For two decades, scientists have been warning that chlorofluorocarbons (CFCs) and Halons (bromine-containing fluorocarbons) may deplete the stratospheric ozone shield that screens out some of the sun's harmful ultraviolet rays and thus regulates the amounts which reach the Earth's surface. CFCs have been used as refrigerants, solvents, foam blowing agents, and outside the United States as aerosol propellants; Halons are used primarily as firefighting agents. Increased radiation could result in an increase in skin cancers, suppression of the human immune system, and decreased productivity of terrestrial and aquatic organisms, including some commercially important crops.