Risk Assessment of Radon in Drinking Water (1999) / Chapter Skim
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4 Dosimetry of Ingested Radon and its Associated Risk
4 Dosimetry of Ingested Radon and its Associated Risk
Pages 59-81
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From page 59... ...
Studies of the behavior in the body of inhaled and ingested radon indicate that radon is readily absorbed by blood and is rapidly eliminated from the body in exhaled air. Because of the wide range in dose coefficients reported in the literature, the committee has undertaken an independent dosimetric analysis using the methods of contemporary radiation dosimetry.
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From page 60... ...
They estimated water intake on the basis of data collected by the US Department of Agriculture 1977-1978 Nationwide Food Consumption Survey and calculated daily intake and total water intake by various age groups of males, females, and both sexes combined. They defined tapwater as "all water from the household tap consumed directly as a beverage or used to prepare foods and beverages" and defined total water intake as tapwater plus "water intrinsic to foods and beverages." Table 4.1 summarizes data from the Ershow and Cantor study.
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From page 61... ...
DOSIMETRY OF INGESTED RADON AND ITS ASSOCIATED RISK TABLE 4.1 Tapwater Intake by Sex and Agea 61 Daily Tapwater Intake (mL) Age Group (y)
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From page 62... ...
However, ingested and inhaled radon is known to be promptly removed from the body by exhalation. Biologic removal processes are also applicable to the decay products formed within the body, but the short half-life of some decay products limits the importance of these removal processes.
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From page 63... ...
Adipose tissue is the major tissue of deposition of radon that has entered the systemic circulation. Estimates of Dose from Ingested Radon The inhalation hazard of radon and its short-lived decay products has long been of concern in occupational radiation protection and public health.
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From page 64... ...
, Sharma and others (1996~. In vitro studies have provided data on the solubilities and partition coefficients of the noble gases and other chemically inert substances in human blood, adipose tissue, and individual tissues; the data have been summarized by Steward and others (1973~.
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From page 65... ...
to estimate the dose from dissolved radon. They make no statements regarding the diffusion of radon into the stomach wall and computed, in a most unusual manner, the stomach dose as the alpha energy emitted within the stomach contents divided by the mass of the stomach wall and contents (C.T.
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From page 66... ...
Ingested radon enters the stomach and is absorbed from the gut as indicated in the upper right of figure 4.3. In figures 4.2 and 4.3, dashed arrows denote the transfer of radon as a gas, and solid arrows correspond to the flow of radon dissolved in arterial (thicker arrows)
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From page 67... ...
Venous blood is pumped by the right side of the heart to the pulmonary region of the lung, the "Pulmonary" compartment of figure 4.2, where radon dissolved in blood exchanges with alveolar air and is exhaled. Ingested 222Rn is readily absorbed and appears promptly in exhaled air.
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From page 68... ...
Further details regarding the radon biokinetic model, including the numerical values of the transfer coefficients of the resulting differential equations, are given in appendix A The equations are solved by assuming that a unit activity (1 Bq)
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From page 69... ...
69 FIGURE 4.4 The fraction of ingested radon remaining in the body (with the contents of the gut and in systemic tissues) at various times following an intake by ingestion.
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From page 70... ...
The upper dashed line represents the retention observed shortly after a subject had ingested a breakfast that included "heavy whipping cream." The other observations were at least 2 h after a "normal light" breakfast. The comparisons in figure 4.6 were limited to the time period of the observations.
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From page 71... ...
Diffusion of Radon in the Stomach As seen from table 4.4, the dose to the stomach depends strongly on whether radon is considered to move into the stomach wall, presumably by diffusion. Alpha particles emitted within the contents of the stomach cannot penetrate the mucus layer lining the epithelium and cannot reach the stem cells at risk (the range of alpha particles in tissue is about 50-60 lam)
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From page 72... ...
The time-integrated concentration was found to be insensitive to the value assumed for the diffusion coefficient and to depend somewhat on the depth to which radon was assumed to diffuse. Although the diffusion model of the stomach does not permit definitive conclusions, it does suggest that both radon concentration and its time integral vary over a rather limited range for a wide range in the diffusion coefficient.
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From page 73... ...
The set of "source regions" specifies the location of radionuclides in the body, and the set of "target regions" consists of organs and tissues for which the radiation doses are to be calculated. The source regions are those anatomical regions involved in the behavior of the radionuclide (and subsequent decay products)
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From page 74... ...
The absorbed dose is the time integral of the absorbed-dose rate. The equivalent dose is the absorbed dose of the various kinds of radiation weighted by a factor that represents their relative contributions to the biologic insult.
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From page 75... ...
Equivalent dose is a dosimetric quantity of radiation protection and is of limited utility in health risk assessments because the radiation weighting factor embodies consideration of the relative biologic insults of the different kinds of radiation. For reference purposes, table 4.5a gives the equivalent dose received by various tissues of adults, assuming an intake of a unit activity of 222Rn dissolved in water.
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From page 76... ...
Thus, despite the higher dose per unit intake at these ages, relative to that of the adult, the lower consumption rates result in intakes in the first 10 years that contribute about 30% to the lifetime risk. CANCER RISK PER UNIT 222RN CONCENTRATION IN DRINKING WATER Estimates of the cancer mortality risk per unit concentration of 222Rn in drinking water were derived with the method of Federal Guidance Report 13 (EPA 1998~.
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From page 77... ...
Absorbed dose rate as a function of time following a unit activity intake at each age FIGURE 4.8 Schematic of method to estimate cancer mortality risk per unit concentration of Rn-222 in drinking water, derived using the methodology of Federal Guidance Report 13 (EPA 1998)
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From page 78... ...
The baseline cancer mortality rates are calculated from US cancer mortality data for 1989-1991 (NCHS 1993a; 1993b; 1992~. The computation of sex- and site-specific values for the lifetime cancer risk per unit absorbed dose involves an integration over age, beginning at the age at which the dose is received, of the product of the age-specific risk-model coefficient (times the baseline mortality of the cancer in the case of a relative-risk model)
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From page 79... ...
are used to convert the calculated absorbed-dose rates to lifetime cancer risks for the case of an acute intake of one unit of activity at each age xi. This calculation involves integration over age of the product of the absorbed-dose rate at age x for a unit intake at age x, the lifetime risk per unit absorbed-dose received at age x, and the value of the survival function at age x divided by the value at age xi.
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From page 80... ...
It is assumed that the lifetime average drinking water consumption rate is 0.6 L d-l. For each cancer site and each sex, the lifetime cancer risk for chronic intakes is obtained by integration over age x of the product of the lifetime cancer risk per unit intake at age x and the expected drinking water consumption at age x.
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From page 81... ...
The asymmetric bounds reflect the judgment that the base case estimate is taken to be conservative; however, at this time sufficient information is not available to further refine the model and its parameter values. Similarly, it was judged that the stomach cancer mortality coefficients are probably not greater than three times the values of table 4.6 while they probably are greater than onetenth the tabulated values.
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