ar fragments of heavy-element fission which are of greatest
concern are those radioactive atoms (also called radionuclides) which
decay by emitting energetic electrons or gamma particles. (See
"Radioactivity" note.) An important characteristic here is the rate of
decay. This is measured in terms of "half-life"--the time required for
one-half of the original substance to decay--which ranges from days to
thousands of years for the bomb-produced radionuclides of principal
interest. (See "Nuclear Half-Life" note.) Another factor which is
critical in determining the hazard of radionuclides is the chemistry of
the atoms. This determines whether they will be taken up by the body
through respiration or the food cycle and incorporated into tissue. If
this occurs, the risk of biological damage from the destructive
ionizing radiation (see "Radioactivity" note) is multiplied.

Probably the most serious threat is cesium-137, a gamma emitter with a
half-life of 30 years. It is a major source of radiation in nuclear
fallout, and since it parallels potassium chemistry, it is readily
taken into the blood of animals and men and may be incorporated into
tissue.

Other hazards are strontium-90, an electron emitter with a half-life of
28 years, and iodine-131 with a half-life of only 8 days. Strontium-90
follows calcium chemistry, so that it is readily incorporated into the
bones and teeth, particularly of young children who have received milk
from cows consuming contaminated forage. Iodine-131 is a similar
threat to infants and children because of its concentration in the
thyroid gland. In addition, there is plutonium-239, frequently used in
nuclear explosives. A bone-seeker like strontium-90, it may also become
lodged in the lungs, where its intense local radiation can cause cancer
or other damage. Plutonium-239 decays through emission of an alpha
particle (helium nucleus) and has a half-life of 24,000 years.

To the extent that hydrogen fusion contributes to the explosive force
of a weapon, two other radionuclides will be released: tritium
(hydrogen-3), an electron emitter with a half-life of 12 years, and
carbon-14, an electron emitter with a half-life of 5,730 years. Both
are taken up through the food cycle and readily incorporated in organic
matter.

Three types of radiation damage may occur: bodily damage (mainly
leukemia and cancers of the thyroid, lung, breast, bone, and
gastrointestinal tract); genetic damage (birth d