The U.S. Pentagon is a major source of perchlorate, the main ingredient of solid rocket fuel and a toxic chemical that can cause neurological problems and developmental damage in infants and children, and possibly cancer and other serious ailments in adults. For decades, millions of Americans have been exposed unknowingly to perchlorate through their local water supplies, but the substance remains only semi-regulated because of fierce debate between the U.S. EPA and the Pentagon over what levels constitute dangerous exposure. The Pentagon contends that small doses of the chemical are safe, and has asked Congress for an exemption from environmental laws concerning the clean up of explosive residues at its sites. The EPA, meanwhile, has identified 75 perchlorate hotspots in 22 states. For example, the Colorado River, which provides water to some 15 million homes in the Southwest, contains roughly seven parts per billion perchlorate, or seven times the level the EPA considers safe.