Nel 1930, una misteriosa nebbia assassina attraversò l’Europa (soprattutto il Belgio)

I never heard about this, it’s an interesting fairly short video.

looks like it was a combination of fairly typical localised smog casualties among older and vulnerable people plus some were apparently also hit by fluorine https://nl.wikipedia.org/wiki/Dodelijke_mist_in_de_Maasvallei?wprov=sfti1

wikipedia doesn’t say the total population in the valley so difficult to say how exceptional this number of casualties truly was