The results of the research have prompted senior author and  Director of King's Environmental Research Group, Professor Frank Kelly, to argue that improving air quality in cities should be a public health priority. The full research findings are published in BMJ Open.

Although air pollution is now an established risk factor for heart disease/stroke and respiratory disease, its potential role in neurodegenerative diseases, such as dementia, hasn’t been clear. 

Researchers used carefully calculated estimates of air and noise pollution levels across Greater London to assess potential links with new dementia diagnoses. To do this, they drew on anonymised patient health records from the Clinical Practice Research Datalink (CPRD). This has been collecting data from participating general practices across the UK since 1987.

The researchers focused on just under 131,000 patients aged 50 to 79 in 2004, who had not been diagnosed with dementia, and who were registered at 75 general practices located within the London orbital M25 motorway. Based on the residential postcodes of these patients, the researchers estimated their annual exposure to air pollutants–specifically nitrogen dioxide (NO2), fine particulate matter (PM2.5), and ozone (O3) – as well as proximity to heavy traffic and road noise.

The health of these patients was then tracked for an average of seven years, until a diagnosis of dementia, death, or deregistration from the practice, whichever came first. During the monitoring period, 2181 patients (1.7%) were diagnosed with dementia, including Alzheimer’s disease. These diagnoses were associated with ambient levels of NO2 and PM2.5, estimated at the patients’ homes at the start of the monitoring period in 2004.

Those living in areas in the top fifth of NO2 levels had a 40 per cent heightened risk of being diagnosed with dementia compared to those living in the bottom fifth. A similar increase in risk was observed for higher PM2.5 levels. These associations were consistent and unexplained by known influential factors, such as smoking and diabetes, although when restricted to specific types of dementia, they remained only for patients diagnosed with Alzheimer’s disease.

Speaking about the research Professor Kelly said: ‘We hypothesise that it is reactions by our body’s immune system  to elevated pollution occurring over and over again that leads to the eventual tissue damage such as to the lungs, blood vessels or brain

‘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.’

Researchers stressed that many factors may be involved in the development of dementia, and the exact cause is still not known, and while there are several plausible pathways for air pollutants to reach the brain, how they might contribute to neurodegeneration isn’t clear.