Sources are mostly burning of coal and some from nature. Limits - Ave. 24 hr 280 ppb. Effects - Forms acids with water, very damaging to lungs. Mitigation - Remove sulfur before burning.
Sources are evaporation from fuels, plants, animals solvents.
Limits - 1 hr 160 ppb. Much of this is from cars. Effects - Combines with hemoglobin. Mitigation - Oxygenated gasoline, microbes, atmospheric oxidation. It has been greatly mitigated by switching to non-leaded gasolines.