Air pollution may increase the risk of developing dementia, according to a new study.
Researchers found that people living in the most polluted parts of London had a 40 per cent higher risk of getting the degenerative brain disease within seven years compared to those who lived in areas with the cleanest air.
A survey of nearly 131,000 50 to 79 -year-olds found the increased risk could not be explained when taking into account other factors associated with dementia.
Lead author Dr Iain Carey, Population Health Research Institute, St George’s, University of London, said: “We have found evidence of a positive association between residential levels of air pollution across London and being diagnosed with dementia, which is unexplained by known confounding factors.
“The implications of linking exposure to air pollution such as NO2 to the development of dementia, specifically Alzheimer’s disease, raise many questions.
“The cause of these neurodegenerative diseases is still largely unknown and may be multifactorial.
“With the future global burden of dementia likely to be substantial, further epidemiological work is urgently needed to confirm and understand better recent findings linking air pollution to dementia.”
The study recorded when 130,978 50 to 79-year-olds registered in 75 London GP practices in January 2005 were diagnosed dementia.
The findings showed those living in the top fifth of Nitrous dioxide (NO2) levels ran a 40 per cent heightened risk of being diagnosed with dementia than those living in the bottom fifth.
The observational study, published in the online journal BMJ Open, saw a similar increase in risk for other higher air pollutant levels.
The researchers said the associations found couldn’t be explained by factors known to influence the risks of developing the condition.
And said further research is needed to explore the link air pollution has with dementia after it was established as a risk factor for heart disease, stroke and respiratory disease in previous studies.
Dr Carey said: “The association could not be explained by confounding and was consistent within subgroups.
“When we restricted to specific diagnoses, associations were still observed with Alzheimer’s disease but not vascular dementia.
“Traffic related air pollution has been linked to poorer cognitive development in young children, and continued significant exposure may produce neuroinflammation and altered brain innate immune responses in early adulthood.”
His team used anonymised medical health records from The Clinical Practice Research Datalink – a database that has been collecting patient data from participating UK general practices since 1987.
The researchers estimated patients yearly exposure to air pollutants NO2, fine particulate matter (PM2.5) and ozone, and comparing this with their post codes.
They then tracked the patients’ health for an average of seven years until they were diagnosed with dementia, died, or deregistered from the practice.
A total of 2,181 patients were diagnosed with dementia with higher rates among those who lived in more polluted areas.
The correlations between pollution and dementia diagnoses were consistent and unexplained by known influential factors, such as smoking and diabetes.
But when restricted to specific types of dementia, they remained only for patients diagnosed with Alzheimer’s disease, the study found.
The researchers warned their “observational study” cannot establish a cause for the link and that their findings may be applicable only to London.
They warned many factors may be involved in the development of dementia, the exact cause of which is still not known and that there are several ways for air pollutants to reach the brain.
They said that even if the impact of air pollution were relatively modest, the public health gains would be significant if it emerged that curbing exposure to it might delay progression of dementia.
The researchers added that they were not able to glean long term exposures, which may be relevant as Alzheimer’s disease may take many years to develop.