indicate that over 450 mines shipped ore in 1951. Mining companies introduced radon surveys in 1956 and the state programs continued through 1960. Both company and state-sampling efforts were made in work areas for information purposes, not for control purposes, and "are considered to be representative of the areas in the mines in which miners were exposed" (Lundin and others 1971). This early data base is of primary importance in considering the adequacy and precision of miner's exposure estimates as utilized in epidemiology assessments of risks due to radon since a large portion of the cumulative exposure occurred in the 1950's.
By 1960, exposure levels had dropped precipitously in anticipation of Colorado's adoption of a 10 WL shutdown level in 1961. However, regulatory control probably reduced the validity of the measurements in mines for epidemiologic purposes. As outlined in Joint Monograph No. 1, the most complete description of the Colorado Plateau miner data (Lundin and others 1971) "Most radon daughter measurements available from Colorado, Utah, and Wyoming after 1960 were made by mine inspectors who measured air samples primarily for control purposes." This may have led to bias in the estimated exposures. As noted by Lundin and others (Lundin and others 1971), "Proportionately more measurements were made in sections of mines having high levels which tended to yield radon-progeny values greater than would have been obtained by sampling all work areas with equal frequency." In addition more measurements were concentrated in mines having high levels of radon. The U.S.P.H.S. investigators who assembled the data base for estimating cumulative exposures chose to exclude company measurements made after 1960 on the grounds that they might have been "minimized to avoid regulatory action." The aim was clearly ''to assure a consistent direction of bias, that is, over estimation of radon daughter levels" (Lundin and others 1971).
Even though the number of radon-progeny measurements increased during the 1960's, the number per mine increased only slowly from about six in 1960 to almost 12 in 1968 (Figure E Annex 1-1). Measurements of radon progeny in a particular mine were never extensive and, more importantly, were not made on even a once per year basis in the majority of mines. Only 341 miners, about 10% in the Colorado Plateau miner cohort, had their exposure assignments based on measured radon-progeny concentration. For the majority of the miners, information on measured levels was combined with estimates made using a variety of methods as described by Lundin and others (1971).
Many of the uranium miners were also employed as hardrock miners or previous to 1950 some had mined the same ore bodies, where uranium was found, for radium, vanadium etc., particularly in the Urivan Mineral Belt in Colorado. In the epidemiologic study, hardrock miners were assigned an exposure level of 1 WL for mining that occurred before 1935, 0.5 WL for 1935 through 1939, and 0.3 WL for later years (Lundin and others 1971). No information is given as to the basis of these estimates but a statement is included in Joint Monograph No. 1