CHICAGO - Three popular brands of baby mattresses that were marketed in recent months to families and day care centers contained toxic flame retardants linked to increased cancer risk, according to lab tests conducted for the Chicago Tribune.

One member of that family of chemicals, known collectively as chlorinated tris, was removed from children's pajamas over cancer concerns a generation ago.

Yet that same flame retardant turned up in significant amounts in 11 baby mattresses sold recently by retailers under the Babies R Us, Foundations and Angeles brands. Two other mattresses made by Angeles contained a related form of tris.

The test results surprised and alarmed some scientists who have studied the chemicals. Babies and even toddlers can spend 12 or more hours a day in a crib, and foam mattresses can meet federal fire-safety rules without the use of chemicals.

Linda Birnbaum, director of the National Institute of Environmental Health Sciences, said regulators had assured her that chlorinated tris and other toxic flame retardants weren't used in mattresses.

"These are bad chemicals, and we've known they've been bad for a long time," said Birnbaum, a toxicologist. "If these chemicals are in your child's mattress, they are going to be constantly exposed."

In the late 1970s, scientists at the University of California, Berkeley, found that TDCPP, a form of tris, could cause mutations in DNA, and its manufacturer removed it voluntarily from the market for children's pajamas.

The Tribune tested 27 mattresses. All of the mattresses containing chlorinated tris had one thing in common: labels saying they were made in China or imported from China. None of the tested mattresses made domestically contained significant amounts of any form of chlorinated tris.

The response to the test results from manufacturers, importers and retailers varied.

Wayfair, the retailer that filled the Tribune's Wal-Mart order through the retail giant's online marketplace program, halted sales of the Angeles mattress, which fits cribs that are popular at child care centers.

One importer, however, vigorously defended its product.

Summer Infant Inc., the importer of the Babies R Us branded crib and bassinet mattresses that contained chlorinated tris, noted that the mattresses "are in a sealed impermeable plastic covering," which "ensures no exposure of the inner mattress foam to the child."

Responding to questions from the Tribune, the company wrote, "Simply put, the statements made are misleading and reckless in that they imply a health hazard that doesn't actually exist."

But Birnbaum and Heather Stapleton, a Duke University chemist who studies flame retardants, questioned whether any foam product can be sealed completely. They said chemicals escape when they vaporize and seep through seams or holes and get into air or dust.