Even moderate noise may harm hearing

Chronic, low-level sound exposure causes deficits in rats

Constant low-level noise might cause hearing problems, a new study in rats finds. The discovery, published online May 15 in Nature Communications, suggests that extended exposure to noise at levels usually deemed safe for human ears could actually impair sound perception.

Maps of the part of the brain that processes sounds, called the auditory cortex, show that  rats exposed to low-level noise have fewer nerve cells that respond to highly-pulsed sound patterns than rats that lived the quiet life, a new study shows. X. Zhou

The findings are “definitely a warning flag,” says study coauthor Michael Merzenich, an integrative neuroscientist at the University of California, San Francisco. He adds that it will be important to find out whether people employed at factories where continuous low-intensity noise is emitted throughout the workday experience similar consequences.

“The big picture is that there is no safe sound,” says Jos Eggermont, an auditory neuroscientist at the University of Calgary in Canada. Even sounds considered safe can cause damage if delivered in a repetitive way, he says. “There might be not-so-subtle effects that accumulate and affect communication and speech understanding.”

It’s common knowledge that sustained exposure to louder noises — such as that above 85 decibels — or brief exposures to very loud noises above 100 decibels can cause inner ear damage and hearing impairments. But until recently, the impact of chronic, quieter sound hasn’t been well studied.

In the new study, Merzenich and his colleague Xiaoming Zhou of East China Normal University in Shanghai exposed adult rats to 65 decibel sound — roughly at the higher end of normal human speech volume — for 10 hours daily. Because low, monotone hums don’t usually excite the brain, the researchers delivered the noise to rats in three to 18 pulses per second. The researchers also exposed another group of rats to similar low-level sound over 24 hours.

After two months of sound conditioning, the noise-exposed rats did not perform as well on listening tests compared with animals that lived a quieter life, the scientists found. The tests — which involved distinguishing a sequence of 6.3 pulses per second from another sound pattern of 20 pulses per second — assessed how well the animals could pick out slight variations in sounds, which is important for processes like speech understanding.

In exposed rats, nerve cells in the part of the brain where sounds are processed tended to be less responsive to the more rapid sound pulses than nerve cells in unexposed rats. The noise-exposed animals also had fewer nerve cells that are involved in detecting sharp sound and tended to have higher levels of a protein that’s involved in reshaping how the brain handles information. This finding could mean that the brains of these animals are being altered by sound conditioning.

“These are not necessarily lifelong impairments but they are significant ones and it’s a cause for concern,” says auditory neuroscientist Larry Roberts from McMaster University in Hamilton, Canada.

Still, Eggermont cautions that it’s not clear to what extent these findings can be applied to humans. Although some workplace sounds are repetitive, they aren’t as constant as the ones in the experiments. And employees are exposed to a range of other sounds outside of work, such as music and television, that could also affect their hearing.

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