Since 2019 people living in Krakow do not burn any solids for heating, because the city introduced total ban on solid fuels including coal and wood an the ban is quite effective.

But people around the city and, frankly, in most of Poland still burn solid fuels and if you drive around these places the smell can be really terrible and the smoke color and density coming from some chimneys definitely doesn't look like a dry wood smoke. Such smoke is often a product of burning very low quality, super fine-grained coal or rather coal dust, which is the cheapest fuel available.