The study cannot say exposure to the machines' noises is actually impairing children's hearing - just that the devices may be capable of creating sounds loud enough to cause hearing loss after prolonged exposure.
The report's lead author from the Hospital for Sick Children in Toronto told Reuters Health that people should consider sound in terms of dose.
"Everyone knows aspirin is safe, but everyone knows you don't take 40 of them," Dr. Blake Papsin said.
White noise machines, also known as sleep machines, produce sounds to soothe infants to sleep and help mask other noises, Papsin and his colleagues write in the journal Pediatrics.
But noise can cause hearing loss at certain levels, and the ears of young children are different from adult ears. Higher-frequency sounds get amplified by their smaller ear canals.
The researchers write that the Canadian Centre for Occupational Health and Safety and U.S. National Institute for Occupational Safety and Health recommend adults limit workplace sound exposure to no more than 85 A-weighted decibels (dBA) for eight hours.
That's equivalent to a garbage disposal or blender operating about three feet away.
For young children in hospitals, it has been recommended that they not be exposed to more than 50 dBA averaged over an hour. That's equivalent to a dishwasher running in the next room.
For the study, the researchers bought 14 different types of white noise machines off the Internet and in traditional brick-and-mortar stores.
The machines produced a total of 65 different sounds that included white noise and nature, mechanical and heartbeat sounds.
Using a sound booth, the researchers tested each device turned to its maximum volume and placed sound meters at varying distances to simulate the machine hanging on the side of a crib, being placed next to a crib and being across the room.
The microphones for the sound meters were fixed with special attachments to simulate ear canals. The measurements were also adjusted to estimate what a six-month-old child would hear.
Three machines were capable of producing noise in excess of 85 dBA when positioned on the side of the crib. All machines were capable of producing sounds greater than 50 dBA when placed either in or next to the crib. And all but one of the machines were capable of producing sounds greater than 50 dBA from across the room.
"The main message is that off-the-rack machines - three of them - at certain conditions are capable of producing hazardous levels of sounds," Papsin said. "I'm not saying they were (producing hazardous sound), but they were capable."
Based on their results, he and his colleagues suggest that parents place the machines as far away from infants as possible and set the volume as low as possible.
They also recommend that parents limit how long the devices are on. For example, they could have an automatic shut-off or turn the device off once the child falls asleep.
Alison Grimes said she agreed with the researchers' conclusions.
Grimes, the head of audiology and newborn hearing at the University of California, Los Angeles Medical Center, was not involved with the new study.
"I think their recommendations are very appropriate with their caution," she told Reuters Health. "I think the other question to ask is, what is the noise in the environment that needs to be mitigated in this way"
Grimes said children may not be able to learn about the sounds in their environment if they are drowned out by a white noise machine.
"Infants are designed to hear speech and environmental sounds," she said. "That's how they do their environmental learning."
A representative from Graco, a maker of baby accessories including sound machines, said the company didn't have a comment on the new report.
Papsin said he's not recommending that parents get rid of their machines.
"That would be foolish," he said. "Parenting is like juggling . . . I just gave the parents another ball."
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