Probabilities coefficient

Large uncertainties exist in the values of these reaction probability coefficients, which tend to vary greatly with both temperature and type of surface. 

Estimates of Probability Coefficients for Carcinogens. The nominal probabilities of a stochastic response (primarily cancers) per unit dose used in risk assessments, which are referred to in this Report as probability coefficients, normally differ for radionuclides and chemical carcinogens in regard to the degree of conservatism incorporated in the assumed values and the number of organs or tissues at risk that are taken into account. 

The nominal probability coefficient for radionuclides normally used in radiation protection is derived mainly from maximum likelihood estimates (MLEs) of observed responses in the Japanese atomic-bomb survivors. A linear or linear-quadratic dose-response model, which is linear at low doses, is used universally to extrapolate the observed responses at high doses and dose rates to the low doses of concern in radiation protection. The probability coefficient at low doses also includes a small adjustment that takes into account an assumed decrease in the response per unit dose at low doses and dose rates compared with the observed responses at high doses and dose rates. 

In contrast, nominal probability coefficients for chemical carcinogens are derived from upper 95 percent confidence limits of observed responses at high doses, mainly in studies in animals. In some studies, the difference between the upper 95 percent confidence limit and MLE of the observed responses at high doses is an order of magnitude or more. Furthermore, several models have been used to extrapolate the observed responses to the low doses of concern in health protection of the public, with the result that estimated probability coefficients at low doses can differ by several orders of magnitude depending on the extrapolation model chosen. 

Thus, the nominal probability coefficients at low doses of chemical carcinogens could be considerably more conservative (more likely to overestimate risk) than the probability coefficient for radionuclides. As a result, potential risks posed by chemical carcinogens could be given a disproportionate weight in classifying waste. 

For the purpose of classifying waste that contains radionuclides, NCRP reaffirms use of the nominal probability coefficient for fatal cancers (i.e., the probability of a fatal cancer per unit effective dose) of 0.05 Sv 1 normally assumed in radiation protection of the public. For chemical carcinogens, NCRP believes that MLEs of probability coefficients obtained from the linearized, multi-stage model should be used in classifying waste, in order to provide reasonable consistency with the probability coefficient for radionuclides. The use of MLEs for chemical carcinogens usually will result in substantially lower probability coefficients than the use of upper 95 percent confidence limits. 

The use of MLEs of probability coefficients, rather than upper confidence limits (UCLs), to classify waste can be justified, in part, on the grounds that the assumed exposure scenarios for hypothetical inadvertent intruders at waste disposal sites are expected to be conservative compared with likely on-site exposures at future times. However, uncertainties in probability coefficients should still be considered in classifying waste. When risk is calculated using MLEs of... 

In general, the relationship between dose and response can be represented by a variety of functional forms. At low doses of substances that cause stochastic effects, the dose-response relationship usually is assumed to be linear and, thus, can be expressed as a single probability coefficient. This coefficient is frequently referred to as a risk (or potency factor or unit risk factor or slope factor) in the literature. However, it is really the response (consequence) resulting from a dose of a hazardous substance, and it should not be confused with risk as defined and used in this Report. 

In estimating the total detriment due to stochastic responses in any organ as described above, the probability coefficient for fatal cancers (F) or severe hereditary responses is based on data in humans and animals described in Section 3.2.2.2, and the lethality fraction (k) and relative length of life lost per fatal response (.II) are based on data on responses from all causes in various national populations. The values of F, k, and t/l for different organs, as well as the probability coefficient for severe hereditary responses, assumed by ICRP (1991) and the resulting estimates of total detriment, F((/T)(2 - k ), are summarized in Table 3.2. The two entries for Total in the last row represent the probability coefficient for... 

For routine exposures of the public, ICRP recommends a total detriment per unit equivalent dose from uniform whole-body irradiation of 7.3 X 10 2 Sv 1, as shown in Table 3.2. Of this, the recommended probability coefficient for fatal cancers is 5.0 X 10 2 Sv-1, or about two-thirds of the total detriment, and the contributions from severe hereditary responses and weighted nonfatal cancers are 1.3 X 10 2 Sv-1 and 1.0 X 10 2 Sv, respectively. These probability coefficients are summarized in Table 3.3, and their use in radiation protection is discussed in the following section. As noted previously, the probability coefficient for weighted nonfatal cancers is not the same as the probability coefficient for incidence of nonfatal cancers. The probability coefficient for fatal cancers also gives the probability of a fatal cancer per unit effective dose. The effective dose was developed to describe nonuniform irradiations of the body and is discussed below. 

Table 3.3—Nominal probability coefficients for stochastic responses due to radiation exposure of the general public.a...

It also is important to note that the total detriment developed by ICRP (1991) is intended to be used mainly in obtaining wTs in the effective dose (the values of wT in Table 3.4 are roughly proportional to the corresponding total detriments in Table 3.2). However, total detriment is not normally used in estimating responses from a given effective dose. ICRP and NCRP have continued to emphasize fatal cancers as the health effect of primary concern and have used the probability coefficient for fatal cancers of 5 X 10 2 Sv 1 given in Table 3.3 for this purpose. Total detriment is not used in estimating responses because, as noted previously, the detriment due to nonfatal cancers in Table 3.3 is not the probability of a nonfatal cancer and the detriment due to severe hereditary effects is not experienced by exposed individuals. 

