Long-term exposure to residential road traffic noise is associated with an increased risk of Parkinson's disease, study finds

The study by Sørensen and colleagues provides, to date, the most robust evidence on the link between road traffic noise and the risk of Parkinson’s disease. It was carried out on a Danish cohort of over three million people, followed for almost two decades, with a complete residential history dating back to 1990 and an estimate of noise exposure using a validated model applied to both the façade most exposed to noise and the least exposed. One of the study’s main strengths is that it adjusts the analyses for relevant confounding factors such as air pollution, green space, socioeconomic status and, in a sub-cohort, smoking and physical activity. Sørensen et al. found a modest increase in the risk of Parkinson’s disease for each interquartile increase in noise (around 3–4 per cent), but this increase was consistent across subgroups and consistent with the pathophysiological mechanisms involved in the disease, such as oxidative stress, microglial activation and disruption of circadian rhythms. I find it particularly interesting that having a quiet façade—with a difference of at least 10 dB compared to the most exposed one—attenuates the association, as this suggests that good urban planning can help reduce exposure to this risk factor.

Although the link between traffic noise and the risk of Parkinson’s disease had already been studied in other European and Canadian cohorts, with mixed results, Sørensen’s study stands out due to the size of the population studied, the quality of the exposure estimates and the protective effect of a quiet façade.

However, the results should be interpreted with the caution typical of an observational study. Furthermore, it should be borne in mind that exposure was estimated solely on the basis of outdoor noise, without taking into account noise inside homes or exposure in other settings, such as the workplace. However, this does not detract from the relevance of the results presented in the study. Given that millions of people are exposed to noise levels exceeding those recommended by the WHO, even a small increase in risk can have a major impact on public health.

Parkinson’s disease is the second most common neurodegenerative disorder and the one expected to see the greatest increase in incidence over the coming decades. Consequently, identifying potentially modifiable environmental factors is a public health priority. This study provides strong evidence that chronic exposure to traffic noise is associated with an increased risk of developing Parkinson’s disease. A particularly interesting finding is that having a façade or area of the home with a noise level more than 10 dB lower than the area most exposed to noise appears to attenuate this association, which reinforces the hypothesis that noise pollution, particularly during periods of night-time rest, could play a significant role.

The main strength of the study lies in its methodological design. It is a national cohort study involving more than three million people followed for almost two decades, with a detailed assessment of noise exposure and thorough adjustment for potential confounding factors, including air pollution, access to green spaces and socio-economic variables. Furthermore, the results are biologically plausible, as environmental noise has previously been linked to chronic activation of the sympathetic nervous system, oxidative stress, inflammation and sleep disturbances – mechanisms also implicated in neurodegeneration. One limitation is that, as this is an observational study, it cannot establish a definitive causal relationship and lacks information on actual noise exposure inside homes or on individual factors that might influence risk. Nevertheless, the study represents some of the most robust epidemiological evidence published to date on the possible role of noise pollution as an environmental risk factor for Parkinson’s disease.

Although the observed increase in risk is relatively modest at an individual level, the huge proportion of the population exposed to traffic noise means that its potential impact on public health could be considerable. In this context, the findings reinforce the importance of policies aimed at reducing urban noise pollution, through strategies such as urban planning, the installation of noise barriers, reducing traffic in residential areas, or designing homes with façades and recreational spaces that are less exposed to noise.

"Parkinson’s disease is currently the fastest-growing neurodegenerative disorder. According to the World Health Organisation (WHO), the number of cases of Parkinson’s has doubled over the last 25 years, and the organisation warns that this figure could triple over the next 25 years. As the cause of this disease is not yet known, it is of great importance to identify and minimise the risk factors associated with Parkinson’s.

There is considerable evidence linking exposure to certain environmental agents – particularly pesticides used in agriculture and, more recently, industrial pollutants – with an increased risk of developing Parkinson’s disease. Against this backdrop, the study explores whether exposure to road traffic noise might also be associated with this risk, reinforcing the importance of investigating potentially modifiable environmental factors that could contribute to the prevention of the disease.

This is one of the first studies to analyse the role of road traffic noise as a risk factor for Parkinson’s disease. The results indicate that prolonged exposure to traffic noise increases the risk of developing the disease. Compared with previous studies carried out in the Netherlands and Canada, this study, conducted in Denmark, goes a step further, as it includes data from a large number of participants – more than three million people – and takes into account the possible influence of additional variables, including demographic, socio-economic and environmental factors.

The study indicates that exposure of residential building façades to road traffic noise is associated with an increased risk of developing Parkinson's disease. It also suggests that noise may have a particularly important impact during periods of rest, especially in homes that lack a quieter façade, where bedrooms and other resting areas are typically located. These findings highlight the importance of considering such factors in urban planning.

Although the study does not investigate the biological mechanisms that might link noise exposure to the degeneration of dopaminergic neurons characteristic of Parkinson's disease, it suggests that this association could be related to sleep disruption and disturbances of the circadian rhythm. We now know that during sleep, the brain activates systems that help clear toxic molecules, thereby maintaining brain health, and that these processes are disrupted when sleep is fragmented. In addition, noise exposure may contribute to increased inflammation and oxidative stress, both within and outside the nervous system, which are key processes involved in the development of Parkinson's disease. Although it is unlikely that noise directly causes the death of dopaminergic neurons, it may impair the brain's protective mechanisms and promote biological processes involved in disease development. Therefore, further studies are needed to better understand the biological mechanisms that could explain the association between noise exposure and Parkinson's disease. At the same time, additional epidemiological studies will be essential to confirm these findings, identify new risk factors, and provide the evidence needed to inform prevention strategies and guide public health decision-making in light of the growing impact that Parkinson's disease is expected to have in the coming decades.

 Is the study of good quality?

“The study has been published in a highly prestigious international medical journal, which follows a rigorous, high-quality peer-review process, thereby supporting the scientific rigour of the methodology and the results described.”

Are there any limitations that need to be taken into account?

“This is a long-term longitudinal study, which includes data from a large number of participants, employs a validated methodology and analyses various demographic, socio-economic and environmental variables, combined with exposure to road traffic noise. However, it does have some limitations, which the authors also acknowledge, such as the need to take into account other indicators, including the structural aspects of housing, participants’ exposure to noise outside the home – for example, in the workplace – or lifestyle factors such as smoking or physical exercise. It would also be interesting to consider other factors that may influence the risk of developing the disease, such as the presence of other health conditions among participants, including obesity, diabetes or high blood pressure. We now know that these conditions increase the risk of developing the disease and could influence the results of this type of study.”

What are the implications of this result, and how does it fit in with the existing evidence? Is traffic noise a new risk factor?

“Whilst exposure to certain environmental factors, such as pesticides, is a well-known risk factor for Parkinson’s disease, the evidence pointing to traffic noise as a possible risk factor is much more recent. Until now, only two previous studies had been published, one using data from the Dutch population and the other from Canada, with conflicting results, partly explained by the methodology and type of study. This is therefore a promising line of research, although it is still at an early stage. Studies such as this one can help to identify modifiable factors which, once confirmed, will contribute to the design of strategies to prevent or reduce the risk of developing the disease.”