More than 50 different pieces of research examining the relationship bettered air pollution and dementia have been analysed to gain an improved perspective on the risk.
Out of more than 15,000 initial records, the team from by a team from the Medical Research Council Epidemiology Unit at the University of Cambridge found 51 studies that met their criteria for data extraction, with 32 of these providing suitable data for statistical pooling.
The analysis focused on pollutants for which three or more independent studies reported comparable results, allowing the researchers to combine effect estimates and assess broader patterns.
The researchers focused on observational studies involving adults aged 18 and older, specifically those that investigated the relationship between at least one year of exposure to outdoor air pollutants and a subsequent medical diagnosis of dementia. Only studies that provided quantitative data and met certain quality thresholds were included in the final meta-analysis.
The findings showed a statistically significant link between dementia and long-term exposure to several common air pollutants. PM2.5 was one of the most clearly associated pollutants, with analysis of 21 studies involving more than 24 million people showing an 8% increased risk of dementia for every 5 µg/m³ increase in exposure.
Similarly, NO₂ exposure was associated with a 3% increased risk per 10 µg/m³, based on data from 16 studies.
Black carbon, a component of PM2.5 linked to vehicle emissions, also showed a notable effect: a 13% increased risk per 1 µg/m³, across six studies.
However, other pollutants, such as nitrogen oxides (NOx), PM10 and ozone, did not show a statistically significant association with dementia in the pooled results. These findings were more variable, and the number of studies contributing data for these pollutants was smaller, making it harder to draw firm conclusions.
Senior author Dr Haneen Khreis from the MRC Epidemiology Unit said: ‘Epidemiological evidence plays a crucial role in allowing us to determine whether or not air pollution increases the risk of dementia and by how much. Our work provides 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.
‘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.’
The full report can be read here.
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