[Big Dave]

I'll defer on the incidence of asthma, but as for the overall ambient concentrations I refer to the report the EPA is required to submit to congress every four years by the statute enabling the formation of the EPA.

For all my dislike of the EPA, one thing thing they do well is monitor air quality. They have a network of thousands of monitors scattered about the country, mostly in urban/suburban areas. These monitors automatically measure concentration of "criteria" pollutants every twenty minutes. The national average is the average of 26000 samples a year at each location - literally millions of samples annually. They sample for the following:

PM10 (particulate matter of less than 10 microns diameter)
SO2
NOx
Ozone (surrogate for LA-style smog)
CO
Lead

Since 1976, when monitoring really got going, the national average concentration for each pollutant is way, way down. Lead has almost disappeared from the air. PM10 is down 60%. SO2 is down 70%, Ozone is down 40%. Carbon monoxide is down 90%.

A smaller particle called PM2.5 (2.5 microns or less) has become an item of interest. As a rule PM2.5 concentrations are about 5% of the PM10 concentrations.

All trends have been steadily down but are tailing to flat as they hit the asymptote. From here on, any improvements will be very small and very, very expensive. Two exceptions. Air quality could substantially improve if the failed New Source Review program were replaced with what was called Clear Skies. The big exception is that a huge improvement in air quality would be realized if the nation substituted nuclear energy for fossil fuel for electric power generation. PM10, SO2, NOx, and ozone would effectively disappear if nuclear took over.

The diesel particulate thing is an example of very small improvements for a hideous price. Not only does Tier II wreck the efficiency of diesel engines, but the ultra-low sulfur requirements mean that diesel fuel (and jet fuel, heating oil, and kerosene) have to be run through the Klaus Train four times instead of the previous two times. This has caused diesel to become more expensive than unleaded gas by 30-50 cents per gallon.