Scientists have also linked 428 of the 4,863 cancer cases that cropped up in atomic bomb survivors between 1950 and 1990 to genetic damage from the bomb blasts. Researchers think of radiation dosages as the amount of energy absorbed per unit of body mass, usually expressed in scientific units as joules per kilogram, says Keith Eckerman, a dosimetry expert at Oak Ridge National Laboratory in Tennessee. Some isotopes are more likely to affect human health following exposure, because they emit more radiation than other isotopes. It takes uranium 235 approximately 4 billion years to lose half of its radioactivity, and another 4 billion years for the remaining half of the isotope to lose half of its remaining radioactivity, and so on. Polonium 210 is far more radioactive than uranium 235.
Scientists measure the amount of radioactive energy deposited in tissue using a unit called a gray.
One grey is enough to radiation sickness, marked by nausea, vomiting, diarrhea, fever and the sloughing off of damaged tissue in the gut. But people become ill at much lower doses. A single absorbed dose of about 0.15 gray to the genitals can cause temporary sterility in men, while 0.25 grays delivered to a fetus at 28 days gestation can cause birth defects and other developmental problems. Studies of the survivors of the atomic bombings in Hiroshima and Nagasaki have shown, experts say, that a dose greater than 0.2 gray is enough to significantly increase the number of cancers that emerge in a population. Hoffman says his firm now is under contract to the National Cancer Institute to update 1995 tables reflecting the probability that a given disease was caused by a given exposure.