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A paper presented on Jul. 13, 1959 , at the Annual Conference, San Francisco, Calif., by Richard L. Blanchard, Asst. Sanitarian, Div. of Radiological Health, Occupational Health Field Sta., USPHS, Salt Lake City, Utah; George W . Leddicotte, Supervisor, Activation Analysis Group, Analytical Chemistry Div., Oak Ridge National Lab., Oak Ridge, Tenn.; and Dade W . Moeller, Chief, Radiological Health Training Activities, Div. of Radiological Health, Robert A. Taft San. Eng. Center, USPHS, Cincinnati, Ohio. This study was jointly sponsored by USPHS and Oak Ridge National Lab., and is based on the work performed by the authors at Oak Ridge National Lab.

THE radioactive gas radon-222, produced by the decay of radium-226, diffuses from the ground and is transported by atmospheric turbulence far from its origin through the troposphere and into the stratosphere1. The radioactive decay of radon-222 and its short-lived descendants produces measurable quantities of lead-210 in the atmosphere1–3. Lead atoms so formed become attached to dust particles and eventually return to the Earth's surface, principally in rainfall1,4,5. Hill's4 report of a correlation coefficient of 0.87 between the quantity of rainfall and the concentration of lead-210 in grass clearly indicates a linear relationship. Holtzman6 reported a relationship between the concentration of lead-210 in the human skeleton and in the atmosphere. Whether the lead-210 enters the body principally through precipitation on foodstuffs that are later eaten5 or through direct inhalation, or by both means7, is at present uncertain. It is therefore feasible to view the occurrence of lead-210 as a “natural fall-out” product, similar, in this respect, to fission product fall-out, artificially produced from a nuclear detonation. In fact, very strong similarities between lead-210 and fission product activities have been reported with respect to their behaviour in the atmosphere1, to their rate and mode of deposition1,5, and to their concentration in grasses4. The similarities between lead-210 and fission product activities do not imply any relationship in their production. The formation of lead-210 in nuclear weapons explosions is improbable, and if any were so formed it would be negligible in comparison with the amount formed by natural processes8. The observed similarities between lead-210 and fall-out are caused by their similar behaviour in the atmosphere.

The thorium-230: uranium-234 method of dating corals and oolites has been evaluated in detail for reliability, and various criteria have been established. Reliable ages for extensive coral formations of about 6000 and 120,000 years were obtained. A hiatus in the development of coral between 6000 and 120,000 years ago on the Pactfic atoll of Eniwetok implies that conditions did not permit coral growth during this period. The record prior to 120,000 years ago is not clear, probably because of a lack of unaltered samples.

THE principal radiation exposure of a uranium miner is to the lungs and results from inhaling short-lived 222Rn daughter products (Fig. 1) in the atmosphere1. The short-lived daughters (RaA, RaB, RaC and RaC′) decay in the lung to 22-year 210Pb, of which a fraction is absorbed into the blood stream and stored in the skeleton. Correlation coefficients of 0.84 and 0.77 between exposure to the short-lived daughter products and the 210Pb skeletal burden have been reported2,3, but these studies indicate that the concentration of 210Pb in bone may be as much as four times that expected from the inhalation of the short-lived radon daughter atoms alone3. Less than 10 per cent of additional skeletal 210Pb can be expected from inhaled 222Rn stored in body fat. In order that the concentration of 210Pb in bone should be a valid index of exposure to the 222Rn daughter products, it must be shown that 210Pb atoms supplied by other sources are proportional to the 222Rn daughter concentration.