The three pollutants that have decreased the most since 1990 are sulfur dioxide (SO2), lead (Pb), and carbon monoxide (CO). Specifically, sulfur dioxide has seen a decrease of about 88-93%, lead emissions have dropped by about 80-88%, and carbon monoxide has decreased by roughly 65-77% since 1990.
These reductions are attributed to regulatory measures such as the Clean Air Act, technological advances, and stricter emissions controls on vehicles and industrial sources.
