TUESDAY, Jan. 25 (HealthDay News)-- Prolonged exposure to loud traffic noise is strongly associated with stroke in people aged 65 and older, a new Danish study finds.

The researchers discovered that for every 10 decibels or more of road noise, a person's risk of having a stroke increased by 14 percent.

For people younger than 65, that did not translate into a statistically significant risk, but researchers found that among those 65 or older, stroke risk increased significantly, by 27 percent for every additional 10 decibels of noise. Also, in older people the researchers found evidence of a threshold limit around 60 decibels, above which the risk of stroke increased still more.

The study involved more than 57,000 people between the ages of 50 and 64 living near Copenhagen and Aarhus who were recruited for the Danish Diet, Cancer and Health cohort study between 1993 and 1997, according to study author Mette Sorensen, senior researcher at the Institute of Cancer Epidemiology of Danish Cancer Society in Copenhagen. The average follow-up time was 10 years.

"Although our study was the first study on traffic noise and stroke, I was not surprised of the results as earlier studies have found traffic noise to be associated with other cardiovascular diseases," Sorensen said. The study showed only an association between loud noise and stroke, she noted, adding that more research would need to be done to confirm the findings.

The study authors noted that their study took place in an urban area and so was not representative of the whole country. But by taking noise exposures at different dwellings into account and applying the findings across Denmark, Sorensen estimates that 600 new cases of stroke each year in her country could be attributed to road traffic noise. The 5.5 million inhabitants of Denmark suffer 12,400 new cases of stroke annually.

Although the study does not determine how road noise influences health, Sorensen believes stress is likely a factor.

"Exposure to traffic noise is believed to provoke a stress response and disturb sleep, which might increase the risk for stroke, through mechanisms including increased level of stress hormones, increased heart rate and blood pressure and impaired immune system," Sorensen said.

Researchers analyzed participants' medical and residential histories during an average 10-year period. A total of 1,881 suffered a stroke.

The researchers then measured their exposure to traffic sound over time using a noise calculation program designed to map noise levels in a variety of locations in Scandinavia for several years. The program takes into account a variety of factors that could amplify or blunt road noise, including traffic composition and speed, road surface and road type, nearby buildings and the position and heights of homes above the roads.

At the time the volunteers joined the study, 35 percent of them were exposed to noise levels greater than 60 decibels, and the majority (72 percent) lived at the same address throughout the study. Researchers estimated the lowest noise exposure to be 40 decibels and the highest to be 82 decibels.

They also accounted for a variety of other risk factors for stroke, such as air pollution, railway and aircraft noise, smoking, diet, alcohol and caffeine consumption.

The study is published in the current issue of the European Heart Journal.

The researchers "did a reasonable job of trying to control for other potential things that could affect the outcome," said Dr. Larry B. Goldstein, director of the Duke Stroke Center at Duke University in Durham, N.C.

However, Goldstein noted that there are likely a number of additional factors that could not have been accounted for. "These epidemiological studies are always subject to a variety of unmeasured biases that could potentially affect the outcome," he said.

For example, people living in areas with loud traffic noise are also likely have better access to fast food restaurants, and probably have a lower socioeconomic status than people in quieter suburban and rural areas, he noted. (The researchers also reported that a higher percentage of those exposed to noise over 60 decibels were from a lower income group, and since lower socioeconomic class is itself a predictor of stroke, they concluded they could not rule out the possibility that that skewed the results.)

"The other thing is, what do you do about it even if it's true? Move to electric cars and no horns?" Goldstein added.

The researchers had an answer for that.

Sorensen said the study could be used to argue for construction methods that limit exposure to traffic noise, such as roads built with noise-reducing asphalt and noise barriers, or homes built with noise-isolating windows and other materials that dampen loud sounds.

More information

To find out more about how loud noise affects your health, visit the World Health Organization.