The highest ultraviolet (UV) index ever recorded on Earth was 43.3. This record-breaking UV index was measured on December 29, 2003, at the summit of Bolivia's Licancabur volcano, located in the Andes at an altitude of about 5,917 meters (19,423 feet)

. This extreme UV level is far beyond typical dangerous levels, as a UV index above 11 is already considered to pose an extreme risk of harm to unprotected skin and eyes. The 43.3 reading is more similar to conditions on Mars than on Earth, highlighting the intensity of UV radiation at high altitudes near the equator combined with ozone depletion and solar activity factors

. The unusually high UV index was influenced by a combination of factors including the high elevation, proximity to the equator, seasonal ozone depletion possibly caused by aerosols from storms and fires, and a large solar flare two weeks prior that may have further thinned the ozone layer

. In summary, the highest UV index ever recorded was 43.3 at Bolivia's Licancabur volcano in 2003, representing an extreme and unprecedented level of ultraviolet radiation on Earth's surface