Exposure to certain forms of air pollution is linked to an increased risk of developing dementia, according to the most comprehensive study of its kind.
The illness is estimated to affect about 57 million people worldwide, with the number expected to increase to at least 150m cases by 2050.
The report, which was produced by researchers at the Medical Research Council’s epidemiology unit at the University of Cambridge involved a systematic review of 51 studies.
It drew on data from more than 29 million participants who had been exposed to air pollutants for at least a year.
Although air pollution has already been identified as a risk factor for dementia, the research, which is the most comprehensive study of its kind to date, found there to be a positive and statistically-significant association between three types of air pollutant and dementia.
The pollutants were: PM2.5, which comes from vehicle emissions, power plants and woodburning stoves and fireplaces; nitrogen dioxide, which arises from the burning of fossil fuels; and soot, which comes from sources such as vehicle exhaust emissions and burning wood.
When inhaled, these pollutants can penetrate deep into the lungs and are associated with various respiratory diseases and an increased risk of certain heart problems.
More specifically, the study found that for every 10 micrograms per cubic metre of PM2.5, an individual’s relative risk of dementia would increase by 17%. Using equivalent figures for soot, the risk rose by 13%.
Soot and PM2.5 levels approached or exceeded these levels in 2023 at roadside locations in central London, Birmingham and Glasgow.
The most common cause of dementia is Alzheimer’s disease and in the UK about 982,000 people have the illness. Symptoms include memory loss, difficulty concentrating and mood changes.
Dr Haneen Khreis, the senior author of the study, said it provided “further evidence to support the observation that long-term exposure to outdoor air pollution is a risk factor for the onset of dementia in previously healthy adults”.
She added: “Tackling air pollution can deliver long-term health, social, climate and economic benefits. It can reduce the immense burden on patients, families, and caregivers, while easing pressure on overstretched healthcare systems.”
Air pollution may cause dementia through causing inflammation in the brain and oxidative stress, which is a chemical process within the body that can cause damage to cells, proteins and DNA.
The researchers acknowledged that the report was limited because the majority of studies analysed included participants who were white and living in high income countries. They said future studies on air pollution should include more participants from marginalised backgrounds.
Dr Isolde Radford, senior policy manager at Alzheimer’s Research UK, said: “This rigorous review adds to mounting evidence that exposure to air pollution – from traffic fumes to wood burners – increases the risk of developing dementia.
“Air pollution is one of the major modifiable risk factors for dementia – but it’s not something individuals can solve alone. That’s where government leadership is vital.
“While the 10-year health plan acknowledges the health harms of air pollution, far more needs to be done to tackle this invisible threat.
“Alzheimer’s Research UK is calling for a bold, cross-government approach to health prevention – one that brings together departments beyond health, including Defra, to take coordinated action on the drivers of dementia risk.”
The report was published in The Lancet Planetary Health.