The recommendations of icrp and ncrp were originally intended for protection of workers exposed to ionizing radiation. Before World War II, there was so little use of radiation that the need for standards to protect the public did not yet arise.

The early students of radiation protection did not have the benefits of the generous governmental grants that exist today, nor did they have the sophisticated laboratory equipment of present-day research. However, they did have an all too ample research resource arising out of the tragic misuses of ionizing radiations. Although before World War II there were relatively few X-ray machines, and the radioactive substances to which people were exposed were limited to about two pounds of radium that had been extracted by then from the earth’s crust, hundreds of deaths and many injuries resulted from an inadequate understanding of the principles of radiation hygiene. Fortunately, the effects of the misuses of these sources of ionizing radiation were studied with such extraordinary dili­gence and perception by the experts of a generation ago, that much of the basic information needed to protect the employees of the atomic energy program was already on hand by the time it was initiated during World War II. Two basic recommendations were already available, pertaining to the upper limit of permissible exposure to external X and gamma radia­tion, and to the maximum permissible body burden of radium.

The recommendation that the permissible body burden of 22eRa be limited to 0.1 pCi has not been changed since it was first established early in World War II. This yardstick had been a strong influence in setting the permissible body burdens of other bone-seeking radionuclides.

The maximum permissible dose of external radiation exposure per­mitted during World War II was 0.1 r/day, based on the scanty informa­tion that was available up to that time; this was equivalent to 20 r/yr. If we allow for the difference between roentgens and rads, and for the fact that the radiations now encountered in the atomic energy program are more penetrating than the 75-125-kv X rays that were the principal source of radiation before World War II, we find that the permissible dose for occupational exposure that was recommended by ncrp as much as 30 years ago is within a factor or two of 5 rem/yr permitted today for occu­pational exposure.

The problem of setting standards for protection of the general public is much more complex for several reasons. Because radiation workers are a relatively small fraction of the total population and because the genetic effects are related to the per capita gonadal dose of the population, genetic effects are less important than somatic effects insofar as occupational ex­posure is concerned. The probability of somatic injury at a given level of exposure in the general population is increased by the fact that children and fetuses are involved. Additionally, one becomes more conservative as the size of the exposed population increases, and in this country the gen­eral population is about one thousand times the size of the population in­dustrially exposed.

Leukemia and genetic mutations are the potential effects of ionizing radiation exposure that are of greatest concern insofar as the general population is concerned, and our discussion of the aec standards will fo­cus on these.

The incidence of leukemia has increased among several groups of humans exposed to relatively high doses of ionizing radiation (United Na­tions Scientific Committee on the Effects of Atomic Radiation, 1964). These include Japanese survivors of the atomic bombings of Hiroshima and Nagasaki, patients irradiated for ankylosing spondylitis, radiologists exposed to ionizing radiation in the course of their work, and children ir­radiated in utero in the course of pelvic X-ray examinations. The Japa­nese received a single dose which is estimated to have varied from 100 to 900 rads. The ankylosing spondylitis patients received fractionated doses of 100-300 rads over a one-month period. The cumulative doses received from day to day by the radiologists in the course of their practice is not known but is believed to be considerably in excess of 100 rads. The fetal dose in pelvic examinations is not known but probably was about 3 rads.

Thus, epidemiological experience involves mainly single or multiple exposures at high dose rates compared with those permitted by existing standards, as we shall see below. To estimate the expected effect of doses of a fraction of a rad delivered in small bits, one must extrapolate from these epidemiological data; in the interest of maximum safety, this is done by assuming that there is no threshold and that the biological response is proportional to the dose, and independent of the dose rate. Both the United Nations Scientific Committee on the Effects of Radiation (1964) and the icrp (1966) have emphasized that the estimates made in this way represent an upper limit of risk and that the actual risk may in fact be very much less.

Subject to these conservative assumptions, the epidemiological evi­dence suggests that a dose of 1 rad delivered to a million people (that is, 106 man rads) may produce a maximum of about 20 cases of leukemia during the lifetime of the population. The incidence of leukemia in the normal population is about 70 cases per million per year.

Insofar as genetic effects are concerned, we have no epidemiological information upon which to draw. However, we do have extensive research with lower animals which suggests there is no threshold for genetic effects and that the frequency of mutations is proportional to dose, but is not independent of dose rate (United Nations Scientific Committee on the Ef­fects of Atomic Radiation, 1966). According to these data, a per capita dose of about 10 rads per generation, delivered to successive generations, will eventually cause the spontaneous mutation rate to double. It has re­cently been shown however, that when the dose is fractionated, the genetic effect is somewhat less, by a factor of 6 (Russell, 1968). Thus, for con­tinuous exposure, a dose of 60 rads per generation delivered to many suc­cessive generations would eventually cause the spontaneous mutation rate to double. For a generation of 30 years, the doubling dose would thus be about 2 rad/yr.

We now come to the point where it is necessary to connect the thread of epidemiological and experimental evidence to the apparatus of public health regulation. Public health regulation is not a science but an art. The public health administrator, as an artisan, must start with a mix of scien­tific information of various grades of quality which he must evaluate according to certain philosophical concepts of permissible risk. He must then fabricate a system of regulation that is understandable, that is prac­tical, and above all, that protects the public health. Whether one is regu­lating radioactive, chemical, or biological contaminants of the environ­ment, the same approach must be taken, and the same questions inevitably arise. Thus, we regulate the biological quality of drinking water by making

assays for an innocuous group of coliform organisms because we are basically interested in certain pathogenic organisms that originate in fecal pollution, for which the coliform organisms are a useful indicator. This system of control does not provide absolute safety, but in most cases it provides more safety than is needed. In any case, for many decades this has been a practical system of control which has lent itself to practical systems of enforcement. Other regulatory mechanisms could be designed which would provide more safety, but they might not be so practical to administer or to enforce. The aec standards, for all their defects and in­consistencies, have served well to protect the public health (Environ­mental Radioactivity Exposure Advisory Committee, 1968).

To illustrate the quality of aec standards for protection of its em­ployees, there is the fact that, after about a quarter century of aec ex­perience, there is no known case of radiation injury among atomic energy workers from chronic exposure to ionizing radiation exposure. This is an excellent record, considering the great size of the program, the long period of time, and the enormous quantities of radioactive materials, compared with the disastrous experience with the relatively limited op­portunities for exposure that existed before World War II.

The basic criteria for the upper limit of permissible occupational exposure is that an employee should not accumulate more than 5 {N —18) rads, where N is the employee’s age in years (10CFR20). Stated another way, the employee should not work with ionizing radiation until he is 18 years old and then should not be exposed to more than an average of 5 rad/yr.

When internal radiation exposure is involved, the icrp methodology introduces the concept of the “critical organ,” the organ in which a given radionuclide tends to accumulate (International Commission on Radio­logical Protection, 1959). For example, the critical organ for iodine 131 is the thyroid and for strontium 90, the skeleton. With a few exceptions, exposure to internal emitters is controlled by limiting the quantities of radionuclides that may be absorbed by ingestion or inhalation to that amount which will result in exposure of the critical organ to less than 5 rad/yr.

The maximum permissible dose for exposure of the public, according to aec regulations, is one-thirtieth of the permissible occupational dose. The regulations assume that this average will not be exceeded if the most- exposed individual of a given population receives no more than one-tenth of the permissible occupational dose. In short, the mean exposure of a given population should not exceed 0.17 rad, and the maximum exposure of any individual in that population should not exceed 0.5 rad.