Exposure to fine particulate matter (PM2.5) air pollution is linked to a heightened risk of dementia, even at levels below current US, UK and European air quality standards, finds research published by The BMJ.
More limited data suggests that exposure to nitrogen dioxide and nitrogen oxide might also be a risk factor for dementia.
Many uncertainties remain, so caution is needed when interpreting these findings, but the researchers say the results “strengthen the evidence that air pollutants are risk factors for dementia.”
More than 57 million people worldwide are living with dementia and the global burden continues to increase. But interventions to delay or prevent the onset of dementia are scarce.
Growing evidence suggests that air pollutants may contribute to the risk of dementia, but studies have used different approaches and none have included detailed assessment of bias, making it difficult to draw firm conclusions.
To address this, a team of US researchers set out to investigate the role of air pollutants in dementia risk, accounting for study differences that could influence findings.
Using scientific databases, they identified 51 studies reporting associations between air pollutants averaged over a year or more and dementia cases in adults.
After assessing study quality and risk of bias, they were able to include 16 studies in their main quantitative analysis, mostly from North America and Europe.
The results show that higher exposure to fine particulate pollution was associated with an increased risk of dementia.
In 14 studies that specifically examined the potential effects of PM2.5 on dementia, they found that for every 2 micrograms per cubic metre (µg/m3) increase in average annual PM2.5 concentration, the overall risk of dementia rose by 4%.
Studies that actively assessed participants reported a stronger association between dementia risk and air pollution than studies using passive surveillance methods, such as electronic health records.
Among studies with active assessment, results suggested a 42% greater risk of dementia for every 2 µg/m3 increase in average annual PM2.5 concentration. The most conservative estimate was a 17% greater risk.
The results also suggest an elevated but small increase in dementia risk with exposure to nitrogen dioxide (2% for every 10 μg/m3 increase) and nitrogen oxide (5% for every 10 μg/m3 increase), but this was based on more limited data.
The study did not find an association between ozone and dementia.
The researchers acknowledge that most studies had concerns related to risk of bias which, alongside other limitations, may have influenced the results.
But they say the findings suggest consistent evidence of an association between ambient air pollution and clinical dementia, particularly for PM2.5, even below the current US Environmental Protection Agency (EPA) annual standard of 12 μg/m3, and well below the limits of the UK (20 μg/m3) and the European Union (25 μg/m3).
These findings support the public health importance of limiting exposure to PM2.5 and other air pollutants and provide regulatory agencies and others with a best estimate of effect for use in burden of disease and policy deliberations, they conclude.
In a linked editorial, researchers note that PM2.5 concentrations in major cities vary considerably, from below 10µg/m3 in some cities (eg. Toronto, Canada) to more than 100µg/m3 in others (eg. Delhi, India), therefore, air pollution has the potential to substantially affect dementia risk globally.
They also point to several challenges, such as the complex inter-relations between socioeconomic status, ethnic group, air pollution, and dementia, and a lack of studies from lower and middle income countries.
Effective measures to reduce air pollution will likely require global legislation and policy programmes that focus on transition to clean and renewable energy sources, reduced energy consumption, and changes in agriculture, they write.
Any positive effect on dementia and general health would be accompanied by an important impact on climate change and biodiversity, therefore, reducing air pollution should be a global health and humanitarian priority, they conclude.