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