Sixty Three Dead from Meuse Valley smog (1930)

According to the (unsourced) article, casualties were reduced due to the generational memory of farmers knowing of a similar nearby incident in 1911.

Oddly enough, I found an article from an NSW newspaper from 1911 about symptoms that sound vaguely resembling Bovine Spongiform Encephalopathy (BSE) in a cluster of downer cows. Perhaps (v)CJD is an recurring, emergent prion disease that cannot be totally avoided in either cows or humans.