Air pollution is not only damaging lungs and hearts, but it may also be contributing directly to Alzheimer’s disease, the most common form of dementia, according to a major new U.S. study and fresh global data from the State of Global Air (SoGA) 2025 report.

Dementia is a broad term used to describe a decline in memory, thinking, and reasoning severe enough to interfere with daily life. Alzheimer’s disease is the most common type of dementia, accounting for the majority of cases worldwide.

The new study, led by researchers at Emory University and published in PLOS Medicine, tracked 27.8 million Americans aged 65 and older between 2000 and 2018. It found that long-term exposure to fine particulate matter, known as PM2.5, was associated with a significantly higher risk of developing Alzheimer’s disease.

PM2.5 particles, produced mainly by burning fossil fuels, industrial activity, wildfires and agricultural burning, are small enough to penetrate deep into the lungs and enter the bloodstream. Scientists believe they can trigger inflammation, oxidative stress and damage to blood vessels in the brain.

Even after adjusting for conditions such as hypertension, stroke and depression, the association remained. The authors concluded that air pollution appears to raise Alzheimer’s risk “largely through direct effects on the brain rather than through common chronic conditions.”

The findings align with the latest SoGA 2025 estimates, which show that in 2023 alone, dementia attributable to air pollution resulted in 626,000 deaths globally and 11.6 million healthy years of life lost. More than one in four dementia deaths worldwide is now linked to air pollution exposure.

“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,” the authors said.

The findings suggest pollution may increase Alzheimer’s risk primarily through direct biological effects, including inflammation, oxidative stress, and damage to blood vessels in the brain, rather than indirectly by causing other chronic diseases.

However, the study found that individuals with a history of stroke were particularly vulnerable.

“Our findings suggest that individuals with a history of stroke may be particularly vulnerable to the harmful effects of air pollution on brain health, highlighting an important intersection between environmental and vascular risk factors,” the researchers noted.

The average pollution levels in the studied areas were roughly twice the World Health Organization’s recommended limit for PM2.5 exposure.

When researchers combined results from 20 previous studies across the United States, Europe, and Asia, they found a consistent pattern: for every additional 10 micrograms of PM2.5 per cubic metre of air, dementia risk rose by about 40 percent. Alzheimer’s risk increased by roughly 47 percent, while vascular dementia risk doubled.

PM2.5 levels are especially high in parts of Africa, India, and China, regions where dementia rates are also rising sharply. Experts warn that without aggressive pollution control, countries could face mounting human and economic costs linked to cognitive decline.

The new findings strengthen calls for stricter air quality standards, not just to prevent heart and lung disease, but to protect ageing brains. Cleaner air, researchers say, may be one of the most overlooked tools in the fight against dementia.

This story was a collaboration with New Narratives. Funding was provided by the Clean Air Fund, which had no say in the story’s content.