ODENSE, Denmark —According to new research, noise pollution can significantly increase the risk of developing Alzheimer’s disease. Scientists say road and railway traffic alone accounts for an astonishing one-in-seven dementia cases. Constant racket from cars, buses, and trains, as well as dirty air, speeds up the brain’s decline. This leads to the conclusion that people who live in busy urban areas are much more vulnerable.
“Long-term residential exposure to road traffic and railway noise was associated with increased risk of dementia – especially Alzheimer’s,” corresponding author Dr. Manuella Lech Cantuaria and researchers write in The BMJ.
Estimates warn that the global number of dementia patients will triple to 150 million by 2050. With no cure in sight, there is an increasing focus on preventative lifestyle changes patients can make.
The findings suggest noise prevention strategies would help combat the costly and growing health crisis. The results come from a review of more than two million Danish residents over the age of 60, who researchers tracked for up to 14 years. It is the biggest analysis of its kind.
How loud is too loud?
Those exposed for a decade to traffic noise of 55 decibels (dB) – the recommended limit – were 27 percent more likely to develop Alzheimer’s. Rumbling from railways of only 50 dB was enough to raise the risk by 24 percent — in comparison to others with homes in areas hearing less than 40 dB.
Noise disturbs sleep and releases vessel blocking stress hormones into the bloodstream, thereby reducing oxygen to the brain. Dr. Cantuaria explains that it can also lead to changes in the immune system and inflammation – which are all risk factors for the onset of dementia.
The team followed the Danish residents between 2004 and 2017. During that time, doctors diagnosed 103,500 with dementia. Study authors estimated road traffic and railway noise at their highest and lowest levels for all residential addresses in Denmark. They then used national health registers to identify new cases including Alzheimer’s, vascular dementia, and Parkinson’s-related dementia over an average of 8.5 years.
In 2017, the final year of the study period, researchers conclude that transportation noise contributed to as many as 1,216 out of the 8,475 registered cases of dementia. It’s the second-worst environmental risk factor for public health in Europe, behind air pollution, according to the study. A fifth of the population encounters noise levels above the guideline levels of 55 dB – equivalent to an open office. This is about five decibels lower than a conversation between people at a distance of about three feet.
How does noise damage the human body?
“Epidemiological studies have consistently linked transportation noise to various diseases and health conditions, such as coronary heart disease, obesity, and diabetes. The proposed biological mechanisms are noise-induced reactions, with activation of the autonomic nervous and the endocrine (hormone) system and subsequent release of stress hormones, affecting several physiological functions. Exposure to noise during the night can also lead to sleep disturbance and fragmented sleep,” writes the team, led by researchers at Southern Denmark University.
“Experimental studies have found associations between transportation noise at night time and endothelial dysfunction, increased oxidative stress, alterations in the immune system, and increased systemic inflammation.”
There have been few previous investigations into the connection between noise and dementia and the results have also been inconsistent. Dr. Cantuaria and the team accounted for other potentially influential personal and neighborhood factors, including air pollution. A general pattern emerged, showing that the higher the noise exposure, the higher the risk of mental decline.
“Air pollution and transportation noise share a common source. Including high-quality estimations of air pollution in our models is important to disentangle the effect of the two exposures. Our results were robust to adjustment for air pollution, thus indicating an independent effect of noise on the biological mechanisms leading to dementia,” Dr. Cantuaria team says.
“If these findings are confirmed, they might have a large effect on the estimation of the burden of disease and healthcare costs attributed to transportation noise. Expanding our knowledge on the harmful effects of noise on health is essential for setting priorities and implementing effective policies and public health strategies focused on the prevention and control of diseases, including dementia.”
Noise bad for blood pressure too?
The findings do not present the full picture of possible harm to the aging brain from long-term exposure. Professor Beate Ritz, an epidemiologist at California University in Los Angeles who did not take part in the study, still describes the findings as “comprehensive.”
“The residential noise estimates accounted only for road traffic and railway noise – not noise from airports, industrial activities, or occupational exposure. Exposure to noise at work might influence vulnerability to exposures to ambient noise in later life, for example. Noise might also affect the risk of other chronic disorders, such as high blood pressure, through which noise contributes indirectly to dementia risk,” Prof. Ritz says.
“Noise pollution is not only an environmental nuisance but also a threat to public health. Reducing noise through transportation and land use programs or building codes should become a public health priority,” study authors conclude in a media release.
South West News Service writer Mark Waghorn contributed to this report.