Review Of Epidemiological Studies On Hazards Of Radon Daughters

INTRODUCTION Radon-222 is an inert, radioactive gas formed by the radioactive decay of radium-226, a long-lived member of the uranium-238 decay chain. Radium is present in varying amounts in virtually all soils and, on the average, about 36,000 pCi (1330 Bq) of radon per square meter of soil diffuse into the atmosphere each day (UN 1977). Radon decays with a half-life of 3.6 days through four short-lived daughters to lead210 and it is these short-lived daughters[ [Ra A, (218Po, t, = 3.0 min) , Ra B, (214 Pb, t;, = 27 min), Ra C, (214Bi, t] = 20 min) and Ra C1 (214Po, t] = 2.5 x 10-6 min)] ]which cause the major health hazard associated with radon (Bale 1951). Atoms of these daughters, either unattached or attached to the ever-present particles in air, are deposited on the surfaces of the respiratory tract; alpha particles emitted in their decay can result in large doses to the cells of the bronchial epithelium lining the respiratory tract. These daughters will be present in air in varying relative concentrations depending on the "age" of the air (time since radon emanated into it) and on the amount of mixing of radon and radon daughter contaminated air with clean air. [The practical unit developed to quantify the amount of radon daughters in air is the Working Level (WL). This unit was historically related to the equilibrium concentration of 100 pCi (3.7 Bq) of the short-lived daughters of radon in one liter of air (cf. Holaday 1969) and is defined as any mixture of the short-lived daughters in a liter of air that have a-potential alpha energy of 1.3 x 105 Mev (2.08 x 10-5J) in their decay to lead-210. The working level month (WLM) was developed along with the WL, and was defined as an exposure to one WL for a working month (170 h). This is equivalent to 2.2 x 107 MeV•h•L-1 (3.54 x 10-3 J.h.m-3). If the average breathing rate is taken as 1.2 m3.h•1 (ICRP 1975), then one WLM is equivalent to inhalation of 4.24 mJ of potential alpha energy.] It is now generally agreed that the inhalation of radon daughters is the major potential radiation hazard in uranium mining, and contributes a substantial fraction to the natural radiation exposure of the general population due to the accumulation of radon and radon daughters from natural sources in buildings. Radon daughter concentrations in modern mines are controlled by ventilation, and by blocking off old working areas (cf. Simpson, 1959). However, before the hazard from radon daughters was recognized, considerably higher concentrations of radon daughters were present in some uranium and non-uranium mines. These high radon daughter concentrations resulted in an increase in lung cancers in the mining population, and it is these results that are our main source of information on the risk of inhaling radon daughters. HISTORICAL REVIEW Many excellent reviews of the history of understanding the health effects of inhalation of radon daughters are available (see, for example, Hueper 1942, Lorenz 1944, Sikl 1950, Stewart 1964, Holaday 1969, Lundin 1971, Cross 1979) but a brief summary of some of the highlights in this area may be of interest. [a] 1556: Agricola describes an unusual and fatal chest disease occurring among underground miners in the region of Schneeberg and Joachimsthal (Jachymov) in the Erz mountains in Central Europe. (It is of some historical interest to note that Agricola's book was translated from Latin into English by a mining engineer and his wife; the engineer later became President of the U.S.A.) [b] 1879: Haerting and Hesse indicated that the majority of deaths among Schneeberg miners were due to lung cancer; the lung cancers in these miners (who were incidentally not cigarette smokers) were observed twenty to fifty years after they began working in the mines. [c] 1896: Discovery of natural radioactivity by Becquerel, followed by discovery of radon by Dorn in 1900. [d] 1924: Ludewig and Lorenser report high concentrations (400 - 15000 pCi or 15 - 570 Bq per liter) of radon in the air in the Schneeberg mines and suggest that radon could be responsible for the high rate of lung cancer among miners. However, the reason why radon should cause lung cancer specifically was still not really understood up to twenty years later (Lorenz 1944). High concentrations of radon in the air in the Joachimsthal mines were reported by Behounek in 1927 and a high incidence of lung cancer among Joachimsthal miners (similar to that among miners at Schneeberg, which is only some 30 km distant but in a different political district) was noted at about the same time (Sikl 1930; cf. Sikl 1950). It is estimated that about half of the Joachimsthal miners died from lung cancer and about half from silicosis and tuberculosis (Sikl 1950). [e] 1930's and 1940's: Radioactive ores are deliberately mined in the U.S.A., Canada and other countries primarily as a source of radium for medical purposes and for luminescent dials; other radioactive ores are mined as a source of several non-radioactive minerals, while extensive uranium mining did not begin until the late 1940's. [f] 1940: Based on crude epidemiology and dose calculations, Evans and Goodman propose 10 pCi L-1 as a maximum permissible concentration of radon in continued human exposure. This is the first known recommended maximum permissible concentration for radon. This recommendation was adopted by the U.S. National Bureau of Standards in 1941 and reconfirmed in 1953. [g] 1945: Mitchell identifies the short-lived daughters in radon as a likely cause of the increased lung cancers in the Schneeberg-Joachimsthal miners