Exposure to fine particulate matter (PM2.5) air pollution has been linked to a higher risk of dementia, even at levels lower than existing US, UK, and European air quality regulations.
The results of the study were published by The BMJ.
More limited data suggest that exposure to nitrogen dioxide and nitrogen oxides may also be risk factors for dementia.
Many uncertainties remain, so caution is needed when interpreting these findings, but the researchers said 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 is increasing. But interventions to delay or prevent the onset of dementia are rare.
Growing evidence suggests that air pollutants may contribute to dementia risk, but studies have used different approaches and none have included detailed assessments 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 may have influenced the findings.
Using scientific databases, they identified 51 studies reporting associations between exposure to air pollutants lasting an average of one year or more and cases of dementia 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 showed 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 meter (ug/m3) increase in average annual PM2.5 concentration, the overall risk of dementia increased by 4%.
Studies that actively assessed participants reported stronger associations between dementia risk and air pollution than studies that used passive monitoring methods, such as electronic health records.
In studies with active evaluation, results suggested a 42% greater risk of dementia for every 2 ug/m3 increase in average annual PM2.5 concentration. The most conservative estimate was a 17% greater risk.
The results also suggest a higher but smaller increase in dementia risk with exposure to nitrogen dioxide (2% for each 10 mg/m3 increase) and nitrogen oxide (5% for each 10 mg/m3 increase), but this was based on more limited. data.
The study did not find a link between ozone and dementia. The researchers acknowledge that most studies had concerns related to risk of bias that, along with other limitations, may have affected the results.
But they say the findings suggest consistent evidence of a link between ambient air pollution and clinical dementia, particularly for PM2.5, below the current US Environmental Protection Agency (EPA) annual standard of 12 mg/m3, and below the limit. UK (20 mg/m3) and European Union (25 mg/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 better estimates of disease burden and impact for use in policy discussions, they conclude.
In the linked editorial, the researchers note that PM2.5 concentrations vary widely among major cities, from less than 10ug/m3 in some cities (eg Toronto, Canada) to over 100ug/m3 in others (eg Delhi, India). , air pollution is likely to influence dementia risk worldwide.
They also point to several challenges, such as the complex interrelationships between socioeconomic status, ethnic groups, air pollution, and dementia, and the paucity of studies from low- and middle-income countries.
Effective measures to reduce air pollution will likely require global legislation and policy programs that focus on the transition to clean and renewable energy sources, reduced energy consumption, and changes in agriculture, they write.
Any positive impact on dementia and general health would be accompanied by significant impacts on climate change and biodiversity, so reducing air pollution should be a global health and humanitarian priority, they conclude.
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