Breathing air laden with particles would not only damage the lungs. A large American study suggests that
air pollution could directly increase the risk of
Alzheimer’s diseasewithout going through only hypertension or depression. A major challenge when dementia already affects around 57 million people worldwide.
This new research led by Emory University in the United States focused on
fine particles PM2.5emitted by road traffic, wood heating, industry or forest fires. The results show a clear link between these particles and Alzheimer’s disease, with a particular vulnerability in people who have already suffered a stroke. Enough to attract the attention of health authorities.
Air pollution and Alzheimer’s: a direct impact on the brain
Yanling Deng’s team tracked more than 27.8 million Medicare beneficiaries aged 65 and older between 2000 and 2018, almost the entire U.S. elderly population. About 3 million new cases of Alzheimer’s were identified from medical records and then compared to the estimated PM2.5 level by zip code. The average concentration was 10.1 µg/m³, almost double the annual limit recommended by the World Health Organization (5 µg/m³).
© Yanling Deng (CC-BY 4.0, https://creativecommons.org/licenses/by/4.0/)
The researchers calculated that an increase of 3.8 µg/m³ of PM2.5 over the five years preceding diagnosis was associated with an increase of approximately 8.5% in the risk of Alzheimer’s. In other words, a senior living in a slightly more polluted area has a significantly higher probability of developing the disease. “In this large national study of older adults, we found that prolonged exposure to fine particle air pollution was associated with an increased risk of Alzheimer’s disease, primarily through direct effects on the brain rather than through common chronic conditions such as hypertension, stroke or depression“, the authors wrote in the journal PLOS Medicine.
An especially direct impact on the brain, with an increased risk after a stroke
The study examined three common comorbidities – hypertension, stroke and depression – to see if they served as intermediaries between pollution and Alzheimer’s. PM2.5 increased the risk of these pathologies, themselves linked to dementia. However, only 1.6% of the pollution-Alzheimer’s association was explained by hypertension, 4.2% by stroke and 2.1% by depression, which mainly points to a direct effect of pollution on the brain.
People with a history of stroke, however, appeared more sensitive: for the same increase in PM2.5, their risk of Alzheimer’s increased more (+10.5%) than in those without stroke. “Our findings suggest that people with a history of stroke may be particularly vulnerable to the adverse effects of air pollution on brain health, highlighting an important intersection between environmental and vascular risk factors.“. This observation is consistent with the idea that a brain already weakened by vascular damage is less resistant to the attacks of polluted air.
A puzzle still incomplete, but plausible mechanisms
PM2.5 particles, smaller than 2.5 micrometers, penetrate deep into the lungs, pass into the blood and then reach the brain. Work cited by specialists shows that they can promote chronic inflammation, oxidative stress and cerebral circulation disorders, all phenomena associated with typical Alzheimer’s lesions. A synthesis of 20 studies in America, Europe and Asia cited by British researchers has already shown that for each additional 10 µg/m³ of PM2.5, the risk of dementia increased by around 40%, that of Alzheimer’s by 47%, and that of vascular dementia doubled.
The authors of the Emory study point out that their work remains observational and that certain precise mechanisms continue to be debated, but the convergence of data reinforces the idea that reducing air pollution could help prevent dementia, particularly in the elderly and those who have already suffered a stroke.