Polonium 210, an important natural reference for radiological protection

Natural polonium 210 which has a relatively short decay period (138 days) and is uranium decay series daughter should retain more radiological attention because it induces chronic doses higher than that of traces of caesium 137 and plutonium isotopes (238, 239, 240) in the environment. In the earth's crust, uranium is present together with radium, radon and its daughter products, at a concentration of about 40,000 Bq t-1. As a consequence of radon 222 exhalation from soils within a metric layer and of lead 210 (radon daughter) fall-down, there is an enrichment of polonium 210 radioactivity in top layer soil relatively to radium concentration, within a factor varying from 2.7 to 8.8 according to environmental measurements. Human exposure to polonium 210 by ingestion is not negligible. The induced exposure by ingestion of natural polonium 210 is 30 to 70 limes higher than that of caesium from Chernobyl fall-down at Helsinki in 1986. Moreover, the latter decreases in relation with the 30 years' decay period while that from radon daughters remains. Comparison with plutonium shows a 500 to 2,000 times stronger health effect by ingestion for the same number of deposited becquerels on soil. 'Hot spots' may occur in the environment: polonium 210 is particularly concentrated by marine fauna, as shown by several measurement programs over the ares of north-east Atlantic. (author)