Vol.1 No.1 2008

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Research paper : A strategic approach for comparing different types of health risks (A. Kishimoto)−32−Synthesiology - English edition Vol.1 No.1 (2008) children) and most highly exposed groups in our society. The reference values are derived from NOAEL in animal tests divided by sufficiently large uncertainty factors. Therefore, even if the exposure level slightly exceeds the reference values, no adverse effects will appear in most people.Second is to conduct screening-level risk assessments. Examples are: the Environmental Risk Assessment prepared by the Ministry of the Environment; the Initial Risk Assessment prepared by the Ministry of Economy, Trade and Industry; and the various risk assessments prepared by the Food Safety Commission. Worst-case scenarios are applied not only to toxicity assessment but also to exposure assessment, in which the 95 percentile of measured or estimated valuesTerm 1 are often adopted as the exposure levels. The social demands that they cover include delisting substances that pose no or very small risk to society even if their toxicity or exposure is overestimated. These are screening-level risk assessments. The substances that are not delisted should be subject to detailed risk assessment, and precautionary measures are sometimes taken at this point.These methodologies of exposure assessment, toxicity assessment, and risk characterization were developed to cover the social demands for setting reference values or for screening purposes. In contrast to the risk assessment procedures (gray arrows in Figure 1, from bottom to top), each step was developed in the reverse direction (white arrows in Figure 1, from top to bottom). The bottom line is that the current elemental techniques have been optimized to address particular social demand, and are not necessarily optimal responses to other social demands.Today, there are emerging social demands in the field of chemical risk management. We must compare the health risk of a chemical substance being used now with the risk of a proposed substitute. We must deal with the trade-offs between chronic disease risks and accident risks. We must also assess the cost-effectiveness of risk reduction measures to maximize risk reduction within budget constraints. Current toxicity assessment with conservative assumptions (that are used to overestimate risks in case of uncertainty and variability) and exposure assessment with worst-case scenarios cannot be used to address these new social demands. When a risk assessor does not recognize the close relationship between methodology and social demand, he or she may naively apply the current procedures (Figure 1) to the assessment for some other purposes. As a result, the risk is overestimated, the effectiveness of risk reduction measures is also overestimated, and the cost-effectiveness ratio turns out much better than the expected (average) one. Since the extent of conservativeness (overestimation) is different in each case, we cannot compare the risks for different chemical substances. To address the third social demand in addition to the first and the second, it is necessary to travel downstream of risk assessment procedures, i.e. the social demands, and reconsider proper methods for each step upstream from scratch, just as when the current risk assessment methodology was first developed.3 Quantification strategy to compare different kinds of risks3.1 Casting back from social demandsWe considered the conditions necessary for each step of risk assessment to address the social demands, in reverse direction of risk assessment operations. The white arrows in Figure 2 show this review process. It is necessary to present the magnitude of various risks with common metric to compare them with other types of risks and set priorities for various risk reduction measures. The necessary condition of the common metric for health risks is to combine mortality risks (loss of life years) and morbidity risks (loss of quality of life). We decided to adopt quality-adjusted life years (QALYs) proposed and applied in the area Fig. 1 Conventional process of risk assessment and social demandsFig. 2 Reexamination of elemental techniques for new social demandsExisting social demandsScreening of risksEstimates of personal exposures (upper bound)Measurement of concentrations of environmental media (upper bound)Derivation of reference valuesUncertainty factorsDetermination of NOAELLiterature review of toxicity assessment Risk comparison / cost- effectivenessNew social demandsQuantification of human health risksEmission sources and volumesLiterature review of toxicity assessmentDose response functions of subjective symptomsDistribution of personal exposures (annual average)Estimation of the distribution of concentrations in environmental media

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