Exposure to high levels of air pollution over a decade is associated with an increased risk of developing dementia, according to a long-running study.

Researchers said their ability to estimate exposures over 40 years in a specific area made their work “unprecedented” and “unique”.

The study, led by the University of Washington, found that just a slight increase in the levels of fine particulate matter – PM2.5 – over a decade at specific addresses in Seattle in the US was associated with a greater risk of dementia for people living there.

Using data from two large, long-running study projects in the Puget Sound region – one dating back to the late 1970s measuring air pollution and another that began in 1994 on risk factors for dementia – the researchers found that a one microgram per cubic metre difference in pollution levels between residences was associated with 16% higher incidence of dementia.

This research provides support for policies that reduce exposure to particulate air pollution

Lead author Dr Rachel Shaffer said: “In other words, individuals exposed to elevated long-term PM2.5 had a higher risk of developing dementia.”

The study, published in the journal Environmental Health Perspectives, concluded: “These results add to a growing body of both epidemiological and toxicological evidence on the neurodegenerative effects of air pollution and suggest that reducing exposures across the population could contribute to reducing the burden of dementia.”

The research looked at more than 4,000 Seattle-area residents enrolled in the 1994 study and, of those, more than 1,000 people who had been diagnosed with dementia at some point since the study began were identified.

Dr Shaffer said: “We know dementia develops over a long period of time. It takes years – even decades – for these pathologies to develop in the brain, and so we needed to look at exposures that covered that extended period.”

She said that due to the availability of detailed databases of air pollution in the region, they were able to estimate exposures dating back four decades in the area.

Dr Shaffer added: “That is unprecedented in this research area and a unique aspect of our study.”

She said the findings support the need for policies which focus on cutting air pollution exposure.

Dr Shaffer said: “This research provides support for policies that reduce exposure to particulate air pollution and suggests that reducing exposures across the population could contribute to reducing the burden of dementia.”

Dr Sara Imarisio, head of research at Alzheimer’s Research UK, said: “This study adds to growing evidence of a relationship between air pollution and dementia risk.

“We know that the diseases that cause dementia can begin up to two decades before symptoms appear. While this research looked at exposure to air pollution over 10 years, the volunteers had an average age of 75 at the start of the study, and future research should explore how air pollution throughout our lives may affect the risk of dementia.

“As individuals, air pollution is a dementia risk factor that is hard to avoid but can be addressed by wider societal action and legislative change.

“The findings underlie the importance that reducing air pollution should be a priority for public health authorities.”