Halons used to extinguish fire are placed in another class of ozone-destroying chemicals. Because they contain bromine, they are three to ten times more dangerous than CFCs in ozone destruction potential. Halons include CF 2 BrCl, CF 3 Br, and C 2 F 4 Br 2 . However, one reason they are less harmful is that they are more unstable than CFCs and often enter into reactions in the troposphere. This can shorten their lifetimes and the lifetimes of the equipment and fluids with which they interact.
Hydrofluorocarbons are ozone-friendly because fluorine forms stable compounds in the stratosphere.
However, they have the potential to contribute to global warming. Many other substances involved in ozone depletion have both natural and manmade sources. The oceans and biomass burning are sources of these compounds. Methyl bromide is particularly destructive and is used as a pesticide, where it is injected into soil to kill pests. Oxides of nitrogen also have both natural and manmade sources.