PM2.5 Particulate Pollution
This started to be regulated in the late 1990's in the US. The term refers to particles less than 2.5 microns long, which can penetrate to the alveoli in the lungs and perhaps cause harm there. The precise form of harm seems not to be known, but Wikipedia and the media are very sure it causes many deaths based on dubious epidemiological studies. One of those found that increased levels in the months before death increased cancer deaths by 2.5%, which seems like a good placebo test of coincidental effects. But maybe they're right. It's just that the channel is unknown. And, many people do die of lung diseases, and maybe this tips them over the edge, even if it does not cause any disease. How it causes heart attacks is less obvious. I wonder if it would be interesting to see if lung disease deaths fall *below* normal levels in the months after an episode of high PM2.5. That sort of study should be done on covid too.