Editing Ozone–oxygen cycle (section)

===Additional reactions===
In addition to these five reactions, certain [[free radical]]s - the most important being [[hydroxyl]] (OH), [[nitric oxide]] (NO), and atomic [[chlorine]] (Cl) and [[bromine]] (Br) -  [[Catalysis|catalyze]] the recombination [[Chemical reaction|reaction]], leading to an ozone layer that is thinner than it would be if the catalysts were not present.

Most OH and NO are naturally present in the stratosphere, but human activity - especially emissions of chlorofluorocarbons ([[Chlorofluorocarbon|CFC]]s) and [[Haloalkane|halons]] - has greatly increased the concentration of Cl and Br, leading to [[ozone depletion]]. Each Cl or Br atom can catalyze tens of thousands of decomposition reactions before it is removed from the stratosphere.