One of two oxides of chlorine, either Cl₂O or ClO. When used in environmental science,it usually refers to the latter. Chlorine monoxide (ClO) is formed in the atmosphere when free chlorine atoms react with ozone. In the reaction, ozone is converted to normal, diatomic oxygen (O₂). This process appears to be a major factor in the destruction of ozone in the stratosphere observed by scientists in recent years. The most important sources of chlorine by which this reaction is initiated appear to be chlorofluorocarbons and other synthetic chlorine compounds released by human activities.