Air-quality tests are expected to begin late next month on government-issued trailers in Mississippi as federal officials probe concerns that the temporary homes for disaster victims are contaminated by a carcinogen.

Scientists from the U.S. Centers for Disease Control and Prevention in Atlanta traveled to Mississippi on Tuesday to take samples that will help them develop protocols for testing trailers for levels of formaldehyde, which can cause respiratory problems.

This first round of tests is only on trailers that were stored in Mississippi after Hurricane Katrina and haven’t been lived in by Gulf Coast residents displaced by the 2005 storm, CDC spokesman Charles Green said Tuesday.

The CDC hasn’t set a timetable for testing some of the occupied trailers that the Federal Emergency Management Agency provided to tens of thousands of people in Mississippi and Louisiana displaced by hurricanes Katrina and Rita, Green added.

“The protocols for testing occupied trailers is still in the approval process,” he said.

Green said CDC scientists are “optimistic” that tests on unoccupied trailers will start in three to four weeks.

“Good science takes time,” he said. “It may sound like it’s slow, but we’re crossing our T’s and dotting our I’s so that we can have a scientifically accurate report.”

In the meantime, hundreds of Gulf Coast families have asked FEMA to move them out of trailers amid concerns that the units are exposing them to dangerous levels of formaldehyde.

A total of 12,999 trailers in Mississippi were occupied as of last Friday, but 723 families have asked to be relocated, according to a FEMA spokesman. The agency already has moved 256 of those families out of trailers and into apartments, hotel rooms, mobile homes or government-issued “cottages” that are billed as roomier alternatives to FEMA trailers.

In Louisiana, 38,459 trailers and mobile homes are still occupied more than two years after Katrina and Rita. Of the 2,880 families that have asked to move out of trailers, FEMA has relocated 302, mostly to apartments, an agency spokesman said.

FEMA has temporarily suspended the sale of its used trailers and said it won’t shelter victims of future disasters in them until safety concerns are resolved. On Tuesday, FEMA spokesman Aaron Walker referred questions about the air-quality tests to the CDC.

The Sierra Club says it tested the air quality in about 80 FEMA trailers and found that around 90 percent of them had formaldehyde levels above recommended limits set by the Occupational Safety and Health Administration, club spokeswoman Becky Gillette said.

Gillette said many trailer occupants are reluctant to move out of trailers because they want to stay on their property.

“It seems more like a home to some people than a hotel does,” she said. “Unfortunately, there are not a lot of options for these people.”

Hundreds of Gulf Coast trailer occupants are suing trailer manufacturers for allegedly providing FEMA with poorly constructed units that have jeopardized their health.

Peter Taaffe, a lawyer for more than 500 Louisiana residents who sued trailer makers in federal court last month, said his law firm has commissioned its own set of air-quality tests that found high levels of formaldehyde in trailers.

Taaffe said he doesn’t know what impact, if any, the CDC tests will have on the litigation.

“It really depends on the conditions for the tests that are run,” he said.

Larry Hesler, 63, lives in a FEMA travel trailer on the lot where his sister’s home used to stand in Biloxi, Miss. The retired commercial fisherman said he has been in the tiny trailer since shortly after the storm — “two years too long,” he said with a bit of a chuckle.

“The formaldehyde was a little bad at first,” Hesler said. “Just to be on the safe side, I left it open for three or four months.”

He said he left the trailer’s windows open, even when he was at work, and would close them only on winter nights when it was too cold to sleep with fresh air blowing through the trailer.