Ambient levels of all air pollutants targeted by regulations have declined since the 1970s. The air in most metropolitan areas is improving. Between 1975 and 1995, ambient levels of NOx in the U.S. declined by 37.3 percent, and ambient levels of ozone decreased by 25.7 percent between 1976 and 1995. Acid rain is not a threat. It has no observable effect on human health, nor has it damaged North American crops.
Sulfur dioxide is the principal component of acid rain.
Ambient levels of PM, more commonly known as dust and soot, have decreased 22 percent since 1988. Water quality, though difficult to measure, is improving. Industrial pollution is dropping. Organic wastes have fallen by 46 percent, toxic organics by 99 percent, and toxic metals by 98 percent. Rivers and streams are cleaner.