New Delhi, February 17 A study conducted on 27.8 million people in the US has shown that exposure to air pollution can directly contribute to an increased risk of Alzheimer's disease, rather than causing it indirectly through chronic conditions such as hypertension and stroke.
The findings, published in the journal PLOS Medicine, also reveal that people with a history of stroke may be particularly susceptible to the harmful effects of air pollution on brain health, highlighting an important intersection between environmental and vascular risk factors, researchers said.
The team from Emory University said that improving air quality could be an important way to prevent dementia – of which Alzheimer's disease is the most common form – and protect older adults.
They added that while air pollution is known to increase one's risk of Alzheimer's disease and several chronic conditions, including hypertension and depression, it was previously unclear whether air pollution causes the chronic conditions, which then lead to dementia, or whether the chronic conditions amplify pollution's effects on brain health.
The study examined beneficiaries of Medicare, the US's federal health insurance, aged 65 and above during 2000-2018. The participants' exposure to air pollution and whether they developed Alzheimer's disease was analyzed, along with the role of comorbid chronic conditions.
The authors "identified approximately 3.0 million incident AD (Alzheimer's disease) cases."
Exposure to PM 2.5 (five-year moving average prior to onset) was associated with an increased risk of AD in the overall population, with the risk rising for every 3.8 micrograms per cubic metre increase in exposure to air pollution, the authors said.
The link was slightly stronger in individuals with stroke, but there was little effect modification for hypertension and depression.
Exposure to PM2.5 was also significantly associated with higher risks of hypertension, depression, and stroke, all of which were also linked to an increased Alzheimer's disease risk.
"However, the mediation effects were minimal, with 1.6 per cent of the association between PM2.5 and incident AD mediated by hypertension, 4.2 per cent by stroke, and 2.1 per cent by depression," the authors wrote.
They added, "In this large national study of older adults, we found that long-term exposure to fine particulate air pollution was associated with a higher risk of Alzheimer's disease, largely through direct effects on the brain rather than through common chronic conditions such as hypertension, stroke, or depression."