NEW ORLEANS - The Federal Emergency Management Agency is barring employees from entering thousands of stored travel trailers over concerns about hazardous fumes, while more than 48,000 other trailers continue to be used by hurricane victims in Louisiana and Mississippi.

FEMA is advising employees not to enter any of the roughly 70,000 trailers in storage areas across the country, but the directive does not apply to other trailers still in use, agency spokeswoman Mary Margaret Walker said Thursday.

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

Sen. Mary Landrieu, D-La., accused FEMA of using a double standard, and said it “defies logic” that occupied trailers are safer than those in storage.

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

Many trailer occupants have asked to be moved because of concerns about formaldehyde contamination, and hundreds are suing trailer manufacturers, accusing the companies of jeopardizing their health by providing FEMA with poorly constructed campers.

Last week, FEMA indefinitely postponed plans to test for formaldehyde levels in the air inside occupied trailers, saying it needed more time to prepare. 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.

Formaldehyde, a common preservative and embalming fluid, sometimes is found in building materials that are used in manufactured homes. It can cause respiratory problems and has been classified as a carcinogen by the International Agency for Research on Cancer and as a probable carcinogen by the U.S. Environmental Protection Agency.

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 agency regulations.

Jon Byrd, director of FEMA’s Baton Rouge field office, responds minutes later by saying agency officials “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 workplace safety monitoring.

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

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.