If the entire ozone layer were compressed to sea level pressure, it would be only 3 millimeters thick.

The thickness of the ozone layer is measured in Dobson Units (DU), named after G.M.B. Dobson, who developed the first instrument to measure stratospheric ozone in the 1920s. One Dobson Unit represents a layer of pure ozone that would be 0.01 millimeters thick at standard temperature and pressure. On average, the Earth's atmosphere has an ozone concentration of about 300 Dobson Units, which equates to a layer just 3 millimeters thick. Most atmospheric ozone is found in the stratosphere, between 10 and 50 kilometers above the Earth's surface. At these high altitudes, the air pressure is extremely low, causing the ozone molecules to be widely dispersed. If these same molecules were brought down to the Earth's surface where the pressure is 1 atmosphere, they would collapse into the remarkably thin layer described. This thin veil is critical for biological survival because it specifically filters out UV-B radiation. According to NASA and the World Meteorological Organization, the ozone layer prevents high-energy photons from reaching the surface, where they would otherwise cause DNA damage and skin cancer. The 1987 Montreal Protocol remains the primary international treaty dedicated to protecting this layer by phasing out ozone-depleting substances like chlorofluorocarbons.