Negligible risk Acceptable

The negligible risk concept considered here is based strictly on individual risk rates and exposure. It does not include other economic or technical considerations that are part of setting the enforceable standards (i.e, the MCLs). The levels for the MCLs (not RMCLs) would thus be considered to be the upper limits of risk that are considered to be acceptable on the basis of current evaluations of the feasibility and costs of controls. 

Waste that exceeds concentration limits for exempt waste would be classified as low-hazard if it poses no more than an acceptable (i.e., barely tolerable) risk to a hypothetical inadvertent intruder at a dedicated near-surface disposal facility for hazardous wastes, with the important condition that an acceptable risk or dose used to determine low-hazard waste should be substantially higher than a negligible risk or dose used to determine exempt waste. 

The waste classification system should be based on the distinct concepts of negligible and acceptable ii.e., barely tolerable) risks to the public that arise from waste disposal. Precedents for specifying negligible or acceptable risks that could be used in classifying waste, such as other NCRP recommendations, would be cited, but specific recommendations would not be presented in this Report. 

Exempt Waste. Waste classified as exempt would be regulated as if it were nonhazardous, and would be generally acceptable for disposition as nonhazardous material (e.g., disposal in a municipal/ industrial landfill). As noted in Section 1.4.1, disposal is the only disposition of exempt materials considered in this Report. Limits on concentrations of hazardous substances in exempt waste would be derived based on an assumption that the risk or dose to a hypothetical inadvertent intruder at a disposal site should not exceed negligible levels. The use of a negligible risk or dose to determine exempt waste is based on an assumption that a disposal facility for nonhazardous waste could be released for unrestricted use by the public soon after the facility is closed. 

The risk index for any hazardous substance in Equation 1.1 or 1.2 (see Section 1.5.1) is calculated based on assumed exposure scenarios for hypothetical inadvertent intruders at near-surface waste disposal sites and a specified negligible risk or dose in the case of exempt waste or acceptable (barely tolerable) risk or dose in the case of low-hazard waste. Calculation of the risk index also requires consideration of the appropriate measure of risk (health-effect endpoint), especially for carcinogens, and the appropriate approaches to estimating the probability of a stochastic response per unit dose for carcinogens and the thresholds for deterministic responses for noncarcinogens. Given a calculated risk index for each hazardous substance in a particular waste, the waste then would be classified using Equation 1.3. 

Many details would need to be considered in developing a new waste classification system based on the framework presented in this Report. Assumptions about generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites to be used in classifying waste and the time frames for applying the scenarios would be required. Decisions would need to be made about negligible and acceptable (barely tolerable) doses or risks that would be used in classifying waste as exempt or low-hazard, respectively. 

It is important to emphasize that achieving a negligible risk is not the goal of ALARA, because any non-negligible risk between intolerable and negligible levels is acceptable if it is ALARA. 

A proper reconciliation of the radiation and chemical paradigms for risk management is important to the development of a comprehensive and risk-based hazardous waste classification system. In particular, the proposed waste classification system developed in Sections 6.2 and 6.3 of this Report is based fundamentally on the concept that an acceptable risk generally can be substantially greater than a negligible risk. This distinction is used to define different classes of waste that pose an increasing hazard. 

Establishing Allowable Risks or Doses of Substances That Cause Stochastic Responses. Given the assumption of a linear dose-response relationship for substances that cause stochastic responses without threshold, either risk or dose may be used to calculate the risk index. The following two sections discuss suitable approaches to establishing negligible and acceptable risks or doses of substances that cause stochastic responses. 

NCRP notes that the modifying factor in the risk index should be applied independently of the requirement to achieve a negligible risk or dose for exempt waste or an acceptable (barely tolerable) risk or dose for nonexempt waste, in order to provide regulatory flexibility in classifying particular wastes. NCRP believes that such flexibility is highly desirable to promote cost-effective management and disposal of waste, provided it is applied in a transparent manner. 

For the purpose of illustrating how the composite risk index in Equation 6.6 would be used to classify a hypothetical waste, it is helpful to simplify Equations 6.4 and 6.5. This is done by assuming that the summation over all responses (index r) has been calculated, that only one waste classification boundary represented by the index j is being considered (i.e., the boundary between exempt and low-hazard waste, based on a negligible risk, or the boundary between low-hazard and high-hazard waste, based on an acceptable risk), and that the modifying factor (F) is unity. Further, the calculated dose in the numerator of the risk index is denoted by D and the allowable dose in the denominator is denoted by L. Then, the composite risk index for all hazardous substances in the waste, expressed in the form of Equation 6.6, can be written as ... 

For substances that cause stochastic effects (radionuclides and hazardous chemicals), specify negligible and acceptable (barely tolerable) risks to be used in classifying waste. Then, establish the corresponding negligible and acceptable dose of each substance of concern based on an assumed probability coefficient (risk per unit dose). 

For each substance of concern that causes deterministic effects (hazardous chemicals only), establish nominal thresholds for induction of deterministic responses in humans, taking into account all organs and tissues at risk. Then, establish organ-specific negligible and acceptable doses of each substance by applying appropriate safety and uncertainty factors to the assumed thresholds. 

The risk-based waste classification system developed in this Report is based fundamentally on the concepts of negligible (de minimis) and acceptable (barely tolerable) risks from exposure to radionuclides and hazardous chemicals, with the crucial distinction that acceptable risks generally can be considerably higher than negligible risks. Therefore, in implementing the waste classification system, decisions would need to be made by regulatory authorities about... 

