Atmospheric nuclear tests were conducted in
unpopulated areas, and led to exposure at downwind
locations from radioactive fallout carried by wind
currents rather than from direct gamma-ray radiation,
which is emitted at the explosion site in the fi rst seconds
after a detonation. Each nuclear explosion creates
hundreds of radionuclides, many with half-lives of
3 months or less, so that their energy is mostly delivered
within the fi rst few months after a detonation. The
health risk to the public from nuclear weapons testing
arose from exposure to the radioactive byproducts.
Radioactive fallout from atmospheric nuclear
weapons testing was dispersed worldwide, resulting in
measurable radioactivity on every continent.5
Nuclear
tests varied considerably in size. In comparison with the
Hiroshima bomb that had an explosive energy of about
15 000 tons of trinitrotoluene (TNT), tests ranged in
size from less than 1000 tons to 50 million tons of TNT.
Although larger nuclear tests produced more radioactive
debris, the very large tests sent their contamination
into the very high atmosphere where much of the
radioactivity decayed before falling to the ground.