Risk Assessment of Radon in Drinking Water (1999) / Chapter Skim
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5 Dosimetry of Inhaled Radon and its Associated Risks
5 Dosimetry of Inhaled Radon and its Associated Risks
Pages 82-104
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From page 82... ...
The evaluation of lung-cancer risk in miners is summarized later in this chapter. RISK POSED BY INHALATION OF 222RN DECAY PRODUCTS The assessment of risk in miners did not rely on internal dosimetry but was based on the air exposure to decay products in units of potential alpha energy concentration (PAEC)
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From page 83... ...
LUNG DOSE FROM 222RN GAS The alpha dose delivered to target cells in bronchial epithelium arises mainly from the short-lived decay products deposited on the bronchial airway surfaces. The alpha dose from radon gas itself is smaller than that from its decay products because of the location of radon as it decays in the airway; there is a low probability that an alpha particle will interact with a cell.
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From page 84... ...
222RN DECAY-PRODUCT DOSE DURING SHOWERING The most important variables in the alpha dose to cell nuclei in the bronchial airways are aerosol size distribution, breathing rate, and location of the target-cell nuclei. The most extensive activity-weighted size distributions that have been measured in homes were reported by Hopke and others (1995a)
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From page 85... ...
The exposure assessment of waterborne radon includes both its contributions to long-term average indoor radon concentrations and the short-term, perturbed conditions that exist as a result of showering. Showering Conditions It is necessary to provide the radon-progeny activity size distribution as a function of time during and after showering.
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From page 86... ...
o.~s 0.30 0.25 non 0.15 0.10 0.05 PAEC 10 100 Particle Diameter (nary) FIGURE 5.1 Average values of the activity fractions for each of the decay products and for PAEC (no smokers present)
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From page 87... ...
its decay products are provided by Fitzgerald and others (19971. Figure 5.3 shows the radon concentration as a function of time under several ventilation conditions.
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From page 88... ...
FIGURE 5.3 Radon concentration as a function of time under several ventilation conditions. 3500 3000 2500 zooo _/ V 1500 1000 500 o 4 ~o M ~ _~ .
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From page 89... ...
However, the system available for such activity-weighted size distribution measurements draws 90 L 3000 2500 2000 1 500 1 000 500 O 1 r r 1 r 1 -- - Before Shower A -- During Shower - / \ After Shower as/ \ /\ \ / \ \ I \ - j \ / i\\\\ \ \ \ ~ \ \ \ \ \ \ \ \ \ 1 \ \ 1 20 30 50 70 100 200 300 500 Particle Diameter (nary) FIGURE 5.5 A series of number-weighted particle size distributions based on measurements with a scanning electrical mobility spectrometer.
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From page 90... ...
Using the equations given by Porstendorfer and others (1979) , the attachment coefficients for 222Rn decay products to any size of particle can be calculated.
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From page 91... ...
This is not unexpected, because as radon gas is released from water droplets and decay products form, they interact with the normal home aerosol particles. Integrated Measured 222Rn Concentration in the Shower The 222Rn released to a shower from water was monitored in an ultra-high energy efficiency home with a private well (N.
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From page 92... ...
This is compared with the normal home equilibrium of 0.40 and a bronchial dose factor of 0.032 mGy yr-i per Bq m-3. For typical home conditions, the annual shower exposure is a few percent of the whole house exposure attributed to water use.
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From page 93... ...
Although the 222Rn decay-product exposure data are universally weak in all the miner epidemiologic followup studies, they are the only human data available from which one can derive numerical estimates of occupational and domestic lung-cancer risk. Some generalizations are possible and necessary to quantitate this risk.
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From page 94... ...
Linear regression fit to data from 8 domestic studies aExposure assumes a home concentration of 148 Bq m-3 (4 psi L-1 or 0.56 WLM) , calculated with 40% decay product equilibrium, and actual exposure is 70% of the home exposure.
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From page 95... ...
The relative-risk model assumes that radon decay product exposure increases the age-specific lung-cancer mortality rate in the population by a constant fraction per WLM of exposure. However, in all cohorts, there was an obvious reduction in lung-cancer relative risk with time after exposure.
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From page 96... ...
The values of lifetime risk as calculated by the BEIR IV TSE model are also shown in table 5.5. NCI Model The National Cancer Institute coordinated an effort to pool the epidemiologic data from 11 underground-mining studies.
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From page 97... ...
That observation is erroneously called the inverse-exposure effect. It is usually stated that the increase in lung-cancer risk per unit exposure is higher for low exposures than for high exposures.
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From page 98... ...
and relative risk (RR) of lung cancer (right side)
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From page 99... ...
= W5-14+015-24W15-24+025+W25+ · Here the subscript a denotes categories of attained age and the subscript z denotes categories of either exposure duration (in years) or radon concentration in WL.
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From page 100... ...
. Concern has been expressed that the same might be true for lung cancer that results from exposure to radon and its decay products at early ages.
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From page 101... ...
The trend of county lung cancer mortality with increasing home radon concentration is strikingly negative, even when attempts are made to adjust for smoking prevalence, and 54 socioeconomic factors. The measured average county radon concentrations do not exceed 300 Bq m-3, thus the typical low home exposure region is studied.
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From page 102... ...
In the West Bohemia study of 4,320 uranium miners, 28 sites of cancer mortality were evaluated. No statistically significant cancers other than lung cancer occurred.
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From page 103... ...
EVALUATION OF RISK PER UNIT EXPOSURE FROM INHALED 222RN IN AIR The lifetime risk of lung cancer associated with indoor radon concentration of 150 Bq m-3, calculated from the various models, is summarized in table 5.5. The most recent estimate of risk of lung cancer in the United States due to inhalation of radon decay products is from the model published in the BEIR VI report.
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From page 104... ...
and never-smokers models for exposure in homes derived directly from the mines is considered valid for predictive purposes (National Research Council 1999~. The population estimate, 1.6 x 10~, for the lifetime risk of lung cancer for lifetime exposure in the home to 1 Bq 222Rn m-3 in air, as derived from BEIR VI, is the value adopted by this committee.
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