The authors inform that theirs is the first study to examine a link between stroke risk and traffic noise. In a study involving 51,485 individuals, the scientists worked out that the risk of stroke rose by 14% for every 10 decibel rise in noise levels.
On closer examination, they found that the risk of focused on individuals aged over 65 years. There was no statistically significant increased risk for those under 65, but a 27% higher risk of stroke for over 65s for every 10 decibel rise in noise.
The risk of stroke among the elderly rose at a much higher rate when decibel levels were over 60.
Study leader, Dr Mette Sørensen, at the Institute of Cancer Epidemiology, Danish Cancer Society, said:
"Our study shows that exposure to road traffic noise seems to increase the risk of stroke. Previous studies have linked traffic noise with raised blood pressure and heart attacks, and our study adds to the accumulating evidence that traffic noise may cause a range of cardiovascular diseases. These studies highlight the need for action to reduce people's exposure to noise.
"This is the first study ever to investigate the association between exposure to road traffic noise and risk of stroke, and, therefore, more research is needed before any firm conclusions can be made."
They had based their study on the Diet, Cancer, and Health cohort study, which included individuals aged from 50 to 64 (1993-1997). Of the 57,053 people on the database, they gathered medical and residential histories on 51,485 of them. They were followed up, on average, for ten years. 1,881 of them suffered a stroke during that ten-year period.
The scientists explained that they factored out several variables which had the potential to distort their findings, such as air pollution, aircraft and railway noise, smoking status, eating habits, and alcohol and caffeine consumption.
They linked a noise calculation program to study participants and their addresses. The program had been mapping noise levels in several Scandinavian areas for a number of years. The program takes into account traffic speed, composition, road type, road surface, building polygons and where exactly people's homes were in relation to the roads (height, position, etc.).
35% of individuals were exposed to noise levels of at least 60 decibels at time of joining. 72% of them remained at their same address throughout the whole study period. Noise exposure in this study ranged from 40 to 82 decibels.
Dr Sørensen said:
"If we assume that our findings represent the true risk, and the association between traffic noise and stroke is causal, then an estimated eight percent of all stroke cases, and 19% of cases in those aged over 65, could be attributed to road traffic noise. The population in this study, however, lived mainly in urban areas and is, therefore, not representative of the whole population in terms of exposure to road traffic noise. However, if we take the exposure distribution of all dwellings in Denmark into account, we find that about 600 new cases of stroke could be attributed to road traffic noise in Denmark each year. There are 5.5 million inhabitants in Denmark and a total of 12,400 new cases of stroke each year."
This is an epidemiological study, and cannot therefore demonstrate that the cause of higher stroke risk is traffic. All it can do is reveal a link. Epidemiology examines the patterns of health and disease and associated factors at population level.
The authors say that they are not certain why this link exists.
Dr Sørensen said:
"The mechanisms involved are probably the same mechanisms believed to be involved in noise-induced hypertension and heart attacks, namely that noise acts as a stressor and disturbs sleep, which results in increased blood pressure and heart rate, as well as increased level of stress hormones. Taken together, all of these could increase the risk for cardiovascular diseases.
In addition, older people tend to have more fragmented sleep patterns and are more susceptible to sleep disturbances. This could explain why the association between road traffic noise and risk of stroke was seen mainly in the oldest participants."
The team say they plan to carry out future studies to find out whether there might be a link between traffic noise and hypertension (high blood pressure) and some cardiovascular diseases.
"Road traffic noise and stroke: a prospective cohort study"
Mette Sørensen1, Martin Hvidberg, Zorana J. Andersen, Rikke B. Nordsborg, Kenneth G. Lillelund, Jørgen Jakobsen, Anne Tjønneland, Kim Overvad, and Ole Raaschou-Nielsen
Eur Heart J (2011) doi: 10.1093/eurheartj/ehq466
Written by Christian Nordqvist