Science and Judgment in Risk Assessment (1994) / Chapter Skim
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Part III Implementation of Findings: 12 Implementation
Part III Implementation of Findings: 12 Implementation
Pages 243-268
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From page 243... ...
are set in accordance with a Congressional mandate to provide "an adequate margin of safety." Where data do not exist (particularly with respect to responses to low doses and mechanisms of toxicity) , EPA has generally chosen default options that, in addition to being in keeping with current scientific knowledge, are intended to be conservative (i.e., health protective)
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From page 244... ...
A stepwise process that replaces default assumptions with specific data can be expected to yield more and more firmly established standards (regulatory doses) ; i.e., uncertainty should be reduced as a consequence of having more information.
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From page 245... ...
Neither the resources nor the scientific data exist to perform a full-scale risk assessment on each of the 189 chemicals listed as hazardous air pollutants by Section 112. Nor, as we noted in Part II, is such an assessment needed in many cases.
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From page 246... ...
In the absence of sufficient data or resources to characterize each risk-assessment parameter accurately, EPA deliberately uses default options that are intended to yield health-protective risk estimates. Lower-tier risk assessments that are used for preliminary screening rely heavily on default options, and their results should be health-protective.
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From page 247... ...
has proposed a tiered scheme for using health risk assessments to delist source categories and eliminate residual risk. EPA asserts that this scheme provides health-protective estimates of risk by assuming maximal exposure levels, except for cases related to complex terrains (for which an alternative dispersion model should be selected from the complex-terrain models available to EPA to estimate maximal concentrations of chemicals in air and hence maximal exposure levels)
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From page 248... ...
Maximum offsite cancer risk (based on unit risk estimate, URE) Chronic noncancer hazard index (based on chronic health thresholds)
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From page 249... ...
The committee does not recommend any specific model, but suggests that EPA look beyond its set of existing models to find the best possible ones for the dispersion of hazardous air pollutants in the particular type of complex terrain that applies in each case. In addition, models should be considered that account for the possibility of a negative buoyancy plume (i.e., gas heavier than air)
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From page 250... ...
For many of the 189 chemicals now on the Section 112 list, there are no IRIS entries, or the existing entries do not include cancer potencies for suspected carcinogens or RfCs for chemicals suspected of causing acute or chronic noncarcinogenic health effects. In these cases, it will be appropriate for EPA to develop crude screening estimates of cancer potencies and RfCs for use in research planning; if the screening values are entered in IRIS, they should be clearly identified as screening values.
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From page 251... ...
First, the tiered approach requires a conservative first level of analysis. EPA asserts that its approach provides a conservative risk estimate, except in the case of complex terrain.
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From page 252... ...
· Use an upper-bound interspecies dose-scaling factor for screening-level estimates. · Fully communicate to the public each risk estimate, the uncertainty in the risk estimates, and the degree of protection.
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From page 253... ...
If the evidence from this step or from reviews of the clinical, epidemiologic, or toxicologic literature suggests potential human health concerns, aggregate emission data and estimates of potentially exposed populations should be reviewed. The completed preliminary analyses, including a description of the assessment process used and the findings, should be placed in the public domain (e.g., IRIS or another mechanism readily accessible to the public)
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From page 254... ...
In summary, this iterative approach to gathering and evaluating the existing evidence is intended to produce a risk assessment for each of the 189 chemicals (or mixtures) that is appropriate to the quality and quantity of available evidence, the estimated size of the problem, and the most realistic scientific judgment of potential human-health risks based on that evidence.
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From page 255... ...
In addition, attempting to incorporate default assumptions of unknown conservatism into each step of a risk assessment can lead to an insufficiently or too conservative analysis.
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From page 256... ...
But EPA operates in a decision-making context that imposes pressures on the conduct of risk assessment, and these contextual pressures have led to recurrent problems of scientific credibility, the most important of which were noted in Chapter 2. Criticisms of EPA's risk assessments take a variety of forms, but many of them focus on three basic decision-making structural and functional problems: unjustified conservatism, often manifested as unwillingness to accept new data or abandon default options; undue reliance on point estimates generated by risk assessment; and a lack of conservatism due to failure to accommodate such issues as synergism, human variability, unusual exposure conditions, and ad hoc departures from established procedures.
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From page 257... ...
Uncertainty Analysis Flowchart 4. Conduct a Monte Carlo analysis by using probability distributions of parameters as input for simplified versions of each model (e.g., emissions, exposure, and dose-response relationship)
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From page 258... ...
Problems of intra-agency coordination that have persisted throughout EPA's history create communication gaps between risk assessors and managers. The firef~ghting mode in which the agency all too often operates hinders the design of effective long-range research programs and even the formulation of the right questions for science to answer.
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From page 259... ...
To remain accountable to the public under these circumstances, regulatory agencies like EPA must assess uncertain science in accordance with principles that are fully and openly articulated and applied in a predictable and consistent manner from case to case. Risk-assessment guidelines and default assumptions were designed to accomplish those objectives, and they have succeeded to a large extent in making EPA's decisions both transparent and predictable.
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From page 260... ...
Protecting the integrity of the risk assessment, while building more productive linkages to make risk assessment more accurate and relevant to risk management, will be essential as the agency proceeds to regulate the residual risks of hazardous air pollutants. Risk assessment should be an adjunct to the Clean Air Act's primary goal of safeguarding public health, not an end in itself.
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From page 261... ...
Situations might be discovered where current risk-assessment practice is underestimating health risk or where the information base for a chemical is not sufficient to allow regulation to proceed. Present EPA practice is to remove IRIS listings while cancer potencies or RfCs are under review.
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From page 262... ...
in these settings can give rise to higher public-health risk in many cases than outdoor exposure due to stationary-source emissions. Focusing regulation on the source, rather than on the overall reduction of the pollutant (and its potential risk to public health)
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From page 263... ...
However, in order for this approach to work properly, EPA needs to provide justification for its current defaults and set up a procedure such as that proposed in the report that permits departures from the default options.
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From page 264... ...
Verification of Amount of Risk-Assessment Conservatism In its tiered approach, EPA plans to use exposure models developed and validated for criteria pollutants, but not fully evaluated for the broader group of situations including hazardous air pollutants. In particular, it has not shown that analysis conducted with a simple, single Gaussian-plume approach with the generic worst-case conditions will necessarily be conservative over all situations in which it would be applied.
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Toxicity Data Development Some of the 189 chemicals lack cancer potencies or RfCs.
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From page 266... ...
Requirements EPA and others often interpret the term risk assessment as a specific methodologic approach to extrapolating from sets of human and animal carcinogenicity data, often obtained in intense exposures, to quantitative estimates of carcinogenic risk associated with the (typically) much lower exposures experienced by human populations.
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From page 267... ...
Iterative Uncertainty Analysis Because EPA often fails to characterize fully the uncertainty in risk assessments, inappropriate decisions and insufficiently or excessively conservative analyses can result. · The committee believes that the uncertainty in a risk estimate can be handled through an iterative process with the following parts: conduct a conservative screening analysis, conduct a default-uncertainty analysis, and conduct testing or analysis to develop site-specific probability distributions for each important input.
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From page 268... ...
· EPA should further develop its methods for risk comparison, taking account of such factors as differing degrees of uncertainty and of conservatism in different categories of risk assessment. Policy Focus on Stationary Sources Title III focuses primarily on outdoor stationary sources of hazardous air pollutants and does not consider indoor or mobile sources of those pollutants.
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