Thursday September 10 2009
Researchers did not measure the road noise levels
“Living near a busy road is more likely to cause stress and raised blood pressure,” The Daily Telegraph has reported. A study involving almost 28,000 people in Sweden has shown that exposure to traffic sounds measuring more than 60 decibels was associated with high blood pressure among the relatively young and middle-aged. High blood pressure is an important risk factor for cardiovascular disease such as stroke or heart attack. The researchers note that elderly people seemed to be least affected.
The survey was large and the effect measured was statistically significant. Unfortunately, there was a high non-response rate to the survey and the potential for inaccuracy as participants reported whether they had ever been told they had blood pressure or if they used blood pressure treatments. These types of limitations, and the fact that the effect of noise on heart disease and stroke were not measured, mean the importance of a 60-decibel threshold as a safety limit is not clear.
Where did the story come from?
This research was carried out by Dr Theo Bodin and colleagues from the Department of Occupational and Environmental Medicine at Lund University Hospital in Sweden. The study was funded by the Swedish Council for Working Life and Social Research. It is currently available online in a provisional, press-only version and is awaiting full publication in the journal Environmental Health.
What kind of scientific study was this?
The researchers explain that studies on exposure to road traffic noise and blood pressure have so far produced mixed results and they wanted to test the supposed link further.
They designed a large cross-sectional survey that collected data from a sample of residents in southern Sweden. They surveyed people aged 18-80 years old living in the Scania region at the end of June 2004. Out of 855,599 people living in this area, the researchers mailed their questionnaire to 46,200, and randomly selected 2,800 people to answer the questionnaire by phone interview. Around 41% of people who the questionnaire was mailed to did not respond, leaving data on 24,238 adults.
The researchers used the participants’ residential addresses as the basis for estimating the average road noise, cross-referencing it with data from a previous mapping of road noise from the Road Administration authority. They did not measure the road noise levels for the purpose of their study.
Road noise levels were adjusted to account for surface type and other factors, producing an estimate for each participant known as the A-weighted equivalent noise level, over a full day (LAeq 24hr). This estimate was measured in decibels and applied to the participant’s current residential address.
Effects on self-reported blood pressure was assessed if the participants answered yes to the questions: 'Do you have hypertension?', or 'Have you, during the last three months, used any drug or preparation against hypertension?' The researchers then measured the association between noise exposure levels and whether or not people reported blood pressure problems. They adjusted the results to account for factors such as sex, age, BMI, alcohol intake, exercise, education, smoking and socioeconomic status.
What were the results of the study?
The researchers found that more people answered yes to the questions about hypertension once their estimated noise exposure increased to more than 64 dB(A) (odds ratio [OR] = 1.45, 95% confidence interval [CI] 1.04 – 2.02).
When looking at different thresholds of traffic noise among the middle-aged participants (40 to 59 years old) there was a stronger link with noise levels above the threshold of 64 dB(A) (OR = 1.91, 95% CI 1.19 – 3.06) than at the range of 60 to 64 dB(A) (OR = 1.27, 95% CI 1.02 – 1.58)
An effect was also indicated among younger adults but not among the elderly.
What interpretations did the researchers draw from these results?
The researchers say that the study supports an association between road traffic noise at high average levels and self-reported high blood pressure in middle-aged people.
What does the NHS Knowledge Service make of this study?
- This was a large study using an extensive questionnaire and the researchers say they have made adjustments for confounders. Confounders are the other factors associated with blood pressure that, if distributed unevenly among the groups, may make the link appear real when it is not. By taking these factors into account in their analyses, the researchers hoped to reduce this bias.
- By not telling the participants what this sub-study was looking for they ensured that the chance of introducing reporting bias into the study was low.
- Unfortunately, there was a low response rate of 59% in this study that may have led to response bias (those who responded may be systematically different from those who did not). The researchers looked at broad features of the non-responders of this study in another publication and say that it is likely that the selective participation contributed to an underestimation of the prevalence of hypertension, which has a high chance of being under-reported anyway, due to the use of self-reporting. Any misclassification would have reduced the size of the effect seen in the study.
Overall, this large study adds to the knowledge about the effects of road noise on health and the extent of the risk to health. However, any conclusions or interpretation about the extent of risk on heart disease or stroke are unwarranted as blood pressure itself was not measured.