Researchers from Arizona State University have found a link between air pollution and Alzheimer's.
Arizona State University's National Bureau of Economic Research (NBER) issued a paper examining the effect of air pollution on dementia. Titled "Hazed and Confused," this analysis linked "fifteen years of Medicare records for 6.9 million adults age 65 and older to the EPA’s air quality monitoring network" and tracked those individuals' health. In particular, they focused on PM2.5, a type of "fine-particulate air pollution" that the State of New York explains can be created by vehicle emissions and the burning of fuel.
As an article by Slate explains, NBER's research found that Alzheimer's "probability goes up more than 1 percentage point for every additional 1 microgram-per-cubic-meter (μg/m3) exposure over 10 years." This is the second study to make this connection between dementia and long-term air pollution exposure. A study published in Proceedings of the National Academy of Sciences (PNAS) focused on the air quality in Chinese cities and found that "polluted air may impede cognitive ability as people become older, especially for less-educated men."
The U.S.'s air pollution levels vary by counties, but Slate does report that fine particle levels have dropped in the U.S. since the year 2000 thanks to Environmental Protection Agency (EPA) regulation. The overall U.S. average concentration of fine particles currently is around 9 μg/m3. Since the World Health Organization (WHO) recommends these levels stay below 10 μg/m3, this means the majority of the United States is not under great risk at this time. If EPA's regulations of PM2.5 become less strict, the average concentration of these fine particles is likely to go up again, leaving elderly individuals at greater risks of dementia. NBER's paper discourages these changes by pointing out that EPA's regulation thus far has lead to $150 billion in dementia-related savings since 2000.