Halocarbons are mostly man-made gases consisting of both carbon and at least one of the halogens (fluorine, chlorine, iodine, and bromine). The majority of them fall into the category of Chlorofluorocarbons or CFCs. These gases are most well known for their ability to destroy stratospheric ozone, but they are also very strong greenhouse gases. On average they are thousands of times more efficient at warming than CO2. Fortunately, their concentrations are very small, so their powerful greenhouse effect is limited. Their atmospheric lifespan can range from 1 to 50,000 years. The sources included refrigerants and propellants. Most of these gases have been highly regulated by the Montreal Protocol (1989) and the vast majority of them are decreasing. Though emissions of CFC-12 have almost stopped, it is such a long lived gas that it is still responsible for some global warming as well as the ozone hole.