Exposure to traffic noise linked to higher dementia risk
Long-term exposure to noise from traffic on roads and trains is related with an increased chance of acquiring dementia, particularly Alzheimer's disease, according to a Danish study published today in The BMJ.
According to the researchers, up to 1,216 of the 8,475 dementia cases documented in Denmark in 2017 could be connected to these noise exposures, showing a significant potential for dementia prevention through noise reduction.
By 2050, the worldwide dementia population is predicted to exceed 130 million, creating a costly and expanding global health catastrophe. Apart from well-established risk factors for dementia, such as cardiovascular disease and an unhealthy lifestyle, environmental exposures may also contribute to dementia development.
Transportation noise is considered the second greatest environmental risk factor for public health in Europe, second only to air pollution, and almost a fifth of the European population is exposed to noise levels above the recommended range of 55 dB. (decibels).
Transportation noise has been regularly related to a variety of diseases and health concerns, including coronary heart disease, obesity, and diabetes. However, there is a dearth of research on the relationship between transportation noise and dementia, and the findings are conflicting.
To address this, researchers examined the relationship between long-term residential exposure to road traffic and train noise and the risk of dementia among Denmark's two million over-60 persons between 2004 and 2017.
The researchers calculated the amount of road traffic and train noise on the most and least exposed sides (or façades) of all Danish residential properties.
They then analysed national health registers for cases of dementia due to any cause and several forms of dementia (Alzheimer's disease, vascular dementia, and Parkinson's disease-related dementia) over an average of 8.5 years. 103,500 additional instances of dementia were discovered during the study period.
After adjusting for potential confounding variables associated with residents and their neighbourhoods, the researchers discovered that a 10-year average exposure to road traffic and railway noise on the most and least exposed sides of buildings was associated with an increased risk of all-cause dementia.
These relationships demonstrated a general trend of increased risk associated with increased noise exposure, but with a plateau or even a slight drop in risk at higher noise levels.
Further analysis by type of dementia revealed that both road traffic and railway noise were related with an increased risk of Alzheimer's disease - up to 27% higher for exposure to 55 dB road traffic noise and up to 24% higher for exposure to 50 dB railway noise, compared to less than 40 dB.
However, only road traffic noise, not railway noise, was associated with an elevated risk of vascular dementia.
Possible explanations for noise's influence on health include the release of stress hormones and sleep disruption, which can result in a type of coronary artery disease, immune system alterations, and inflammation - all of which are thought to be precursors to dementia and Alzheimer's disease.
Because this is an observational study, it cannot establish cause and effect, and the authors acknowledge several limitations, including a lack of information about lifestyle habits that may contribute to a person's risk of developing dementia, and a lack of information about factors such as sound insulation in homes that may affect personal exposure to noise.
The study's benefits, however, are its large sample size, lengthy follow-up period, and high-quality measurement of noise exposure from two distinct modes of transportation.
As a result, the authors conclude: "If these findings are replicated in subsequent investigations, they may have a significant impact on the calculation of the disease burden and healthcare expenditures associated with transportation noise." Expanding our understanding of the detrimental impacts of noise on health is critical for prioritising and implementing effective policies and public health programmes aimed at disease prevention and control, including dementia.”
Although this large and comprehensive study has significant strengths, it does not paint a complete picture of the potential harm to the ageing brain from long-term exposure to noise, such as from airports, industrial activities, or occupational exposure, according to US researchers in a linked editorial.
They add that noise may also affect the likelihood of developing other chronic conditions such as hypertension, implying that noise contributes indirectly to dementia risk.
The widespread and significant exposure to noise on a global scale, the severity of the associated health consequences, and the limited tools available to individuals to protect themselves all strongly support the WHO's argument that "noise pollution is not only an environmental nuisance but also a threat to public health," they write.
“Improving public health by reducing noise through transportation and land use policies or building rules should become a priority,” they conclude.