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1. RISK OVERVIEW 9 or slightly increases. While absolute risk is very important from a public health perspective, we chose to discuss relative risk because of its use in the cited literature, as well as the ease of understanding by an individual person in the RECA population. The most recent and widely recognized lung-cancer risk estimates associated with that exposure were reported in the BEIR VI report (NRC, 1999a). An important finding of the BEIR VI committee relevant to some of the RECA populationsâidentified as uranium miners, uranium millers, and ore transporters âis that the excess relative risk of radiogenic lung cancer decreases with increasing age and time since exposure. Accordingly, the populations now alive and seeking compensation, who are generally over 60 years old and who have been out of the mines for 30 years or more, are at much lower relative risk now than they were shortly after retiring from mining uranium. For example, using data from 11 international cohorts, the BEIR VI committee estimated that uranium miners of 65â74 years old have about 25% of the excess relative risk of radon-induced lung cancer that miners in their 50s have, and the most recent analysis of the Colorado Plateau uranium-miner data (Hornung et al., 1998) estimated that the excess relative risk for miners in their 70s is less than 10% of that of miners in their 50s. Similarly, the BEIR VI committee estimated that miners who have been out of the mines for more than 25 years have less than half the lung-cancer risk of recently retired miners, and the analysis of the Colorado Plateau miner data indicated a 65% reduction in excess relative risk for miners who have been out of the mines for more than 25 years. The RECA population should also be informed that these analyses also have shown a synergistic relationship between exposure to radon and cigarette smoking. That is, the excess relative risk of radiogenic lung cancer for smoking miners is greater than the sum of the excess relative risks of smoking alone and radon alone. Risks to uranium millers and ore transporters have not been nearly as well characterized as risks to miners. A small mortality study of 662 millers from the Colorado Plateau published in 1973 (Archer et al., 1973) found a significantly elevated relative risk for lymphatic and hematopoietic cancers based upon only 4 observed deaths. A later study of millers expanded to 2002 millers in the same area was published in 1983 (Waxweiler et al., 1983). This more powerful study found no statistically significant increase in the mortality risks among the millers for any malignant neoplasms when compared with the risks among the general population. The only statistically significant increased risk for nonmalignant disease was for respiratory diseases but there was no significant trend with duration of employment in the mills. The committee is unaware of any epidemiological studies of ore transporters who would likely have very low exposures to ionizing radiation. Risks to Downwinders and Onsite Nuclear-Test Participants Estimates of the cancer risks associated with external exposure to gamma radiation generally are influenced by age at exposure and time since exposure; the risks are highest for those exposed in childhood and are relatively higher for the radiogenic leukemias than for other cancer types. For leukemias, except chronic lymphocytic leukemia (CLL), and for thyroid (nonmedullary) and lung cancer, the highest risks occur
1. RISK OVERVIEW 10 about 10 and 25 years, respectively, after exposure, and they appear to remain elevated thereafter though they gradually decrease (NRC, 1990; UNSCEAR, 2000a, b). The studiesâ results also suggest that the risk of thyroid cancer is only about 10% of the risk of benign thyroid nodules. The excess risk of thyroid cancer in exposed children declined by a factor of 2 for each 5-year age-at-exposure group under the age of 15 years (Ron et al., 1995). This suggests that excess relative risks to the surviving RECA populations (identified as downwinders and onsite nuclear-test participants) associated with their radiation exposure may now be substantially reduced compared to excess relative risks observed 10 to 25 years after radiation exposure occurred. The risks of most other types of solid cancers appear to reach a peak 20â25 years after exposure and then decrease but remain elevated for at least an additional 25 years. Risks posed by chronic exposure to low doses of sparsely ionizing radiation such as gamma rays, are estimated to be 1/3â1/2 of the risk following acute exposure to high doses, and this has been taken into account in estimating the risk of low doses of such radiation. This finding, however, is in contrast to the inverse dose rate effect found in the Colorado miners where exposure for long periods of time to low doses of densely ionizing