In medieval ages, in the medieval buildings with high ceilings such as, above all churches, there was a large volume of air that took a long time to pollute, perhaps longer than a Church service. Warm polluted air got up in church towers and were placed high up the bell tower openings.
In medieval ages, in deep coal mines air stood completely still, resulting in horribly polluted and sometimes toxic air. The easiest way to get down a little bit better air was to heat mine air with fire, after which it rose in the channels at the same time lowered the fresh air through the other channels further away. The capacity was limited and it required enormous amounts of fuel.