Radioactive fallout from the Nevada tests also is minimized by the manner in which the shots are fired, Most of the Nevada explosions occur well above the surface of the earth, with the result that only small amounts of earth are sucked up into the cloud, This is accomplished by exploding the devices atop towers as high as 500 feet or dropping them from planes, As the basis of its safety criteria for all Nevada tests the Commission uses an "operational guide” of radioactivity which is extremely conservative and far below the actual limit of safety, As was stated in the Commission's February 15 report: "In general, the total amount of radiation received by residents of the United States from a nuclear detonation to date, including che Russian and British tests and ail of our own tests in the United States and the Pacific, has been about one-tenth of one roentren,. This is only about 1/100th of the average radiation exposure inevitably received from natural causes by a person during his or her reproductive lifetime, It is about the same as the exposure received from one chest xeray." - 7 7, wee ed