Air Pollution The Problem Contamination of the atmosphere by gaseous, liquid, or solid wastes or by-products that can endanger human health and the health and welfare of plants and animals, or can attack materials, reduce visibility, or produce undesirable odors. Among air pollutants emitted by natural sources, only the radioactive gas radon is recognized as a major health threat. A byproduct of the radioactive decay of uranium minerals in certain kinds of rock, radon seeps into the basements of homes built on these rocks. According to recent estimates by the U.S. government, 20 percent of the homes in the U.S. harbor radon concentrations that are high enough t