During the 1951-1963 period of testing at the Nevada Test Site, nearly 100 nuclear weapons tests were conducted above ground (DOE 1994), and another dozen or so were conducted underground at depths from which some atmospheric release of radioactive material was possible. Well before the Nevada tests began, the United States had the world's first test of a nuclear weapon at Alamogordo, New Mexico in July 1945. Soon thereafter, U.S. bombers dropped atomic bombs on Hiroshima and Nagasaki as part of a strategy to end World War II.

Following the war's end in 1945, the United States began testing nuclear weapons in the Pacific. From 1946 to 1958, 66 nuclear tests were conducted in the Marshall Islands, 12 near Johnson Atoll and 24 near Christmas Island (Simon and Robison 1997). All were atmospheric tests, except for four that were detonated underwater. After the first five Pacific tests, the United States began nuclear testing in Nevada.

Because testing in the Pacific was logistically complex and expensive, the government eventually moved all tests to the Nevada Test Site (NTS), which is located northwest of Las Vegas. This location was selected on the basis of such factors as safety, climate, geology, security, population density, and transportation. A few tests were also conducted at other locations in Nevada, New Mexico, Colorado, Mississippi, and Alaska. In addition to the U.S. testing during the 1950s, the Soviet Union and the United Kingdom also conducted atmospheric and other tests that produced a global fallout of radioactive materials. Subsequently, other nations including France, China, India, and, most recently, Pakistan conducted tests (the latter two involved underground tests only). This later testing has not added greatly to the exposure of the American public, but the additional testing in the 1950s (including a large program of American tests in the Pacific) has complicated efforts to estimate both the fallout of radioactive materials from the Nevada tests and the health consequences of that fallout. In addition to nuclear weapons tests, the backdrop for current concerns about exposure to I-131 includes the 1986 nuclear accident at Chernobyl and its aftermath, which includes many cases of thyroid cancer in exposed children.

Concern about radioactive fallout north and east of the Nevada Test Site began to emerge soon after weapons testing began. During the earliest years—before 1954—surveillance offsite of the NTS was performed by the Los Alamos Scientific Laboratory and the U.S. Army. The U.S. Public Health Service continued the offsite monitoring during the years 1954 through 1970. After 1970, monitoring activities offsite of the NTS became the responsibility of the Environmental Protection Agency under interagency agreements. Monitoring techniques included exposure-rate measurements along the roads in the counties surrounding the NTS, a fallout collection program using gummed-film devices in nearly every state, aerial monitoring by aircraft (begun in 1963), and other types of air and precipitation measurement (EPA 1984). Systematic studies of fallout patterns

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