The loss of the Earth’s protective ozone layer would result in several years of extremely high ultraviolet (UV) light at the surface, a hazard to human health and food production. Most recent estimates indicate that the ozone loss after a global nuclear war would lead to a tropical UV index above 35, starting three years after the war and lasting for four years. The US Environmental Protection Agency considers a UV index of 11 to pose an “extreme” danger; 15 minutes of exposure to a UV index of 12 causes unprotected human skin to experience sunburn. Globally, the average sunlight in the UV-B range would increase by 20 percent. High levels of UV-B radiation are known to cause sunburn, photoaging, skin cancer, and cataracts in humans. They also inhibit the photolysis reaction required for leaf expansion and plant growth.