Long-term exposure to outdoor air pollution, including car exhaust emissions, can be linked to an increased risk of dementia, according to a large-scale study.
The Medical Research Council (MRC) Epidemiology Unit and University of Cambridge researchers said there was an “urgent need” for interventions to combat air pollution.
They analysed 51 studies and data from more than 29 million people to discover links between dementia and three types of air pollutant.
They said it was possible for pollutants to reach the brain directly or through the bloodstream, to cause widespread inflammation similar to that seen in lung and heart diseases.
Previous studies have highlighted pollutants as a potential risk factor for dementia, but the strength of evidence had been varied.
Clare Rogowski, from the MRC Epidemiology Unit, said: “Stricter limits for several pollutants are likely to be necessary targeting major contributors such as the transport and industry sectors.
“Given the extent of air pollution, there is an urgent need for regional, national, and international policy interventions to combat air pollution.”
The study looked at more than 29 million people who had been exposed to air pollutants for at least one year, mostly from high-income countries.
It found PM2.5, a particulate matter with a diameter of 2.5 micrometres or less, could increase the risk of dementia by 17%.
It was small enough to be inhaled deep into the lungs and came from sources including vehicle emissions, power plants, wood burning and construction dust.
Nitrogen dioxide (NO₂) was also mentioned as creating a 3% higher risk.
This came from burning fossil fuels, especially diesel vehicles, industrial emissions, and gas appliances, researchers added.
Soot, from sources like vehicle exhaust emissions and burning wood, was found to increase the relative risk of dementia by 13%.
Dr Haneen Khreis, from the MRC Epidemiology Unit, said the research was further evidence to support that long-term exposure to outdoor air pollution was a risk factor for dementia in previously “healthy” adults.
Dr Christiaan Bredell, from the University of Cambridge and North West Anglia NHS Foundation Trust, added: “These findings underscore the need for an interdisciplinary approach to dementia prevention.
“Preventing dementia is not just the responsibility of healthcare: this study strengthens the case that urban planning, transport policy, and environmental regulation all have a significant role to play.”
Most of the study participants were white and from high-income countries, despite marginalised groups facing higher pollution exposure, the researchers added.
They urged future studies to ensure better representation.