If operated as sometimes suggested — with the device close to the child, on a loud volume and running continuously — some of the machines exceed the noise limits occupational safety agencies have set for adults, the study notes.
The authors suggest the noise could damage delicate ears and perhaps even hinder auditory development.
"The effects, potentially, on auditory development and speech and language development are theoretical, at best," said senior author Dr. Blake Papsin, otolaryngologist in chief at Toronto's Hospital for Sick Children.
"There are animal models in rodents that show that this happens. But the whole concept of replacing periodic, lovely, incredibly complex environmental sound for the young, developing, processing brain with a babbling brook or" — he makes the sound of static — "is foolish to me."
The study is being published Monday in the journal Pediatrics.
The idea for the study on infant sleep machines, as this line of products is called, came from a check Papsin made on a young child who had undergone surgery at Sick Kids.
A sleep machine was operating loudly when he walked into the patient's room. The parents informed the startled Papsin that they had been told to use the machine by a sleep doula.
He left the room and fetched one of his students and a sound level meter.
"In the room she records 85 decibels — the sound of a car wash. In the kid's ear! To mask the noisy ward, which was probably 85 decibels itself."
"I said 'I'm going to go study this. This is crazy.'"
His team bought as many different types of sleep machines as they could find, both from baby supply stores and online sources. They found 14 different models which projected a variety of sounds such as a heartbeat, rain, crashing waves and white noise.
They tested each at its maximum volume, at three different distances: 30 centimetres, which would be the typical distance to a baby when devices are attached to the slats of a crib; 100 cms, which would be like a placement on a surface near the crib and 200 cms, the equivalent of across the room.
Occupational health standards set 85 decibels of noise for eight hours as the limit for adult ears; after that point, hearing loss could occur. For every three decibels over that limit, the allowed exposure time is halved. So at 88 decibels, the exposure limit is four hours.
No one has done studies to figure out what a safe exposure equivalent is for young children, nor is such a study likely to be done. It would be considered unethical.
But because of the physical traits of an infant's ear — the small ear canal amplifies higher frequency sounds — neonatal intensive care units have recommended that babies should not be exposed to more than the equivalent of 50 decibels averaged over an hour.
All but one of the machines, at all three distances, exceeded the 50 decibel level, Papsin and his team found. The one exception registered 49.2 decibels — and then only at the 200 cm distance.
In fact, at 30 cm — in the crib — three devices emitted sounds over 85 decibels, with one hitting just under 93 decibels. At the 100 cm distance all the machines registered at 60 decibels and upward. The sound levels ranged from 49.2 to 76.2 decibels at the 200 cm distance.
Papsin noted that these sound levels were in addition to whatever noise they were being used to mask.
"Philosophically, have we come to this? Have we come to this as a society in 2014, that you need an app for cuddling your baby?" he asked incredulously.
"Maybe if the baby's comfortable falling asleep to a heartbeat, go to the crib and grab them, put them against your chest. Use the factory installed one. Slow down here. Let's have a conversation, society."
Papsin may not be a fan, but many young sleep-deprived parents are. They rave about the machines to friends, buy the machines for baby showers and pass them on when their children outgrow them.
That's how Melanie Hunter came to use a sleep machine to help her two young sons, now 5 and 2, sleep. When she was pregnant with her older son, friends gave her and her husband one of the machines. "(They) swore by it and bought it for us as a gift."
It became part of the sleep routine, and even accompanied them when the family travelled.
Did it help her children sleep better?
"You never really know," she said with a laugh. "You don't know if it's a placebo effect or if it just makes the parents feel like it's doing something. ... I just thought: Well, it doesn't seem to be hurting. And if it is helping, why not?"
"We didn't have it cranked," she added, noting their device came with a timer that shut the machine off after 15 or 45 minutes.
Hunter just gave the sleep machine to a friend who is expecting her first child.
Papsin hopes she and other parents using the machines make note of his findings and the study's recommendations.
For parents intent on using a sleep machine, they recommend that the device be situated as far as possible from the infant, never in or attached to the crib. He also urges people to use the machines at the lowest possible volume for the shortest period of time.
The study also suggests manufacturers should have to take steps to make the machines safer. Those include limiting the maximum volume output, printing warnings of the potential for hearing loss on packaging and building a timer into all models so that they can be programmed to turn off after a set period.
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