More than a year of exposure to certain air pollutants is associated with an increased risk of dementia, according to a recently published study.

Dementia, a group of diseases including Alzheimer’s, results in the loss of memory and a decline in cognitive abilities like thinking, reasoning and language skills. In 2019, dementia was estimated to affect at least 57.4 million people worldwide and was the eighth leading cause of death globally in 2021.

Previous studies have identified air pollution as a risk factor for dementia. Researchers reviewed 51 of those studies to investigate if there was an association between exposure to outdoor air pollution for at least a year and a subsequent clinical diagnosis of dementia. Those studies — 20 from Europe, 17 from North America, 12 from Asia and two from Australia — involved more than 29 million participants.

The analysis found a strong link between dementia and exposure to three kinds of air pollutants: nitrogen dioxide; soot; and PM2.5, or particulate matter that’s 2.5 microns in diameter or smaller, several times finer than human hair, which means it can be inhaled deep into the lungs.

PM2.5 can come from various sources, including vehicle emissions, wood burning, power plants and other industrial processes, and cooking activities. The researchers found that an individual’s relative risk of dementia can increase by 17% with every 10 micrograms per cubic meter (μg/m³) of PM2.5 exposure.

Nitrogen dioxide (NO2), which is produced from burning fossil fuels, including from vehicle exhaust, industrial emissions and gas stoves, can irritate the respiratory system. The study found that the relative risk of dementia rises by 3% for every 10 μg/m3 of NO2.

Similarly, the relative risk of dementia increased by 13% for each 1 μg/m³ of soot exposure.

For context, the average roadside measurement for PM2.5 in central London in 2023 was 10 μg/m³, and for NO2 it was 33 μg/m³. Meanwhile, annual mean soot concentration in London in 2023 was 0.93 μg/m³; in Birmingham it was 1.51 μg/m³, and in Glasgow it was 0.65 μg/m³.

In short, years of exposure to PM2.5, NO2 and soot can increase one’s risk of dementia in the future.

“This rigorous review adds to mounting evidence that exposure to air pollution — from traffic fumes to wood burners — increases the risk of developing dementia,” Isolde Radford, senior policy manager at Alzheimer’s Research U.K., who wasn’t involved in the study, told The Guardian.

Study co-author Clare Rogowski, from the Medical Research Council (MRC) Epidemiology Unit of the University of Cambridge, said in a press release: “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 equitably.”

This article originally appeared on Mongabay.