It will be helpful to review what is known about the radioactivity of the natural environment (Eisenbud, 1963), since this provides a yard­stick with which to compare the aec standards. An appreciation of the kinds and amounts of ionizing radiation exposure from natural sources will be relevant to my subsequent discussion of the significance of reactor produced radiation.

Radioactive substances are naturally present in the air we breathe and the food we eat. They become incorporated into our tissues in such amounts that on the average our bodies are literally disintegrating at a rate of about 500,000 times per minute due to radioactive decay.

The total body irradiation received by man from natural sources in most parts of the world is about 0.1 rad per year. This figure varies some­what from place to place, with an addition of about.028 rad per year for each 1,500 meters of altitude above sea level. Further deviations from the norm occur in places where the thorium or uranium content of the rocks and soils is above normal —in one village in Brazil, some people are exposed to as much as 12 rad/yr.

The lung and skeleton are selectively exposed over and above the dose received by the body as a whole. A large component of the dose to lung is due to the presence of atmospheric radon, the concentration of which varies from about 10"u uc/ml to about 2 X Ю’10 uc/ml in different parts of the world. A concentration of 1010 uc/ml will deliver a dose of about 1.3 rem/yr to the basal cells of the bronchial epithelium (the tissue of the lung, which is known to be particularly radiosensitive). Doses as high as ten times this value are possible indoors, particularly when the building is made of materials with a high radium content, such as the granites, radium-bearing shales, or concrete.

Radon, which has a half-life of 3.8 days, decays progressively through several shorter-lived progeny to 210Pb, which has a half-life of 22 years and which ultimately deposits on the earth’s surface. Only in the last few years have we begun to appreciate that mankind has always been subject to this form of natural fallout and that broad-leafed plants in particular have relatively high concentration of this isotope because of foliar deposition of 210Pb. According to one investigator, this phenomenon alone contributes an additional 41 mrem/yr to the lungs of individuals smoking one pack of cigarettes per day (Rajewsky & Stahlhofen, 1966).

Two naturally occurring nuclides, 226Ra and 228Ra, which are chemi­cally similar to calcium, enter our bodies through the foods we eat and are deposited with calcium in our skeletons. The daily radium ingestion of individuals in the United States is about 5 pCi/day, approximately equally divided between the two nuclides. Studies of food and water in various parts of the world have shown that there are wide variations from these mean values. In certain parts of the Middle West the radium intake is elevated by the presence of abnormally high amounts of radium in the drinking water, and the dose to the skeleton is increased by about 0.06 rem/yr. Considerably higher doses have been reported from Brazil and India, where there are radioactive anomalies of the type mentioned earlier (Penna Franca, 1965).

Thus, we can conclude that the whole body dose from natural radio­activity in most parts of the world is about 0.1 rem/yr. The lung receives a greater dose, owing to the superimposed radiation from atmospheric radon, and so does the skeleton in certain geographical areas where the radium content of food and water is elevated above normal.