Stochastic Responses. A basic principle of health protection for both radionuclides and hazardous chemicals is that the probability of a stochastic response, primarily cancers, should be limited to acceptable levels. For any substance that causes stochastic responses, a linear dose-response relationship, without threshold, generally is assumed for purposes of health protection. However, the probability coefficients for radionuclides and chemicals that induce stochastic responses that are generally assumed for purposes of health protection differ in two potentially important ways. 

First, the dose-response relationships for radiation used for purposes of health protection and the probability coefficients derived from those relationships are intended to be MLEs. In contrast, the dose-response relationships and probability coefficients for chemicals that induce stochastic responses are intended to be upper-bound estimates (UCLs), although MLEs also are available. In animal data from which the probability coefficients for most chemicals that cause stochastic responses are obtained, UCL can be greater than MLE by a factor that ranges from 5 to 100 or more. 

Radiation-induced cancer incidence also could be estimated using calculations of the probability of cancer incidence per unit activity intake of specific radionuclides by particular ingestion and inhalation pathways or the probability per unit activity concentration of specific radionuclides in the environment by particular pathways of external exposure (Eckerman etal., 1999) probabilities of fatal cancers for the different exposure pathways also have been calculated. These probability coefficients differ from those developed by ICRP (see Section 3.2.2.3.2) in that they are calculated with respect to activity of specific radionuclides rather than dose, and they thus bypass the need to estimate the effective dose. For external exposure, the methods used by Eckerman etal. (1999) and ICRP (1991) to estimate responses essentially are equivalent. However, there are significant differences in the methods used to estimate responses from intakes of radionuclides, and the results obtained by Eckerman et al. (1999) differ substantially in a few cases (e.g., intakes of 232Th)... 

The recommended dose limits for the public define limits on the probability of stochastic responses that are regarded as necessary for protection of public health. Doses above the limits are regarded as intolerable and normally must be reduced regardless of cost or other circumstances, except in the case of accidents or emergencies (see Section 3.3.1). For continuous exposure over a 70 y lifetime, and assuming a nominal probability coefficient for fatal cancers (i.e., the probability of a fatal cancer per unit effective dose) of 5 X 10 2 Sv 1 (ICRP, 1991 NCRP, 1993a), the dose limit for continuous exposure corresponds to an estimated lifetime fatal cancer risk of about 4 X 10 3. However, meeting the dose limits is not sufficient to ensure that routine exposures of the public to man-made sources would be acceptable. 

However, this option presents some difficulties for radionuclides, because studies of radiation effects in human populations have focused on cancer fatalities as the measure of response and probability coefficients for radiation-induced cancer incidence have not yet been developed by ICRP or NCRP for use in radiation protection. Probabilities of cancer incidence in the Japanese atomic-bomb survivors have been obtained in recent studies (see Section 3.2.3.2), but probability coefficients for cancer incidence appropriate for use in radiation protection would need to take into account available data on cancer incidence rates from all causes in human populations of concern, which may not be as reliable as data on cancer fatalities. Thus, in effect, if incidence were used as the measure of stochastic response for radionuclides, the most technically defensible database on radiation effects in human populations available at the present time (the data on fatalities in the Japanese atomic-bomb survivors) would be given less weight in classifying waste. 

Stochastic Responses. Consideration of the dose-response relationships and the nominal probability coefficients for induction of stochastic responses at low doses is important for both radionuclides and hazardous chemicals. 

For radionuclides, NCRP reaffirms use of a best estimate (MLE) of the response probability obtained from a linear or linear-quadratic model as derived from data in humans, principally the Japanese atomic-bomb survivors. This model essentially is linear at the low doses of concern to waste classification. Specifically, for purposes of health protection of the public, NCRP reaffirms use of a probability coefficient for fatal cancers (probability per unit effective dose) of 0.05 Sv 1 (ICRP, 1991 NCRP, 1993a). Although this probability coefficient is less rigorous for intakes of some long-lived radionuclides that are tenaciously retained in the body than for other exposure situations, such as external exposure or intakes of short-lived radionuclides (Eckerman et al., 1999), it is adequate for the purpose of generally classifying waste, especially when the lack of data on cancer risks in humans for most chemicals is considered. 

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). 

The use of MLEs of probability coefficients for radionuclides but UCLs for chemicals that induce stochastic responses is the most important issue that would need to be resolved to achieve a consistent approach to estimating risks for the purpose of waste classification. For some chemicals, the difference between MLE and UCL can be a factor of 100 or more. The difference between using fatalities or incidence as the measure of response is unlikely to be important. Use of the linearized, multistage model to extrapolate the dose-response relationship for chemicals that induce stochastic effects, as recommended by NCRP, should be reasonably consistent with estimates of the dose-response relationship for radionuclides, and this model has been used widely in estimating probability coefficients in chemical risk assessments. The difference in the number of organs or tissues that are taken into account, although it cannot be reconciled at the present time, should be unimportant. 

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. 

The stochastic risk corresponding to the estimated dose is obtained using the nominal probability coefficient for fatal cancers of 0.05 Sv-1 (ICRP, 1991 NCRP, 1993a). Since this coefficient is intended to represent a best estimate, rather than a conservative upper bound, the value is not increased by a factor of 10, as in the adjustment of the slope factors for chemicals that induce stochastic effects (Section 7.1.7.5). Therefore, the calculated stochastic risk due to 137Cs in the waste is (5 X 10 4 Sv)(5 X 10 2 Sv-1) = 2.5 X 10 5. 

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