How does a thin amount of ozone high above Earth reduce ultraviolet exposure?

Sunlight reaching the ground contains less of certain ultraviolet wavelengths because ozone high in the stratosphere absorbs them. This region lies well above ordinary weather. Each ozone molecule contains three oxygen atoms; even a relatively small atmospheric amount can intercept selected ultraviolet radiation along sunlight's path.

An ultraviolet photon is a packet of light energy. Ozone absorbs selected photons, and the transferred energy can split an ozone molecule into an oxygen molecule and an oxygen atom. The atom can later combine with an oxygen molecule to make ozone again. This cycle repeatedly changes individual molecules; the protective region is not a solid screen that catches light without changing.

Through molecular reactions and collisions, absorbed radiant energy can become thermal energy in the surrounding air. Less of that ultraviolet continues toward the surface, while the stratosphere is warmed. A relatively small ozone supply can therefore matter because many molecules absorb across a large atmospheric path. Visible light and some ultraviolet still pass through: absorption depends on wavelength rather than blocking every kind of sunlight equally.

Ozone's effects depend on location and interaction: high above Earth it absorbs selected ultraviolet, while near the ground it can damage lungs and plants. Some released chemicals also speed ozone destruction by upsetting its chemical balance. The greatly exaggerated atmospheric drawing highlights a diffuse absorbing region, whose protection depends on molecular processes rather than a solid shield blocking all sunlight.