Ozone, O3, is an important trace constituent of the stratosphere, where it forms the ozone layer. It consists of three atoms of oxygen arranged as an open triangle. Ozone is produced in the three-body reaction O + O2 + M = O3 + M, where M represents a stabilizing molecule (N2 or O2) and O is produced by the photochemical dissociation of O2. Some other components (NO, Cl, CFCs) catalyze the reaction of ozone destruction O3 + O = 2O2. The importance of ozone for life preservation lies in its unique capability to block solar UV radiation in a range particularly harmful to organic macromolecules (230–290 nm). It is also a greenhouse gas.