The Morro do Ferro, a hill in the State of Minas Gerais, Brazil, is characterized by natural levels of ionizing radiation that are the highest reported of any natural habitats. The external radiation levels range from 0.1 to 3.2 mR/hour. The average concentrations of220 Rn and${}^{212}{\rm Pb}$ in rodent burrows are 27 nCi/liter and 23 pCi/liter. Because the dose to pulmonary tissue is highly dependent upon the metabolic fate of inhaled${}^{212}{\rm Pb}$, laboratory experiments were performed to determine the distribution of this nuclide in the rat after exposure to220 Rn and its daughters. The data obtained indicate that exposure to the average concentration of220 Rn and its daughters in rodent burrows on the Morro do Ferro result in an annual dose from alpha radiation of about 100 rads/year to the tracheal-bronchial epithelium and about 25 rads/year to the alveolar epithelium.

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