Long-term exposure to PM2.5 is associated with an increased risk of Alzheimer’s disease, and this link appears to be direct rather than acting through other common health conditions, according to a nationwide study of more than 27 million older Americans.
The research is one of the largest ever investigations into the relationship between air pollution, comorbidities, and dementia. It followed Medicare beneficiaries aged 65 and older across the United States from 2000 to 2018, identifying approximately 3 million new cases of Alzheimer’s disease.
Researchers found that for every 3.8 μg/m³ increase in average PM2.5 exposure over five years, the risk of developing Alzheimer’s rose by 8.5%. This association remained robust after accounting for a wide range of individual and area-level factors, including age, sex, race, socioeconomic status and smoking rates.
The study then examined whether three common comorbidities – hypertension, stroke and depression – might explain or amplify this relationship. All three conditions are known risk factors for Alzheimer’s and have also been linked to air pollution exposure.
While PM2.5 was associated with higher rates of hypertension, stroke and depression, and these conditions were in turn linked to increased Alzheimer’s risk, they played only a minor role in explaining the link. Hypertension explained just 1.6% of the pollution-Alzheimer’s association, depression 2.1% and stroke 4.2%.
The authors concluded: ‘The effect of air pollution on Alzheimer’s disease is largely independent of these comorbidities. PM2.5 increases the risk of developing AD largely through direct pathways.’
However, the study did find evidence that stroke may increase vulnerability. Among individuals who had experienced a stroke, the association between PM2.5 and Alzheimer’s was slightly stronger, a statistically significant difference suggesting that damage to the brain’s blood vessels may make it more vulnerable to harm from pollution
The authors add: ‘This observed effect modification by stroke may reflect an underlying biological vulnerability in cerebrovascular pathways.’
Stroke-related injury can breach the blood-brain barrier, creating an opening for airborne pollutants to enter, where they can fuel inflammation and accelerate the accumulation of amyloid plaques, a hallmark of Alzheimer’s.
The authors say: ‘Our findings suggest that reducing air pollution could benefit cognitive health broadly across older adults, while targeted interventions may be especially important for those with cerebrovascular disease or multiple chronic conditions.’
The full research can be read here.
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