"Living in a neighbourhood with noisy road traffic may ... increase the risk of stroke," The Guardian reports. Researchers looked at noise levels across London and found a link between high levels of noise and increased risk of hospital admission for stroke, with the risk slightly higher in older people.
This ecological study included the 8.6 million inhabitants of London and assessed day and night-time exposure to levels of road traffic noise in excess of 55 decibels (dB), which is roughly equivalent to the sort of background conversation you would hear in a restaurant.
This study is now complete and has found some small associations, mainly in terms of increased stroke risk. The population level findings observed were small and could not account for all possible confounders. They may also not represent findings on an individual level.
There are steps you can take to compensate for any small increased risk of stroke, such as eating a healthy diet, taking regular exercise, stopping smoking if you smoke and sticking to the recommended guidelines for alcohol consumption.
Read more about stroke prevention.
Where did the story come from?
The study was carried out by researchers from the London School of Hygiene and Tropical Medicine, Imperial College London, Imperial College Healthcare Trust, and Kings College London. Funding was provided by the UK Natural Environment Research Council; Medical Research Council; Economic and Social Research Council; Department of Environment, Food and Rural Affairs; and the Department of Health.
Generally, the UK media reported the story accurately, with most sources making clear that a cause and effect relationship had not been proven, and that more research is needed.
What kind of research was this?
This was an ecological study designed to assess whether higher noise levels are associated with greater risk of cardiovascular disease and death at a population level. This study design is suitable for assessing this kind of research question, but will not provide conclusive answers.
What did the research involve?
The study included the 8.61 million inhabitants of London (within the M25) from 2003 to 2010. It examined the effects of their exposure to road traffic noise, independent of air pollution, on all-cause cardiovascular death, as well as on cardiovascular hospital admissions in adult and elderly populations.
Associations of day (7:00 to 22:59) and night-time (23:00 to 06:59) road traffic noise with cardiovascular hospital admissions and all-cause and cardiovascular death in all adults (≥25 years) and elderly (≥75 years) were assessed through modelling. The researchers made adjustments for the possible confounding effects of:
- area-level socioeconomic deprivation
- air pollution
- “neighbourhood spatial structure” – the actual physical environment of the region being studied
Traffic noise exposure was categorised in five-decibel increments:
- less than 55 (reference)
- 55 to 60
- more than 60
Hospital admission data was taken from Hospital Episode Statistics and are held by the UK Small Area Health Statistics Unit (SAHSU). Death and population data was supplied by the Office for National Statistics, derived from the national mortality registrations and the Census, and are held by SAHSU.
For assessing outcomes, the first registered emergency hospital episode of each year for all cardiovascular causes, coronary heart disease and stroke were used.
Deaths were classified according to the underlying cause on the death certificate; causes included in this analysis were from all natural causes, all cardiovascular causes, coronary heart disease and stroke.
Data was also collected of the person’s age, sex and postcode of residential address at the time of admission or death.
What were the basic results?
The total number of hospital admissions from cardiovascular causes was 400,494 among adults, and 179,163 among the elderly. There were 442,560 adult and 291,139 elderly deaths.
The average (median) daytime exposure to road traffic noise was 55.6dB.
Daytime road traffic noise increased the risk of hospital admission for stroke by 5% in adults, and 9% in the elderly in areas >60 compared with <55dB (baseline). Similar levels were observed when comparing 55 to 60dB to baseline; this was 4% in adults and 6% in the elderly. A small increased risk of hospital admissions for all cardiovascular diseases was seen in the elderly group exposed to daytime road traffic noise of 55 to 60dB when compared to the lower level group, but not for above 60dB.
Night-time road traffic noise of between 55 and 60dB was associated with a 5% increased risk of stroke among the elderly. Levels above this were not significant.
Daytime road traffic noise was associated with a 3-4% increased risk of death from any cause in adults and the elderly in areas exposed to more than 55dB.
How did the researchers interpret the results?
The researchers conclude: “Results suggested small increased population risks of all-cause mortality and cardiovascular mortality and morbidity, particularly of stroke in the elderly, at moderate levels of road noise exposure”.
This modelling study has examined the associations of exposure to traffic noise, independent of air pollution, on all-cause and cardiovascular mortality, as well as on cardiovascular hospital admissions in adult and elderly populations.
It has shown a link between increased noise from traffic pollution and risk of hospital admission for stroke and death. Possible reasons for deaths were most likely to be linked to heart or blood vessel disease, which could be due to increased blood pressure, sleep problems and stress from the noise.
The limitations of this study are that the exposure model used is likely to overestimate noise at low exposure levels and underestimate noise in areas with heavy traffic on minor roads. This may result in bias when analysing dose-response relationships.
The model did not take into account population activities, such as working and commuting outside residential areas, or residence characteristics, such as windows towards roads or building materials. The researchers did not have data on residential histories, which may have introduced further exposure misclassification.
Associations found in this study are in agreement with some, but not all, other previous work in this area, so caution should be taken with interpreting this small increased risk. There was often a lack of dose-response relationship, which requires further investigation. The whole populations study used London inhabitants as their population, which may reduce the ability to generalise the findings to other populations and also on an individual level.
If this association was found to be true, changes would have to be made by legislation; however, to reduce your own risk of cardiovascular disease, it is important to make the right lifestyle choices which protect against both heart disease and stroke.
Read more about cardiovascular disease prevention.