Many babies whose deaths were blamed on the mysterious sudden infant death syndrome may have suffocated while lying face down on ordinary bedding, a new study suggests.

The study was performed by the same researchers who demonstrated the suffocation hazard of polystyrene bead-filled beanbag cushions made for infants.The U.S. Consumer Product Safety Commission recalled 950,000 such cushions.

The new study concludes that "a wide variety of ordinary types of bedding may be implicated in face-down SIDS deaths."

The study was presented Tuesday in Anaheim during the annual meeting of the Federation of American Societies for Experimental Biology. It was conducted by James Kemp and Bradley Thach, pediatricians at the Washington University School of Medicine in St. Louis.

"Perhaps one in four of sudden, unexplained infant deaths may be explained by exhaled carbon dioxide being trapped around the baby's face by bedding such as pillows, comforters and foam beds," Kemp said.

SIDS, also known as crib death, kills about 7,000 infants annually in the United States. Many possible causes have been proposed; none has been proved.

Thach and Kemp said studies around the world indicate at least one-fourth of presumed SIDS victims were found face down in bedding. They should be investigated as possible suffocation deaths, Thach said.

In their study, Kemp and Thach made rabbits breathe through a model of an infant airway pressed against bedding materials on which infants died. That test and a new mechanical test suggested five types of bedding can suffocate infants by trapping exhaled carbon dioxide, allowing children to rebreathe the gas without getting enough oxygen.

The types of bedding were a synthetic-filled adult pillow, a 31/2-inch-thick foam couch cushion, a 11/2-inch-thick foam pad covered with a comforter, a sheepskin sold as an infant bed, and a soft infant bassinet cushion covered by a blanket.

Last summer, the New England Journal of Medicine published a study in which Kemp and Thach used rabbits to show how infants could suffocate on beanbag cushions.