Nuclear weapons: lethal radius by bomb size and shelter type

In the event of an air burst nuclear attack, an underground shelter provides significantly more protection to nearby populations than a basement-level shelter. For a one megaton nuclear bomb, the lethal radius for those in an underground shelter would be over eight times smaller than the lethal radius for those in basement shelters. In fact, basement shelters provide relatively little protection in comparison to underground shelters, as their lethal radii is less than 20 percent smaller than that of populations in unprotected areas, whereas underground shelters' radii are over 90 percent smaller. This is largely due to the impact of thermal radiation, as temperatures from a one megaton bomb would exceed several thousand degrees Celsius at the epicenter, and would cause third degree burns up to six miles from ground zero. Additionally, shelters must be able to withstand the blast force of the explosion.

Thereafter, populations would be subject to the nuclear radiation from both the blast and fallout, where protection is dependent on the shelter's material, thickness, and quality. The figures given are based on the assumption that the basement shelter is built from eight-inch thick unreinforced concrete blocks, whereas the underground shelter is built with 12 inches of concrete and under 30 inches of earth. Different materials offer different levels of protection, for example, a one inch layer of lead would be equal to 4.4 inches of concrete, or 17.6 inches of wood. However, due to the toxic side effects of lead exposure (which was less-understood during the cold war), most modern underground fallout shelters would be constructed with a combination of concrete and steel, with several engineering considerations, such as blast doors or entrance corridors with corners (as gamma rays travel in straight lines).