Air pollution is also known to be a potential reason why the Flynn effect (newer generations of test-takers performing substantially better on older tests, accounting also for test familiarity) is declining; with the "IQ curve" over time plateauing. We know that increased presence of CO and CO2 reduces cognitive capacity on many areas.
Carbon monoxide pollution has decreased (or been stable) in the countries I've bothered to check over the last decades (while the Flynn effect was in full swing):
