During the 1996 Atlanta Olympic Games, incidents of hospitalization for asthma declined by 41% according to an article in the Journal of the American Medical Association.
The air quality in Beijing over the past two weeks has apparently been better than it has in decades. One day had an index reading of 12, down from an all time high in December of last year of 500 (according to this report on Price Tags)
In Atlanta, the city dramatically curtailed private automobile use. In Beijing both private automobiles and manufacturing were shut down in order to improve the air quality.
What all this suggests to me is that irrespective of concerns about global warming, our urbanizing planet needs to cut pollution now simply to look after the health of its citizens. Or, if you want to think about the economy, consider the lost productivity from people unable to breathe.