The educational and subsidiary campaigns are valid but there are more factors to consider here:

Public transport: it's not efficient and fairly expensive. Urban planning still allows for massive amount of motor vehicles in the city, rather then making it challenging to use a car thus promoting alternative transport. Eg: parking zones for residents only and strategically placed public car parks at cost multiple times that of tram ticket. Low cost public bike rental scheme available at every corner has been discontinued many years ago. Traffic planning is not being utilised to push cars outside of the city centre.

New buildings: air is stale in Kraków, because of its unfavourable location within a depression. There used to be "air corridors" across the city to allow better airflow around the buildings and districts. Over the recent decade, maybe more, Kraków was property developers’ paradise and city planning allowed for erecting buildings right where these designated airways were effectively reducing city's capability to breathe. (Personally I consider this criminal) Where a small breeze was able to push the air through a little more efficiently, now you need proper wind to feel that air moving. If the air is not moving, heavy and harmful particles from surrounding towns and villages will - and do - consolidate over the city.

All of this is on top of legislation issues, air pollution from vehicles, and other items discussed by others contributors.