Cesium-137 biospheric contamination from nuclear weapons tests.

Abstract

Weapons tests to date (end of 1957) have produced about 6 and 11 Mc of Sr90 and Cs137, respectively, of which about 1.9 and 3.4 Mc have been deposited as long-range fallout. Average surface deposition levels of Sr90 and Cs137 in the north temperate population belt may be about 20 and 36 mc/mile2, respectively, and the stratospheric reservoir may contain an additional 2.3 Mc of Sr90 and 4.2 Mc of Cs137. Cesium-137 contamination of the atmosphere and of drinking water, assuming they are in equilibrium with the rate of stratospheric fallout, does not appear to be a serious potential hazard to man's health even though weapons tests continue indefinitely at the past rate. The ingestion of contaminated foods is the principal source of Cs137 in the population. At present it is not possible to determine whether cesium is entering the biosphere and man via direct contamination of vegetation or via soil integration and plant uptake. If its mode of entry is through the soil, the potential radiation hazard from continued tests may be a factor of about 4 greater than if it is entering by direct contamination of vegetation. Because of metabolic similarity of cesium and potassium, cesium levels in the biosphere are conveniently expressed as Cs137/g K. The average 1957 Cs137 level in United States milk was about 32 pc/g K, and the level in the rest of the diet was about 20 pc/g K. The average 1956, 1957 and 1958 (through July) levels in the United States population were about 41, 44 and 54 pc/g K, respectively. Correlation of Cs137 levels in the population with levels in milk suggests a dietary discrimination factor of about 2 in favor of cesium over potassium in going from the diet to the body. Correlation of 1957 Cs137 levels in United States milk (expressed as pc/g K per in. of rainfall) with geographic meteorological conditions suggests that the manner in which tropospheric air masses approach or traverse the latitudes of high stratospheric fallout may be a factor in Cs137 and Sr90 surface deposition levels. The increase in Cs137 levels in milk and in the United States population from 1956 to 1957 was not proportional to the increase in integrated fallout, suggesting direct contamination of vegetation as its route of entry into the biosphere. However, measured levels presently in the population are compatible with predictions based on ecological considerations. The average 30 year internal plus external Cs137 radiation dose in equilibrium with a continuing test rate of 10 MT of fission per year is estimated as about 300 mr, or about 10 per cent of natural background, and the average total 70 year bone marrow dose is estimated as about 500 mr (assuming Cs137 enters the biosphere via the soil). Assuming 30–50 r as the genetic doubling dose and a non-threshold response to radiation, indefinite testing at the rate of 10 MT of fission per year may produce 0.5 to 1 per cent increase in the incidence of mutations as a result of Cs137 biospheric contamination. If weapons tests are stopped, the increase in incidence of mutations from Cs137 would be about one-tenth of the above estimate. A comparison of the mean bone marrow dose from Sr90 and Cs137 suggests that (…)