Given the qualitative definitions of the three waste classes, the boundaries of the waste classes would be quantified based on explicit descriptions of how the definitions are related to risk. The boundaries would be expressed in terms of limits on amounts (concentrations) of individual hazardous substances, with specified rules for how to classify waste that contains mixtures of hazardous substances, such as the sum-of-fractions rule for mixtures of substances that induce stochastic effects. Specifically, waste would be classified as exempt if the risk that arises from disposal in a municipal/industrial landfill for nonhazardous waste does not exceed negligible (de minimis) levels. Use of a negligible risk to quantify limits on concentrations of hazardous substances in exempt waste is appropriate because the waste would be managed in all respects as if it were nonhazardous. Nonexempt waste would be classified as low-hazard if the risk that arises from disposal in a dedicated near-surface facility for hazardous wastes does not exceed acceptable (barely tolerable) levels. An essential condition of the definitions of exempt and low-hazard waste is that an acceptable (barely tolerable) risk must be substantially greater than a negligible risk. Waste would be classified as high-hazard if it would pose an unacceptable (de manifestis) risk when placed in a dedicated near-surface facility for hazardous wastes. 

The boundaries between different waste classes would be quantified in terms of limits on concentrations of hazardous substances using a quantity called the risk index, which is defined in Equation 6.1. The risk index essentially is the ratio of a calculated risk that arises from waste disposal to an allowable risk (a negligible or acceptable risk) appropriate to the waste class (disposal system) of concern. The risk index is developed taking into account the two types of hazardous substances of concern substances that cause stochastic responses and have a linear, nonthreshold dose-response relationship, and substances that cause deterministic responses and have a threshold dose-response relationship. The risk index for any substance can be expressed directly in terms of risk, but it is more convenient to use dose instead, especially in the case of substances that cause determinstic responses for which risk is a nonlinear function of dose and the risk at any dose below a nominal threshold is presumed to be zero. The risk index for mixtures of substances that cause stochastic or deterministic responses are given in Equations 6.4 and 6.5, respectively, and the simple rule for combining the two to obtain a composite risk index for all hazardous substances in waste is given in Equation 6.6 or 6.7 and illustrated in Equation 6.8. The risk (dose) that arises from waste disposal in the numerator of the risk index is calculated based on assumed scenarios for exposure of hypothetical... 

Use of the risk index in classifying waste is illustrated in Figure 6.2. Classification of waste essentially is a two-step process. The first step involves a determination of whether a waste can be classified as exempt, based on an assumed negligible risk and an exposure scenario for inadvertent intruders appropriate to disposal of waste in a municipal/industrial landfill for nonhazardous waste. If the waste is not exempt, the second step involves a determination of whether a waste can be classified as low-hazard, based on an assumed acceptable (barely tolerable) risk and an exposure scenario for inadvertent intruders appropriate to disposal in a dedicated nearsurface facility for hazardous wastes. 

In many respects, the foundations and framework of the proposed risk-based hazardous waste classification system and the recommended approaches to implementation are intended to be neutral in regard to the degree of conservatism in protecting public health. With respect to calculations of risk or dose in the numerator of the risk index, important examples include (1) the recommendation that best estimates (MLEs) of probability coefficients for stochastic responses should be used for all substances that cause stochastic responses in classifying waste, rather than upper bounds (UCLs) as normally used in risk assessments for chemicals that induce stochastic effects, and (2) the recommended approach to estimating threshold doses of substances that induce deterministic effects in humans based on lower confidence limits of benchmark doses obtained from studies in humans or animals. Similarly, NCRP believes that the allowable (negligible or acceptable) risks or doses in the denominator of the risk index should be consistent with values used in health protection of the public in other routine exposure situations. NCRP does not believe that the allowable risks or doses assumed for purposes of waste classification should include margins of safety that are not applied in other situations. 

This Section provides example applications of the recommended risk-based waste classification system to a variety of hazardous wastes to illustrate its implementation and potential ramifications. Disposal is the only disposition of waste considered in these examples. In Section 7.1.1, a general set of assumptions for assessing the appropriate classification of hazardous wastes is developed, including a variety of assumed exposure scenarios for inadvertent intruders at waste disposal sites and assumed negligible and acceptable risks or doses from exposure to radionuclides and hazardous chemicals. Subsequent sections apply the methodology to several example wastes. 

Some individuals don t understand comparative risks. They can accept the risk of a lifetime of smoking (a voluntary action), which is gravely serious act, and driving a motorcycle (one of the most dangerous forms of transportation), but they insist in protesting a nuclear power plant that, according to risk experts, has a negligible risk. 

These are unacceptable, acceptable and negligible. In the presence of life-threatening disease and with sufficient information on both the disease and the drug, then decisions, though they may be painful, present relatively obvious problems. But where the disease risk is remote, e.g. mild hypertension, or where drugs are to be used to increase comfort or to suppress symptoms that are, in fact, bearable, or for convenience rather than for need, then the issues of risk acceptance are less obvious. 

Authorisation. The use of chemicals considered to be of very high concern would be subject to authorisation. The aim is for such chemicals to be phased out and substituted, unless industry can show that the use presents negligible risk or that it is acceptable, taking into account its socioeconomic benefits, the lack of safer chemicals and measures to minimise exposure. Chemicals of very high concern are likely to include carcinogens, mutagens or reprotoxic substances (CMRs), particularly persistent, bioaccumulative and toxic substances. 

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