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A new population-based cohort study published in The Lancet reports living close to heavy traffic was associated with a small, yet significant, increase in the incidence of dementia, but not Parkinson’s disease or multiple sclerosis.
Dr Doug Brown, Director of Research at The Alzheimer’s Society, said:
“This study offers some evidence that people who live near major roads in Ontario, Canada have a slightly increased risk of dementia. Although this was a thorough study in a large number of people, the research hasn’t yet shown what causes this link, so we can’t be sure that reducing pollution or noise from traffic would decrease the risk of dementia.
“The causes of dementia are complex. Lifestyle factors such as exercise, eating healthily and avoiding smoking will reduce our chances of developing dementia. Age remains the strongest known risk factor for dementia, but emerging evidence on aspects of the environment could become important to help prevent dementia in future.”
Prof. Martin Rossor, NIHR National Director for Dementia Research at University College London Hospitals (UCLH), said:
“This is an important paper that adds to the evidence that air pollution is bad for health. It is not possible to infer cause and effect from a cross sectional study nor to what extent air pollution contributes to any single case of dementia.
“The effects are small but with a disorder with a high population prevalence such effects can have important public health implications (the fact that no effect was observed for Parkinson’s disease or Multiple Sclerosis may have been due to the fact that these disorders are less common than dementia). Dementia is due to many underlying mechanisms and whilst one can speculate about mechanisms this is premature as the exact cause of the dementia e.g. Alzheimer’s disease is not known.
“The length of time someone is exposed to pollution is also important. For example, previous studies have suggested adverse neurodevelopment effects of leaded petrol in children aged 5-8y. This can reduce educational attainment which is itself protective for dementia.
“Whilst this study does not provide sufficient reason to drive individual choices if available it is an important public health message on the dangers of air pollution and the contribution of the built environment in responding to the dementia challenge.”
Dr David Reynolds, Chief Scientific Officer at Alzheimer’s Research UK, said:
“There are 850,000 people in the UK living with dementia, and therefore interest in the risk factors driving the condition is high. This research is interesting in its identification of an association between dementia and major roads, but if any causal link exists between these two factors, it can’t be confirmed by this study.
“Conditions like dementia have multiple risk factors including age and genetics, and other social factors relating to where people live in cities could also be playing a part here. This study has identified major roads and air pollutants from traffic as possible risk factors for dementia, a finding which will need further investigation before any firm conclusions can be drawn about the relative risks of air pollutants for dementia versus other risks such as smoking, lack of exercise or being overweight.
“Studies like this are valuable in revealing new factors that could be implicated in Alzheimer’s disease and other dementias, and opening new avenues for further research. Alzheimer’s Research UK is funding research focusing on reducing the risk factors surrounding dementia, and is dedicated to finding a way of preventing or slowing down the progression of this devastating condition.”
Prof. Tom Dening, Director of the Centre for Old Age and Dementia at the University of Nottingham, said:
“Interesting and provocative findings! We know from other work that there is a relationship between where people live and their risk of having a diagnosis of dementia, whether this is an urban-rural comparison or whether it is more sophisticated such as looking at patterns of land use and such matters. Place of residence also affects the likelihood of having other mental health conditions such as depression and anxiety.
“It is certainly plausible that air pollution from motor exhaust fumes may contribute to brain pathology that over time may increase the risk of dementia, and this evidence will add to the unease of people who live in areas of high traffic concentration. It is unlikely that Ontario has the worst air quality in the world, so the risks might be even greater in cities that are habitually wrapped in smog.
“However, as the authors acknowledge here, the cause of these findings cannot be easily ascertained from a cross-sectional study. We simply can’t tell if it’s down to pollution or some other reason. It is of course important to make statistical adjustments for factors concerning health and social deprivation. For example, if you live half a mile up a private drive you are probably more prosperous than someone living right by an inner city flyover. The authors have made this adjustment but it is always an open question as to whether the adjustments are sufficient to exclude really big effects from poor health and social deprivation. There may also be reasons why people who are not developing or who do not have dementia may be more likely to have moved away from proximity to traffic, whereas people who are at risk remain where they are, for example they may have less drive and energy to change their circumstances.
“One factor that may be important is there could be a lower diagnosis rate among people who live further away from main roads. The sample was taken across the whole province of Ontario, where some people must live in very remote locations and perhaps they do not consult doctors very often and therefore are less likely to be diagnosed as having dementia than city dwellers.
“However, it is impressive (1) that there seems to be a gradient of risk depending on the distance from the road and (2) that this is seen for dementia but not two other significant neurological conditions (MS and Parkinson’s disease). It is however worth pointing out that the numbers of people developing MS and Parkinson’s were much smaller than those developing dementia, so may not have been large enough to show an effect if there was one. However, this study does ask serious questions about the environments where many people live. Undoubtedly living in conditions of severe air pollution is extremely unpleasant and it is hard to suppose that it is good for anyone.”
Prof. Rob Howard, Professor of Old Age Psychiatry, UCL, said:
“Research to date suggests that the best way to reduce your dementia risk is to look after your general health – stop smoking, control your blood pressure and blood sugar level and take exercise. I tell my patients that what is good for their overall health is generally good for their brain, too. We know that major road air pollution is bad for general health and this latest study doesn’t tell us whether the small increase in dementia risk is driven by indirect effects or whether proximity to traffic directly influences dementia pathology.
“Regardless of the route of causation, this study presents one more important reason why we must clean up the air in our cities.”
Prof. John Hardy, Professor of Neuroscience, UCL, said:
“This study ostensibly reports that living close to a main road has a small but significant effect on dementia risk, but not of either Parkinson’s disease or multiple sclerosis (MS) risk. The analyses are exceedingly complex with many co-variates included in the model and this always leads to concerns that the analytic complexity is hiding confounding factors in the analytic pipeline. Of note the power to detect effects in Parkinson’s disease and MS was lower because the diseases are rarer so the apparent ‘specificity’ of the effect is not certain.
“There are several reasons why one might not want to live near a major road, but this study is not an additional one.”
* ‘Living near major roads and the incidence of dementia, Parkinson’s disease, and multiple sclerosis: a population-based cohort study’ by Chen et al. published in The Lancet at 23:30 UK time on Wednesday 4th January.
Declared interests
Prof Howard: “No declarations of interest”
Prof Hardy: “I consult for Eisai.”
Prof Dening: “I have no conflicts of interest in relation to this story.”