19 May 1953 - Dirty Harry: CTBTO Preparatory Commission

On May 19, 1953, a nuclear explosion in Nevada dropped fall-out on St George, Utah. From the article:

"19 May 1953 - Dirty Harry

Operation Upshot-Knothole is known for the Grable shot on 25 May 1953, a 280mm shell fired from the “Atomic Cannon”.
On 19 May 1953, the United States conducted the "Harry" nuclear test. It was part of a series of 11 nuclear weapon tests dubbed the operation "Upshot-Knothole" at the Nevada Test Site.

The 32 kiloton nuclear bomb was notable for two reasons. First, it was the most destructive pure fission weapon designed until then. Second, due to miscalculation and an unanticipated change in wind direction, this test generated more radioactive fallout than any other continental U.S. test, eventually leading the test to be nicknamed "Dirty Harry".

In many lawsuits, downwinders have sought financial compensation. In a landmark ruling in 1984, families of 10 people who had allegedly contracted cancer as a result of the series of above-ground nuclear tests in Nevada in the 1950s and 1960s were awarded US$ 2.6 million in damages. The court also ruled that the government was guilty of negligence in the way it had conducted the tests.

From measurements at St. George the test group later estimated that the maximum amount of external exposure that could have been received at St. George was 6.0 roentgens and 5.0 roentgens at Cedar City. Richard Hewlett and Jack Holl, Atoms for Peace and War

Communities living downwind of the Nevada Tests Site - the “downwinders” - reported higher cancer rates and loss of farmers’ livestock due to radiation poisoning. A number of studies conducted since have recognized the health hazards of nuclear testing.

The United States conducted its last nuclear test in 1992. It was the first country to sign the Comprehensive Nuclear-Test-Ban Treaty when it opened for signature on 24 September 1996. Today, it is one of the eight countries that have yet to ratify before the Treaty can enter into force. The others are China, the Democratic People's Republic of Korea, Egypt, India, Iran, Israel and Pakistan."