Risk Assessment of Radon in Drinking Water (1999) / Chapter Skim
Currently Skimming:
7 Defining Key Variabilities and Uncertainties
7 Defining Key Variabilities and Uncertainties
Pages 124-140
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 124... ...
In this chapter, the committee evaluates the importance of and methods for addressing uncertainty and variability that arise in the process of assessing multiple-route exposures to and health risks associated with radon. The data, scenarios, and models used to represent human exposures to radon in drinking water include at least five important relationships: · The magnitude of the source-medium concentration, that is, the concentration of radon in the water supply or in ambient air.
|
From page 125... ...
Intake via ingestion , 1 , Dose to the stomach Human anatomy/physiology data Inhalation rate Human activity patterns data Water consumption data Soil contact data Interpretation of estimates FIGURE 7.1 Steps of risk-assessment process for multimedia human exposure to radon, with emphasis on outcome calculated at each step and types of data needed to calculate outcomes.
|
From page 126... ...
That is followed by the committee's consideration of the steps of the risk-assessment process described in earlier chapters and of how uncertainty and variability apply to the assessment and the extent to which they can be quantified. Particular attention is given to the importance of uncertainty across the entire process of characterizing the unmitigated risk associated with radon in drinking water and the risk reduction achieved by various technologies used to reduce radon levels in water supplies.
|
From page 127... ...
ENVIRONMENTAL PROTECTION AGENCY PROCESS FOR ASSESSING AND EVALUATING UNCERTAINTIES IN RADON RISK In support of its proposed rule for radionuclides in drinking water, EPA has developed estimates of the cancer risk associated with radon in drinking water. The risk arises from multiple exposure pathways, including the direct ingestion of water that contains radon, the inhalation of indoor air that contains radon some of which has volatilized from water used in the home, and the inhalation of radon progeny that are introduced into indoor air as a result of radon decay.
|
From page 128... ...
The committee had a particular interest in the radon-ingestion risk model because the ratio of ingestion risk to inhalation risk is an important component of the multimedia approach to radon risk management. The committee observed that the EPA risk assessment used an appropriate approach to obtain the uncertainty factor for the population cancer risk associated with ingestion of radon in water.
|
From page 129... ...
The EPA risk assessment made no effort to assess the contribution of soil relative to that of water to indoor radon levels. Therefore, the study affords little input to the analysis of how any standard or policies can affect the risk associated with all radon exposures.
|
From page 130... ...
· Third, variance-propagation methods (including but not limited to Monte Carlo methods) should be used to map how the overall precision of risk estimates is tied to the variability and uncertainty associated with the models, inputs, and scenarios.
|
From page 131... ...
Neither this committee nor the BEIR VI committee has made risk estimates based directly on the emerging biophysical and cellular models. However, the study of molecular and cellular mechanisms of radiationinduced cancer brings to the risk assessment process important insights about the nature and magnitude of the uncertainties associated with the dose-response models discussed in this report.
|
From page 132... ...
National data on indoor radon, radon in water, and geologic radon potential indicate systematic differences in the distribution of radon across the United States. From geologic-radon potential maps and from statistical modeling of indoor radon exposures, it is clear that the northern United States, the Appalachian and Rocky Mountain states, and states in the glaciated portions of the Great Plains tend to have higher than average indoor radon (see chapter 2~.
|
From page 133... ...
The committee feels that there are problems with both the measurements of the transfer coefficient and the measurements that are the input values into the model. The committee recommends that EPA continue to use 1.0 x 10= as the TABLE 7.1 Parameters of the Lognormal Distributions for the Parameters in the Transfer-Factor Calculation Committee's Values Parameter Geometric Mean Geometric Standard Deviation House volume per occupant, m3 person-1 115 2.0 Ventilation rate 0.77 or 1.07 2.3 Transfer efficiency 0.52 1.3 Water use per capita, m3 person-1 fur-1 9.4 10-3 1.8 Transfer coefficient 5.5 x 10-5 or 3.9 x 10-5 3.5
|
From page 134... ...
Inhalation Risk per Unit of Radon-in-Water Concentration for Inhalation The committee did not conduct its own detailed uncertainty analysis for the risk model used for radon inhalation. Instead it reviewed the uncertainty analyses that have been carried out previously by EPA (1995)
|
From page 135... ...
On the assumption that those uncertainty ranges can be represented by log normal distributions, the BEIR VI committee derived from these ratios a GSD of approximately 1.3 for the exposure-age-concentration model and 1.2 for the exposure-age-duration model. From those results, we select an uncertainty factor of 1.3 to be applied to the inhalation risk factor for situations when the equilibrium factor used by BEIR VI applies.
|
From page 136... ...
Sensitivity and uncertainty analyses with this model helped the committee to bracket the range of risks that could plausibly be associated with ingestion of radon in water. The committee estimated that the diffusion coefficient in the stomach could have a plausible lower bound of 10-7 cm2/s and a plausible upper bound of 10-5 cm2/s (the diffusion coefficient of radon in water)
|
From page 137... ...
· Physiologic processes and histologic structures prevent gastric acids from digesting the stomach, and it is reasonable to assume that they restrict to some degree the movement of gases into the wall. · The basic input data for the calculations of stomach-cancer risk are based on risk factors derived from the Japanese atomic-bomb survivors, and a high background of stomach cancer among the Japanese population is well established.
|
From page 138... ...
Thus, it is concerned that this difficulty will be amplified significantly when there is a need to communicate information about uncertainties to the less technically oriented community groups that must make decisions based on relative-risk estimates. Another National Research Council report, Improving Risk Communication (National Research Council 1989)
|
From page 139... ...
That suggests that the EPA did not underestimate its confidence interval. However, the committee's uncertainty range is at the lower bound of the EPA range, and this suggests the likely upward bias of the EPA risk estimate.
|
From page 140... ...
There remains insufficient information to quantify interindividual variability in the cancer-risk models that are available. As a result, the cancer-risk models for inhalation described in this report are characterized only in terms of uncertainty, not variability.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.