The state air quality bureau reports that new equipment, changes in process and other improvements have cut some of the most dangerous air pollution Iowa in recent decades. Since 1990, sulfur dioxide emissions have dropped 60 percent and nitrogen oxides by 43 percent. Both can cause respiratory trouble, and sulfur dioxide also contributes to acid rain, the Iowa Department of Natural Resources reported. In the 1970s, 13 Iowa counties had areas that violated federal clean air standards; now there are two — Muscatine for sulfur dioxide and Pottawattamie for lead. Read more about Iowa air quality trends.