Minor first-degree burns can occur up to 11 km (6.8 miles) away, and third-degree burns, which destroy and blister skin tissue, can affect anyone up to 8 km (5 miles) away. Third-degree burns that cover more than 24 percent of the body would likely be fatal if people don't get immediate medical attention. The U.S. Department of Health and Human Services recommends staying home for at least 24 hours in the event of a nuclear explosion.
After 48 hours, the exposure rate to a 10-kiloton explosion (the kind that could damage but not destroy a city) drops to just 1%. It is not peculiar to nuclear explosions, as it has been frequently observed in large forest fires and after incendiary incursions during World War II. This notion referred to the nuclear reaction of two atmospheric nitrogen atoms that form carbon and one oxygen atom, with an associated release of energy. If you have a scarf or handkerchief nearby at the time of a nuclear explosion, it is advisable to cover your nose and mouth.
The nuclear fragments of heavy element fission that are of greatest concern are radioactive atoms (also called radionuclides) that break down by emitting energetic electrons or gamma particles. Higher-performing nuclear weapons derive a substantial part of their explosive strength from the fusion of heavy forms of hydrogen: deuterium and tritium. The 1963 Limited Nuclear-Test-Ban Treaty ended atmospheric testing for the United States, Great Britain and the Soviet Union, but two major non-signatories, France and China, continued nuclear testing at a rate of approximately 5 megatons per year. While a serious event, such as a plane crash against a nuclear power plant, could result in the release of radioactive material into the air, a nuclear power plant would not explode like a nuclear weapon.
It is estimated that more than 500 megatons of nuclear performance detonated in the atmosphere between 1945 and 1971, approximately half of this yield was caused by a fission reaction. Depending on how close you are to a nuclear explosion, it may be impossible to avoid the initial burst of light, which can blind you for about 15 seconds to a minute. It is these reaction products and not gamma rays that contain most of the energy of nuclear reactions in the form of kinetic energy. Heat and debris in the air created by a nuclear explosion can cause rain; debris is believed to do this by acting as condensation cores for clouds.
Therefore, all nuclear detonations produce radioactive fission fragments of heavy elements, and larger bursts produce an additional radiation component of the fusion process. Since large doses of radiation of approximately 20 roentgen or more (see radioactivity note) are needed to produce developmental defects, these effects would likely be limited to areas of heavy local rainfall in nuclear warring nations and would not become a global problem. Based on these estimates, the consequences of the more than 500 megatons of nuclear tests until 1970 will produce between 2 and 25 cases of genetic diseases per million live births in the next generation. In a nuclear explosion, injury or death can occur as a result of the explosion itself or as a result of debris thrown by the explosion.
Given the uncertainties about the dynamics of a potential nuclear war, cancers induced by radiation and genetic damage together over 30 years are estimated to range from 1.5 to 30 million for the world's population as a whole.