The role of QSARs and fate models in chemical hazard and risk assessment - Paper prepared for quantitative structure-activity relationships (QSAR) Proceedings of the QSAR 2002 Conference, Ottawa May 2002

A structure is suggested and discussed for the assessment of hazard and risk of chemicals of commerce, starting from a knowledge of molecular structure and proceeding to estimation of chemical properties, environmental fate and presence in organisms. Two metrics of risk are described, the external risk ratio which is based on concentrations external to the organism and the internal risk ratio based on concentrations internal to the organism. Where possible, the latter is preferred. Aspects of this multi-stage strategy are discussed in more detail including the need for more experimental data in support of QSARs, the need for consistency in QSARs describing related properties and the complementary roles of fate models and QSARs. Whereas most screening-level regulatory assessments of large numbers of chemicals focus on hazard, it is argued that the public concern is primarily with risk. Since risk assessment depends on the availability of data on rates of emission and such data are often very uncertain, this stage is often delayed and may only be done for relatively few substances. This is unfortunate because many hazardous substances are used under conditions such that there is minimal risk of exposure and effects. It is suggested that risk assessment can be facilitated by "backtracking" from an arbitrarily assumed risk ratio to calculate a hypothetical "critical" emission rate which would support that ratio. This rate can then be compared with likely emission to give an indication of proximity to levels of concern and thus the sustainability of present chemical emission practices.