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Traffic noise implicated in stroke risk

Exposure to noise from road traffic can increase the risk of stroke in the over 65s,” reported The Daily Telegraph. It said that researchers have found that for every 10 decibel (dB) increase in noise, the risk of stroke increases by more than a quarter (27%).

The aim of this research was to see if exposure to traffic noise is linked to the risk of having a first stroke. The researchers followed 57,000 people between the ages of 50 and 64 for an average of 10 years. Over this time, 1,881 people had their first stroke. People aged 65 or older who were exposed to louder traffic noise had a greater risk of stroke.

This study has shown a link rather than a cause, which needs more exploration. Although it seems plausible that noise might increase stroke risk, especially very high levels of noise, via raised blood pressure or lack of sleep or in other ways, this is quite a weak link. In addition, the people who lived nearer busier roads and were exposed to higher noise levels tended to be on a low income. As socioeconomic status is a known predictor for stroke, further research would need to take this into account.

 

Where did the story come from?

The study was carried out by researchers from the Institute of Cancer Epidemiology, the Environmental Research Institute, Rambøll Danmark, the Danish Environmental Protection Agency, and from Aarhus Hospital and Aarhus University Hospital in Denmark. It was supported by the Danish Environmental Protection Agency, the Research Centre for Environmental Health, the Danish Ministry of the Interior and Health and the Danish Cancer Society. The research paper was published in the peer-reviewed European Heart Journal.

The newspapers report the results accurately, quoting one researcher as saying that the strokes were probably caused by noise disturbing sleep, which in turn raised stress hormones and blood pressure. Cohort studies such as this cannot prove causation but an association between noise and stroke has been demonstrated previously. The researchers say: “As this is the first study of its kind, the results need to be confirmed by other studies before any conclusions can be drawn.”

 

What kind of research was this?

The researchers explain that other studies have suggested that long-term exposure to noise from traffic increases the risk of disorders such as heart disease. However, the exact mechanism and strength of the link is unknown. The aim of this study was to investigate the link between people’s exposure to road traffic noise and their risk of having a first stroke.

This cohort study was well-conducted, collecting a large amount of data over a 10-year period. There are many related factors that contribute to stroke risk. Blood pressure, lifestyle choices, socioeconomic factors and pollution could each or all have had an effect, and the researchers were only able to adjust for some of these. This explains their cautious conclusion that because of the chance of “confounding” factors and the fact that this is the first study of its kind, the results will need checking and corroborating elsewhere in other studies.

 

What did the research involve?

This prospective cohort study was based on the Diet, Cancer, and Health Study, which enrolled people living in the Copenhagen or Aarhus area of Denmark between 1993 and 1997. At enrolment, a random sample of 160,725 people who were free of cancer, born in Denmark and aged between 50 and 64 at the time of invitation were recruited to this study.

Of these, 57,053 people accepted the invitation and completed self-administered questionnaires about lifestyle habits, including smoking, coffee and alcohol intake, diet (by a food frequency questionnaire), health status and other social factors. Trained research staff measured blood pressure, height and weight.

The researchers assessed the participants’ lifetime exposure to noise by taking the complete history of all the addresses where they had lived. This was available for 53,162 of the 57,053 cohort members. Exposure to road traffic noise was calculated for the years 1990, 1995, 2000 and 2005 based on the 61,873 addresses given by the participants.

Noise exposure was estimated using a software programme that predicts noise based on factors such as the floor height of the building, the annual average daily traffic volume and speed, and the road type. Digital maps of expected road and rail noise were produced and the location of each house plotted so that the yearly average (mean) exposure could be estimated. No direct measurements of noise were made.

Stroke events among participants were identified by linking each participant to the Danish National Hospital Registry containing all non-psychiatric hospital admissions in Denmark since 1977 and patients discharged from emergency departments and outpatient clinics from 1995.

The researchers defined a set of possible confounders before they began the study and adjusted for these appropriately in their analyses. This included collecting data on smoking, intake of fruit, intake of vegetables, education, alcohol intake, body mass index and physical activity. They also adjusted for age and blood pressure and address-specific information such as municipality income (average income for the area) and the air pollution in the geographical area. This data was available for 51,485 people.

 

What were the basic results?

Of the 51,485 participants for whom complete data existed, 1,881 (3.7%) were admitted to hospital for their first stroke. The average length of time of follow-up time was 10 years.

The risk of first stroke increased by 14% for every 10 dB increase in road traffic noise, in the range of 55 to 75 dBs among all participants, after adjustments were made for possible confounders (incidence rate ratio (IRR) 1.14 for stroke, 95% confidence interval (CI) 1.03 to 1.25).

The participants’ ages affected the strength of this link, and the association between road traffic noise and stroke was stronger in people over 64.5 years old (IRR 1.27, 95% CI 1.13 to1.43). There was no statistically significant association between exposure to noise and stroke risk for people under 64.5 (IRR, 1.02; 95% CI 0.91 to 1.14).

 

How did the researchers interpret the results?

The researchers say that their study shows a “positive association between residential exposure to road traffic noise and risk for a stroke in a general Danish population among people older than 64.5 years of age”.

They are cautious in their conclusions and call for further studies, saying that as this is the first study of its kind, the results need confirming and they “cannot exclude the possibility of residual confounding by socioeconomic differences”.

 

Conclusion

This study has shown a link rather than a cause, which needs more exploration. Although it seems plausible that noise might increase stroke risk, especially very high levels of noise, via raised blood pressure or lack of sleep or other theoretical mechanisms, this is quite a weak link. As the researchers acknowledge, these differences could be due to unmeasured or incompletely adjusted factors that also increase stroke risk. The researchers point out some other strengths and limitations to their study:

  • The findings are made more robust with the adjustment for air pollution, which is known to correlate with road traffic noise, as both air pollution and noise increased the nearer the participants lived to busy roads.
  • They acknowledge the link between stroke and socioeconomic status by saying that there was a higher proportion of people on a low income among those exposed to high noise levels. As socioeconomic status has been found to be a predictor for stroke, this potential confounder needs further exploration.
  • A potential weakness is that the noise estimates were based on modelled geographic averages and not on measured values.
  • The researchers also say that they had information only on residential addresses and not, for example, work or holiday-home addresses. This could have affected the accuracy of the modelled noise levels.

Overall, the results will be of interest to researchers, but will need to be repeated in further studies (ones preferably fully adjusted for socioeconomic factors) before this association is clearer.

 

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