A new study has found that long-term exposure to air pollution increases the risk of developing Alzheimer’s disease, particularly among older adults with a history of stroke. The research was published in PLOS Medicine.

Alzheimer’s disease is the most common form of dementia, affecting about 57 million people worldwide. Previous research has already linked air pollution to Alzheimer’s and to chronic health conditions like hypertension, stroke, and depression — conditions that themselves are associated with dementia. But it was not clear whether pollution leads to dementia indirectly by causing these illnesses or whether it harms the brain directly.

To find out, the team led by Yanling Deng analysed data from more than 27.8 million U.S. Medicare recipients aged 65 and older from 2000 to 2018. They compared individuals’ long-term exposure to fine particulate air pollution with their likelihood of developing Alzheimer’s disease, while also tracking whether they had conditions such as stroke, hypertension, or depression.

The study found that people with greater exposure to fine particulate matter had a higher risk of Alzheimer’s. This risk was slightly stronger in individuals with a history of stroke, suggesting that stroke survivors may be particularly vulnerable to the effects of pollution on brain health.

These results are important for several reasons. First, they suggest that air pollution contributes to Alzheimer’s disease primarily through direct effects on the brain rather than only through other chronic illnesses. That means improving air quality could directly help protect brain health, not just heart and lung health. Second, identifying groups that are more susceptible — such as people who have had a stroke — can help doctors and carers tailor health advice and monitoring.