radiation, specifically, alpha-particle radiation from radon decay products, was more harmful than exposure to high doses of this type of radiation for short periods of time (Hornung et al., 1998, NRC, 1999a). Estimates of the magnitude of the risk of all solid cancers combined, based on data from a low-dose group (less than 0.5 Sv) of atomic-bomb survivors, are consistent with the estimates derived from data obtained from survivors exposed to wider dose ranges (0â2 Sv; 0â4 Sv) (Pierce and Preston, 2000). Evidence of associations between radiation exposure and increased risks of some cancers (Hodgkinâs disease, non-Hodgkinâs lymphoma, multiple myeloma, and prostatic cancer) is weak or absent (chronic lymphocytic leukemia). Increased risk of thyroid cancer from external radiation to the head and neck has been observed in persons who received x-ray therapy (especially in childhood) and Japanese atomic-bomb survivors who received high whole-body doses. No increase was observed in adults who received even higher doses from the large amounts of 131I given in therapy. Children exposed to fallout from weapons testing at the Marshall Islands and from the Chornobyl accident have had a high incidence of thyroid cancer, but studies of downwinders who received much lower doses from the Nevada Test Site and from releases from Hanford (NRC, 1999b) have not revealed an increased incidence of thyroid cancer or of leukemia. The downwinders and onsite nuclear-test participants also were at risk of exposure to radiation internally from inhaled or ingested radioactive iodines, particularly 131I. Increased incidence of radiation-induced thyroid cancer and leukemia was anticipated based on the experience of other populations exposed externally to gamma radiation. However, epidemiologic studies of populations, mostly adults treated medically, with 131I, have shown no increased risk of leukemia and evidence is equivocal for thyroid cancer. Reports of increases in thyroid cancer incidence among populations less than 20 years old when exposed to radioactive fallout from the 1986 Chornobyl
1. RISK OVERVIEW 11 accident suggest that the risk is significantly greater among those who were less than 6 years old and resident in heavily contaminated areas of the former Soviet Union at the time of the accident. Uncertainties in individual dose estimates and about the influence of iodine deficiency on the induction of radiogenic thyroid cancer are among the factors that currently preclude reliable risk estimates for thyroid cancer in these populations (Ivanov et al., 2003). An unexplained statistically significant increase in the risk of nonmalignant disease mortality (in 1950â1985) with increasing dose was identified among atomic-bomb survivors. Analysis of updated data (1950â1990) has strengthened but still not explained the association with respect to nonmalignant circulatory, respiratory, and digestive system diseases. The increase in relative risk is about 10% of that for radiogenic cancer and appears not to be influenced by age at exposure or attained age. The data further suggest that the risk is negligible below 0.5 Sv (Shimizu et al., 1999). Thus, even if the association eventually proves to be causal, the likelihood that nonmalignant diseases in the RECA populations will be associated with radiation probably will be small in light of the high natural incidence of these diseases. Reviews by the National Research Council (Committee on Health Risks from Exposure to Low Levels of Ionizing Radiation, BEIR VII) of updated and new data from epidemiologic studies of exposed populations are in progress. Those studies might contribute to refinements in existing risk estimates, but they are not expected to identify new radiogenic diseases. In addition, an update of the National Institute for Occupational Safety and Health (NIOSH) mortality study of uranium millers has recently been completed and is under review. Publication of the results of the National Research Council and NIOSH studies is expected in 2004. On the basis of the above discussion, the committeeâs preliminary assessment is that 1. At this time, there is no new physical, biologic, or epidemiologic evidence to suggest a need to revise the estimates of risk for radiogenic cancers among populations previously exposed to ionizing radiation as identified in relation to RECA or the fundamental procedures used to estimate them (UNSCEAR 2000a, b; NCRP, 2001, NRC, 1999a). 2. The excess relative risk of lung cancer from exposure to radon decay products decreases with age and time after the last exposure. Similarly, excess relative risks for most of the cancers associated with onsite participantsâ and downwindersâ exposure decrease with age and time since exposure. Thus, the excess relative risks among the surviving population for developing cancers that may be attributed to radiation are lower today than they were when the exposures occurred several decades ago.