Infant sleep machines are supposed to soothe babies with sound so they can easily fall asleep. They are particularly popular with parents who want their babies to sleep easily and soundly so they can rest as well. A new study, however, suggests that infant sleep machines may cause harm to the baby's hearing.

In a new study published in the journal Pediatrics Monday, researchers tested fourteen machines that play white noise and other sounds. The researchers found that in the 14 infant sleep machines they tested, three had sound output that exceeded 85 decibels, which is equivalent to the workplace safety limits in adults for accumulated sound exposure within an eight-hour period as recommended by the National Institute for Occupational Safety and Health. One of the machines was so loud having the baby listen to it for two hours is equivalents to exposing him to more than the workplace noise limits.

"The main message is that off-the-rack machines - three of them - at certain conditions are capable of producing hazardous levels of sounds," said study author Blake Papsin, chief otolaryngologist at the Hospital for Sick Children in Toronto. "I'm not saying they were (producing hazardous sound), but they were capable."

Papsin got the idea for the study when somebody brought an infant sleep machine to the hospital. White noise machines were supposed to drown out sounds that could disturb babies from their sleep but the particular machine brought to the hospital sounded like a roaring car wash.

"Used too loud or too close or too long, these machines can exceed safety standards and potentially damage the hearing of the infant," Papsin said. "The infant ear has a little straighter tube. It's a little wider open, and it amplifies the higher frequencies."

Based on the results of the research, the researchers advise parents to place the machine as far away from their baby as possible and to set the volume the lowest possible. The researchers even suggest that parents consider not using the machine at all.

"None of our sleep specialists recommend them," Papsin said suggesting that parents should instead help babies fall asleep by reading books or singing lullabies.

Allison Grimes, head of audiology and newborn hearing at the University of California, Los Angeles Medical Center, said that the sound that white noise machines drown actually help the children learn about their environment.

"Infants are designed to hear speech and environmental sounds," she said. "That's how they do their environmental learning."