Some years ago I moved from northern Norway to central Dublin. By any reckoning that's a decrease in air quality -- though Dublin is still much better than most cities -- but it also came with a reduction in allergic reactions.

I suspect simply because there isn't as much nature in the vicinity, which I don't actually appreciate. Oh well.

As someone who has always lived in London, I started getting hayfever around age 22. When asked by doctors if I have any allergies, and I say nothing except maybe hayfever, they always say "oh everyone has hayfever"

Its commonly not allergy e.g. not a histamine response. It can be simple irritation due to particulates in the air. Everybody should respond to that.