Take a look at these articles. One of them says:
A small town with medieval charm and a charming riverside has the country’s cleanest air. Aside from a lovely lungful of O2, there are plenty of other reasons why Dunblane has a reputation as a lovely place to stop if you’re travelling between Edinburgh, Glasgow and the Highlands, or a peaceful destination in its own right for a weekend break.
Spotless medieval town with riverside town is the ‘cleanest in the UK’ – Daily Record