The ozone layer is a region of the stratosphere, approximately 15–35 km above sea level, where ozone (O₃) concentrations reach their maximum — typically 2–8 parts per million. Despite these tiny concentrations, the ozone layer absorbs 97–99 % of the sun's harmful ultraviolet-B radiation (280–315 nm) and virtually all UV-C (<280 nm), protecting life on Earth from DNA damage, skin cancer, cataracts, and damage to marine ecosystems and crops.
Ozone is continuously created and destroyed in a natural cycle: UV radiation splits O₂ molecules into oxygen atoms, which combine with other O₂ to form O₃; ozone is then destroyed by UV absorption, releasing heat that warms the stratosphere. This Chapman cycle maintains a natural equilibrium. However, human-produced chlorofluorocarbons (CFCs) and other halogenated compounds disrupted this balance, catalytically destroying ozone. One chlorine atom can destroy 100,000 ozone molecules before being deactivated.
The most dramatic manifestation was the Antarctic ozone hole, first documented in 1985 — a seasonal depletion where ozone drops by 50–70 % each spring (September–November) due to the extreme cold and unique atmospheric chemistry of the polar vortex. The Montreal Protocol (1987), widely regarded as the most successful environmental treaty ever, banned CFC production and is allowing the ozone layer to slowly recover — full recovery is projected by ~2066 for Antarctica and ~2045 globally. The UV index used by weather services directly reflects ozone column thickness alongside solar elevation and cloud cover.