The Federal Emergency Management Agency is barring employees from entering thousands of stored travel trailers amid concerns they could be exposed to elevated levels of a carcinogen, an agency spokeswoman said Thursday.

The directive doesn’t apply to the more than 48,000 trailers occupied by hurricane victims in Louisiana and Mississippi, FEMA spokeswoman Mary Margaret Walker said.

FEMA last week postponed plans to test for formaldehyde levels in the air inside occupied trailers. Formaldehyde, which can cause respiratory problems and cancer, is found in some materials used to build the trailers. FEMA is advising employees not to enter any of the roughly 70,000 trailers in outdoor storage areas across the country.

“It’s common knowledge that formaldehyde emission levels rise when they are closed in the heat and humidity without any ventilation,” Walker said.

However, U.S. Sen. Mary Landrieu, D-La., called it a double-standard for FEMA to bar employees from entering trailers while allowing residents to live in thousands of units. She said it “defies logic” that occupied trailers are safer than those in storage.

“I don’t really buy that argument,” she said. “It makes no sense, in that most of these (occupied) trailers are closed up and locked during the day.”

FEMA has suspended the sale of used trailers and says it won’t shelter victims of future disasters in them until safety worries are resolved.

The prohibition on entering stored trailers, first reported Wednesday by CBS News, is outlined in a recent string of e-mails between FEMA employees.

In an e-mail dated Oct. 19, a FEMA employee asks if entering a stored trailer at a staging area, in order to close a vent, is “against regulation.”

John Byrd, director of FEMA’s Baton Rouge field office, responds minutes later with an e-mail that starts, “The issue was formaldehyde.”

FEMA officials, Byrd continued, “had directed (although I never saw it in writing) that no one enter any of the (trailers) that had been sitting around in the sun. The idea was that the sun may have baked out high levels of Formaldehyde. We will find out what the policy is.”

Three days later, David Chawaga, a senior industrial hygienist for FEMA, sent an e-mail advising employees not to enter stored travel trailers “until further notice,” based on results from a federal “employee monitoring project.”

Walker said FEMA imposed the ban on entering stored trailers in early August, but some employees apparently weren’t aware of the policy change.

Last Friday, scientists from the Centers for Disease Control and Prevention in Atlanta were scheduled to begin testing FEMA trailers in Mississippi for levels of formaldehyde. But the tests were postponed indefinitely at FEMA’s request, before they started.

Walker said the agency wants to “finalize the testing process” and identify “action levels for responding to the results” before the tests are conducted in around 300 trailers in Mississippi and Louisiana.

Landrieu also criticized FEMA for postponing the tests in occupied trailers.

“Storm victims are suffering from the health effects of formaldehyde exposure while the agency, fully aware of the danger reflected in its own employee policy, is blocking public scrutiny of the extent of the carcinogen in these trailers,” Landrieu said in a written statement.

More than 10,000 trailers in Mississippi and more than 37,000 in Louisiana are occupied by storm victims more than two years after hurricanes Katrina and Rita hit the Gulf Coast. Hundreds of families in both states have asked FEMA to move them out of trailers and into apartments, hotel rooms or other temporary housing.

Hundreds of Gulf Coast residents also have sued trailer manufacturers, accusing the companies of providing FEMA with poorly constructed units that exposed them to dangerous levels of formaldehyde.