EPA Radiogenic Cancer Risk Models and Projections for the U.S. Population

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http://www.epa.gov/japan2011/docs/bluebook/402-r-11-001.pdf

EPA 402-R-11-001 April 2011

EPA Radiogenic Cancer Risk Models and Projections for the U.S. Population

For ingestion or inhalation of radionuclides that concentrate in individual organs, the risk for those specific sites may predominate. In this context, it is important to recognize that the percent uncertainties for site-specific risk coefficients are generally greater than the coefficient for uniform, whole-body irradiation; this is largely due to the smaller number of cancers for specific sites in the LSS and to uncertainties in how radiogenic risks for specific cancer sites in the U.S. might differ from those in a Japanese population of A-bomb survivors.

Cancer sites with large relative changes in the calculated lifetime risk (about 2-fold or more) from EPA’s previous estimates published in Federal Guidance Report 13 (FGR-13) (EPA 1999b) include: kidney, liver, female lung, and female bladder (increased); and female colon (decreased). For both males and females, the estimated risk for all cancers combined increased by about 35%. For mortality, there was a notable change in estimated risk for cancers of the female colon (decreased), and female lung (increased). In general, the new EPA mortality estimates do not differ greatly from those in FGR-13; remarkably, for all sites combined, the estimates changed by less than 2% for both males and females.

Risks for childhood exposures are often of special interest. As shown in Figures 3-6 through 3-8, for most cancer sites, the LAR per unit dose is sub-stantially larger for exposures during childhood (here defined as the time period ending at the 15th birthday) than later on in life. In addition, doses received from ingestion or from inhalation are often larger for children than adults. Table 3-14 compares the average LAR per Gy for cancer incidence for exposures before age 15 to the average LAR for all ages. For uniform, whole-body radiation, the cancer risk coefficient (Gy-1) is 1.16x10-1 for people of all ages. This compares to 2.60x10-1 for exposures before age 15. The corresponding risk coefficients for cancer mortality are 5.80x10-2 (all ages) and 1.15x10-1 (before age 15). Risks from childhood exposures, like those for adults, are generally greater for females (3.29x10-1, incidence; 1.47x10-1, mortality) than for males (1.95x10-1, incidence; 8.51x10-2, mortality).

For example, updated risk coefficients for inhaled radionuclides retained in the lung may be larger than present estimates because the population-averaged lung cancer risk has increased substantially over time.

Among the most important of these studies are: nuclear workers in various countries (Cardis et al. 2005a, 2007, Muirhead et al. 2009); Chernobyl cleanup workers (“liquidators”) (Hatch et al. 2005, Kesminiene et al. 2008, Romanenko et al. 2008); children exposed to radioiodine releases from the Chernobyl accident (Cardis et al. 2005b, Tronko et al. 2006); residents downriver from the Mayak nuclear plant in Russia (Ostroumova et al. 2006, Krestinina et al. 2005); residents downwind from the Semipalatinsk nuclear test site in Kazakhstan (Bauer et al. 2005); and inhabitants of Taiwanese apartments constructed with steel beams contaminated with 60Co (Hwang et al. 2008). Studies on these populations are ongoing and suffer from various shortcomings, including incomplete follow-up, dosimetric uncertainties, limited statistical power and confounding. Nevertheless, results from several of them suggest that radiation risks can be detected and quantified, even in cases where the average dose rate is well below 1 mGy/day, corresponding to less than 1 ionizing track per cell nucleus per day (Puskin 2008).

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