[paulat]

There are cities that announce the Air Quality Index on the evening weather report, with special alerts from time to time.

Mostly cities around mountains...Los Angeles, Seattle, Denver....

The geography traps low temp air under high temp air, creating a temperature inversion. It is a visible effect...we can't see the mountains when it happens.

[William Terrell]

Cities all over the place. Great masses of people live in these cities, and still only one in four get lung disease.