Traffic noise and stroke risk: Study

By Dr. Ananya Mandal, MDJan 26 2011

A new study shows that traffic noise may increase the risk of stroke, particularly for older individuals. The study looked at the association between road traffic noise and stroke in more than 51,000 Danish people and found that for every 10-decibel increase in noise level, the risk of stroke increased by 14 percent. In those over 65, the risk of stroke increased 27 percent.

Researchers found that the increase in risk was most significant for noises louder than 60 decibels, about the loudness of normal conversations. The decibel (dB) scale is logarithmic, so a 60-decibel noise is 10 times louder than a 50-decibel noise.

The study is published on Jan. 26th in the journal European Heart Journal.

This adds to the list of ills that noise pollution can cause. Previous studies have linked traffic noise with an increased risk of heart attacks and high blood pressure. The new study is the first to examine the connection between traffic noise and stroke. Researchers carefully adjusted for other factors that may lead to a high stroke risk in participants like age, gender, smoking status, physical activity, intake of vegetables.

Study researcher Mette Sørensen, of the Institute of Cancer Epidemiology in Copenhagen says this should alert authorities to reduce public exposure to noise. Further studies will be needed to confirm traffic noise ass a contributor to stroke risk Dr. Sørensen added.

The team, for the study analyzed data from Danish citizens in Copenhagen and Aarhus who completed a lifestyle questionnaire between 1993 and 1997, and examined the participants’ medical histories and residential addresses. Road traffic noise was estimated using a model that took into account traffic composition, speed, road surfaces and the heights of homes relative to the roads.

About 35 percent of the participants were exposed to traffic noise levels above 35 decibels. The noise level was estimated to range from about 40 to 80 decibels. Over the course of the study, 1,881 participants suffered a stroke. The researchers estimated that traffic noise could account for 8 percent of all stroke cases in the population, and 19 percent of cases in those aged 65 years and older, Sørensen said.

As an explanation to the risk authors write that noise can raise blood pressure and cause changes in levels of stress hormones, which may contribute to the increased risk of stroke. Also it can lead to sleep disturbances, which can contribute to stroke risk. Older people are more susceptible to sleep disturbances, which could explain why the link was strongest for those 65 and over. Sørensen added, “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.”

The 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.

Dr. Larry B. Goldstein, director of the Duke Stroke Center at Duke University in Durham, N.C. said the team “did a reasonable job of trying to control for other potential things that could affect the outcome” but there are 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. One of the major factors could be place of residence. Like 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.