For many years, analysis of air samples has shown risk levels far below regulatory limits. Since the end of aboveground nuclear weapons testing, the day-to-day radiation in air readings from monitoring sites has fallen. Some aboveground weapons testing by other countries continued until 1980. After the Limited Test Ban Treaty of 1963 was signed by the United States, the Soviet Union and Great Britain, most aboveground tests ceased. Over time the number and size (or yield) of these weapons increased, especially in the late 1950s and early 1960s. Between 19, hundreds of aboveground test took place around the world. The United States conducted the first aboveground nuclear weapon test in southeastern New Mexico on July 16, 1945. Very little radioactivity from weapons testing in the 1950s and 1960s can still be detected in the environment now. Most have very short half-lives, so decay away in a few minutes or a few days, for examples iodine-131, has a half-life of 8 days.
Some stay in the environment for a long time because they have long half-lives, like cesium-137, which has a half-life of about 30 years.
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