noted that there is still no national standard for formaldehyde levels in U.S. housing.

"Until such a time as there is a consensus standard, we will take extraordinary precautions and require that all new-production units that FEMA purchases test below the lowest existing 'standard' and below the midpoint of the range that CDC calls 'typical' for conventional homes," he said.