it is insufficient merely to demonstrate reduction in the risks associated with the original substance. It is necessary also to consider the potential risks associated with the replacement substance. It is necessary to determine whether the overall risk of the replacement substance is less than that of the original substance; and in addition, by comparing the risks before and after substitution, to assess whether the given substitution produces a risk reduction effect that exceeds the cost of countermeasures. Kishimoto[5] argued for the necessity of developing an assessment method for human health risks associated with chemical substance use, which would reflect social requirements, including the new social demand for "comparing risks of different types of chemical substances, and the assessment of cost-effectiveness of emission reduction measures;" and presented a case study involving toluene risk assessment. Here, he proposed a method for quantifying human health risks, using quality of life (QOL) as the risk index, by backcasting from the demand. The method enabled the comparison of the cost-effectiveness of risk reduction measures for toluene with that of risk reduction measures for other chemical substances, infectious diseases, accidents, disasters, etc.However, Kishimoto's proposal is, in practice, applicable only to substances such as toluene, for which abundant toxicity information is available. In risk tradeoff analysis associated with substance substitution, the substitution often involves replacing a substance for which there is relatively abundant information, with one for which there is insufficient information. For such risk tradeoff analysis to become a IntroductionWhile the use of chemical substances is necessary for achieving a prosperous and sustainable society, there is concern about related risks against humans and ecosystems. The production, use, and emission of chemical substances determined to pose significant risks are regulated by laws and through self-management. For example, such substances as polychlorinated biphenyls (PCBs), some chloride agrichemicals, brominated flame retardants, and heavy metals such as mercury, cadmium, and lead are strictly regulated, while the emissions of substances like air pollutants such as toluene are declining every year due to voluntary emission control.[1]Although the risk associated with a particular substance can be reduced by regulating the use and emission of that substance, related measures may be costly; and replacement by different, less hazardous substances is often made, while maintaining the functionality of the product in which the original substance was used. For example, the brominated flame retardant, decabromodiphenyl ether (decaBDE), has been replaced by substances such as bisphenol-A bis(diphenyl phosphate) (BDP); [2] lead solder alloys by 'lead-free solder alloys,' such as tin-silver-copper alloys, which do not contain lead; [3] and chlorinated solvents by carbohydrate or aqueous varieties, for use as industrial cleansers.[4]The occur...

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