- @2l - Crow concludes by stating that "...ionizing radiation is probably not an important factor in animal and plant evolution. If it is important anywhere, species, such as man, it is probably in those that have a long life span, and at least for man it is harmful rather than a potentially beneficial factor." Since the first nuclear explosion in 1945, radiation from artificial radioisotopes has been added to the environment but even today natural radioisotopes irradiate human peings far more intensely than man-made fallout (Arnold and Martell 1959). The estimated total radiation dose for an average resident of the United States for the first thirty years of life is 3 roentgens for background radiation, 3 roentgens for medical radiation to gonads, and 0.3 roentgens for fallout. Recognizing that even low levels of tonizing radi- ations from fallout are potentially hazardous, questions arise such as, "At what levels of radioactivity in the environment should we take positive measures to reduce exposure to radiation?" The general answer is that society must make the decision as to what is or is not an acceptable risk. The reason is explained by Loutit (1959) to be as follows: