According to global air quality monitoring organisation BreezoMeter the air quality in Linz is worse than famously polluted and overcrowded global megalopolises like London, Shanghai and Mexico City.
Scoring 41 in their Air Quality Index this puts Linz at the very bottom bound of what is considered Moderate air quality, verging on the Low quality category, usually reserved for the likes of Mumbai and Delhi.
The low air quality is stems from a very high quantity of fine particle matter. Constituting particles under 2.5µm these typically are caused by combustion processes (including wildfires and domestic fires), mechanical processes and biological particles.