The generic name for several chlorofluorocarbons (CFCs) widely used in refrigerators and air conditioners, including the systems in houses and cars. Freon—comprised of chlorine, fluorine, and carbon atoms—is a non-toxic gas at room temperature. It is environmentally significant because it is extremely long-lived in the atmosphere, with a typical residence time of 70 years. This long life-span permits CFCs to disperse, ultimately reaching the stratosphere 19 mi (30 km) above the earth's surface. Here, high energy photons in sunlight break down freon, and chlorine atoms liberated during this process participate in other chemical reactions that consume ozone. The final result is to decrease the stratospheric ozone layer that shields the earth from damaging ultraviolet radiation. Under the 1987 Montreal Protocol, 31 industrialized countries agreed to phase out CFC freon production. Freon substitutes use bromine atoms to replace the chlorine atoms, providing a substitute refrigeration compound that appears less damaging, although considerably more expensive and less energy efficient.