Over the next 30 years, an aging population will triple the number of dementia cases globally, from 50 million to more than 150 million.

Up to 40% of those cases are linked to potentially avoidable factors like air pollutants.

Now, a paper in the journal BMJ by the Harvard T.H. Chan School of Public Health offers the strongest evidence yet linking dementia risk to fine particulates in the air such as PM-2.5s — particles measuring 2.5 microns or less.

The meta-analysis of 51 studies includes cases found through active case ascertainment methods like community outreach, contact tracing and targeted testing.

These active approaches found a stronger link than passive studies, possibly due to misclassifications in the electronic health records they often use. The fact that many dementia risk factors cluster together — such as socioeconomic group and ethnicity — makes teasing out causes from diagnostic records that much more difficult.

Several studies included in the analysis showed the link stayed strong even when annual exposure fell below the current EPA annual standard of 12 micrograms per cubic meter of air (µg/m3).

Globally, PM2.5 levels range from less than 10 µg/m3 in cities like Toronto, Canada to more than 100 µg/m3 in cities such as Delhi, India.

Analysis of more limited data also suggests links between dementia and nitrogen oxide and nitrogen dioxide, gases released during combustion that cause air pollution, smog and acid rain.

Although particulates pose a smaller risk than factors like smoking, far more people are exposed to air pollution, which kills more than 6.5 million people globally each year.

People can limit exposure by staying indoors and filtering air, but many people live and work in areas where exposure is inescapable. Consequently, experts tend to view regulatory solutions as more effective.

The EPA is now considering a proposal to strengthen PM2.5 regulations.