A year after I moved to Toronto, I developed a chronic cough. A cold moved in and the cough never left. Self-conscious of quite how loud the cough was during the class I was T.A.'ing, I went to the doctor, who referred me to a specialist, who sent me off for various exams which made me appreciate the Canadian health care system, or at least my experience of it, including my speediest hospital visit ever. She offered the diagnosis of "post-nasal drip" and sent me away with a steroid inhalant. After three months, the cough was more or less gone.
But the next time I caught a cold, it came back, in full strength. Several times a week, strangers and acquaintances would ask me with great concern if I was dying. It sounded far worse than it felt.
So the cycle went. About once a year, I'd have another run of steroid inhalants, the cough would recess. Then it would gradually return again, as bad as ever. ("I'm too young to have a chronic cough!" I exclaimed in my more dramatic moments.)
And then, this summer, I left Toronto and moved to London. Within weeks, my cough had entirely vanished. As an added bonus, my skin cleared up beautifully - it hasn't looked this good since I hit puberty. London's air and water is full of pollutants too, but they are not the same pollutants as Toronto; whatever Toronto has, my lungs and skin are less happy with.
If I had any doubts that Toronto was the source of these mild irritants, I had only to return to prove it. After a week in Toronto, the cough was back.