The louder the traffic near people's homes, the greater their risk of heart attack, a new study from Denmark says.

The researchers tracked more than 50,000 study participants for nearly 10 years and found that for every 10 decibels of added roadway traffic noise, the risk of heart attack increased 12 percent.

"We think traffic noise  during the night is especially dangerous, because it disturbs sleep," said lead researcher Mette Sorenson of the Danish Cancer Society. But anytime you’ve been exposed to high levels of noise, "you have increased concentrations of stress hormones in your body,” which could explain the increased heart attack risk, Sorenson said.

Sorenson and her fellow researchers found that the link between heart attacks and roadways held even after accounting for the heightened levels of air pollution near roadways. They estimated that 4 percent of all heart attacks in Denmark are related to traffic noise.

Sorenson suggested choosing a room with a low exposure to traffic noise for sleeping in, or insulating one’s house against noise. It is also possible for officials to pave highways with low-noise asphalt, she said.

The real danger with noise pollution  is that most people don’t realize they are experiencing it, Sorenson said.

“You might wake up thinking that you had a quiet night, but when you look at it in a lab, you see that your sleep stages have been disturbed,” she said.

For the study, the researchers asked participants to report where they lived and whether they had ever had a heart attack, along with other information, including their diets and physical activity habits. The participants’ addresses were used to assess the noise they experienced.

The researchers also accounted for factors that could affect participants' risk of heart attack, including gender, smoking, fruit and vegetable intake, and body mass index.

Noise pollution  is not generally recognized as a health hazard, said Sally Lusk of the University of Michigan, adding that Europeans are generally more concerned about noise levels than U.S. residents are.

Lusk's own research has shown that exposure to high noise levels raises blood pressure; she said the new study's results did not surprise her.

“Almost everyone is listening to something that is louder than it should be,” she said,.

Noise pollution tends to be higher in cities, but Sorenson emphasized that it is possible to “live very quietly in a city but very noisily in a rural area,” particularly depending on proximity to highways.

While the link between noise pollution and heart attack risk has been shown before, the new study is one of the first to demonstrate an incremental correlation between increasing noise and increasing risk. Previous studies have shown that risk increased at noise levels above 60 decibels; this study showed that risk increased between 40 and 80 decibels.

Ten decibels of noise  is enough to interrupt a conversation, while 85 decibels is the minimum level at which hearing protection is required in a workplace, Lusk said.

The study was published today (June 20) in the journal PLoS ONE.

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