DELAYED SOMATIC EFFECTS OF RADIATION

a. A chronic exposure (as opposed to an acute exposure) is one received over a long period of time. Frequent occupational exposures or constant irradiation due to internally deposited isotopes fall within this category. The effects of a chronic exposure are long range and often difficult to determine; it is difficult to prove that such damage as cancer, leukemia, life-shortening, and genetic mutations are induced by radiation in any particular case. Statistical evidence, however, does link damage of this type to radiation exposures. Long-term effects of this type are not limited to being caused by chronic exposures; acute radiation exposures may also result in long-term effects.

b. Cancer or carcinogenesis is one long-term effect of radiation. There is a higher incidence of cancer among those who have received significant doses due to either chronic or acute exposure to radiation than in persons who have not. By 1922, a large percentage of the pioneer radiologists suffered from cancer. It is generally accepted that the probability an individual will suffer from cancer increases proportionally to his total absorbed dose.

c. Another long-term effect is leukemia, or cancer of the white blood cells. The evidence, as with cancer, is statistical, but there is a higher incidence of leukemia among those who have been exposed to excessive amounts of radiation than in the unexposed population.

d. By a similar statistical reasoning process, some studies have shown that radiation exposure decreases the life span of the irradiated population. By exposing large numbers of experimental animals to low levels of radiation over their entire life span, it has been shown that life span-shortening does occur and is probably due to accelerated aging.

e. Finally, one of the most significant long-term effects is the possibility of genetic damage or mutation. A mutation is a hereditary change that can be passed from generation to generation. Most mutations are harmful rather than beneficial; it is therefore not desirable to increase the mutation rate.

       (1) In 1928, it was shown that radiation exposure increased the genetic mutation rate in fruit flies. Since that time, this mutation effect has been observed in many animals, but data in humans are lacking due to the complexity of human genetics.

       (2) The important point to remember in considering the relationship between radiation and genetic effects is that radiation does not create new mutations, but rather increases the rate of mutations already present in the population. Most mutations are recessive; that is, both parents must possess mutant germ (reproductive) cells before the offspring will exhibit the mutant characteristic. For this reason, several generations may pass before the mutation will actually be seen in the population.

f. It is not known whether the effect at low exposures may be determined by assuming that there is a linear relationship between dose and effect, where even very small doses will produce some effect, or whether a threshold dose exists below which no effect will occur. Evidence indicates that cancer would fit the nonthreshold model, while other effects, such as cataracts, are threshold in nature. For radiation protection purposes, the linear, nonthreshold approach is used and all occupational exposures are maintained as low as reasonably achievable (ALARA).