Air pollution link to Alzheimer’s mooted

Chronic exposure to air pollution may be a risk factor in developing dementia, a UK population-based study has found.

If further research throws up a causal, rather than simply observational, link, then neurodegenerative disease can be added to heart disease, stroke and respiratory illness as a known danger of exposure to airborne particles.

The apparent association between air quality and dementia was made by a team led by Iain Carey from the UK’s Population Health Research Institute at the University of London. The research is published in the journal BMJ Open.

To make their finding, the researchers used anonymised patient health records contained in the UK’s Clinical Practice Research Datalink (CPRD), which has been collecting information from participating general practices since 1987.

Carey and colleagues focussed on 131,000 patients who were aged between 50 and 79 in 2004, and who had not been diagnosed with dementia at the time. All of them attended practices that were located within London’s giant orbital M25 motorway.

The patients were tracked for an average of seven years, until they were variously diagnosed with dementia, died, or were deregistered from the practice. Their health progress was correlated with careful analysis of the levels of three specific air pollutants — nitrogen dioxide (NO₂), fine particulate matter (PM_2.5) and ozone (O₃) – recorded around their home addresses.

During the monitoring period, some 2181 patients – 1.7% of the total – were diagnosed with various types of dementia. Comparing these cases to ambient pollution levels revealed a distinct pattern: those living in areas recording the top-fifth of NO₃ or PM_2.5 levels had a 40% higher risk of developing the condition, compared to those in the bottom fifth.

Further analysis showed that the findings remained consistent for Alzheimer’s disease, even when other potential contributing factors, such as smoking and diabetes, were taken into account.

The researchers stress that the results are purely observational, and cannot demonstrate causation. On that basis, the findings may be specific only to London.

Another important limitation is that the analysis covered only seven years. As such it cannot be used to infer a definite link between pollution and Alzheimer’s, because the disease takes many years to fully develop.

Nevertheless, Carey and his colleagues note that the higher levels of dementia in the more polluted areas cannot be satisfactorily explained by other known factors, and that further research should be undertaken.

“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,” they write.