The most biologically harmful of the Sun's ultraviolet (UV) radiation reaching the Earth's surface is UV-B, which spans wavelengths from 290 to 320 nm. In appreciable doses, UV-B harms plants and animals by damaging cellular DNA. The atmosphere's ozone layer absorbs this radiation and protects us from its more harmful effects. Recent theoretical and observational evidence shows that large volcanic eruptions, like that of Mt. Pinatubo in 1991, may cause shor-term ozone depletions, thereby enhancing the amount of UV-B that reaches the Earth's surface.