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Increasingly, health researchers are realizing that noise pollution is more than just a nuisance. A 2012 study found that exposure to the sounds of car traffic can raise the risk of heart attack in people over 50. A more recent study reported that it increases the risk of obesity. Still other work has linked city noise to impaired sleep.

But while these and other studies identify the effects of traffic noise on our bodies, few have looked at how it impacts our minds. New research, published in Environmental Health Perspectives, does just that—providing strong evidence that noise pollution is indeed a mental health problem. The study found that people living in areas with high traffic noise were 25 percent more likely than those in quieter neighborhoods to have symptoms of depression, even when adjusting for socioeconomic factors.

Using data from an ongoing population study, researchers in Germany looked at 3,300 people who showed no signs of depression when they were first surveyed between 2000 and 2003. All lived in three of the most densely populated cities in Western Germany. After participants retook the survey, five years later, the researchers looked for self-reported depressive symptoms, such as feelings of loneliness or sadness, as well as troubles with sleep or concentration. They also looked at whether the participants were taking anti-depressants.

When they compared that data with the noise levels of each participant’s neighborhood, they found that more than a third of the sample were exposed to traffic noise at 55 decibels—equivalent to the noise level of an old dishwasher or chatter in a restaurant—or higher 24 hours a day. About a quarter were exposed to these levels only at night.

Participants with the most depressive symptoms were exposed to “intermediate” traffic noise 24 hours a day.

The mental health effects didn’t differ between those who experienced loud traffic noise around the clock and those who only heard it at night (partly because researchers didn’t have information about what noises each person was exposed to outside their homes). But they did find that participants who reported the most depressive symptoms were not the ones living in the loudest neighborhoods. That distinction went to those who were exposed to “intermediate” traffic noise of 60 to 65 decibels for 24 hours a day.

It could be that people living in extremely loud areas are more likely to take measures to block the noise, Ester Orban, an epidemiologist at University Hospital Essen and lead author of the study, tells CityLab.

The groups most at risk of developing depressive symptoms were those who had lower income and education levels, and who were less likely to be employed. It could be that people who fit into those groups are more likely to live in louder neighborhoods, though the researchers say that particular link needs further study. “Low-income groups, which have been studied before, are more likely to have depression,” Orban says. “But we can’t say why the association between noise and depressive symptom is stronger in this population.”

Orban adds that the link between noise and depression remains strong even after controlling for income and occupational status, which suggests the effect of noise on depression may be independent of monetary factors.

The researchers also saw a stronger correlation with depressive symptoms among those who reported sleep disruptions like insomnia in the first survey, suggesting that impaired sleep is a possible gateway to depression. “Insomnia is risk factor for depression and may be an indicator of mild depression that doesn't quite show up on the CES-D depressive symptom scale,” says Stephen Stansfeld, a psychiatrist at the University of London and author of the book Noise, Noise Sensitivity and Psychiatric Disorder, who wasn’t involved in the study.

Stansfeld says the the most significance recommendation to come out of this study is that more studies are needed. Researchers have evidence that noise leads to anxiety, and that it causes the release of the stress hormone cortisol, but scientists can say very little about its effect on mental health. That makes it hard to come up with public health policies or even better street designs.

”There's very clear evidence that transport noise causes annoyances,” he says, “so it’s always a bit strange not to find strong relationships with mental health.” But of course, he says, mental health outcomes have "lots of potential causes."

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