The destructive power of a nuclear bomb is immense and can cause catastrophic damage to people and property within a certain radius. Depending on the explosive performance and weather conditions, the hazardous rain zone can extend from 10 to 20 miles (15 to 30 kilometers). Those closest to the detonation would face death, while anyone within a distance of up to 5 miles could suffer third-degree burns. 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. In addition to the physical damage caused by a nuclear bomb, there are other effects that can be felt from a much greater distance. People within a maximum distance of 53 miles may experience temporary blindness due to the intense light produced by the explosion. Nuclear explosions can also produce clouds of dust and radioactive sand-like particles that disperse into the atmosphere, known as nuclear fallout.
This persistent radiation hazard could pose a serious threat for up to 1 to 5 years after the attack. Nuclear explosions also produce a powerful phenomenon called a nuclear electromagnetic pulse (PEM), an invisible burst of energy that can cut off power lines, telephone and Internet. This effect can be felt from hundreds of miles away, depending on the size of the bomb. 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.
For more information on the current state of nuclear weapons in the world, including the scale of bombs, you can visit the Nuclear Notebook in the Bulletin of Atomic Scientists.