Five-stage environmental exposure assessment strategy for mixtures: gasoline as a case study

A five-stage strategy is suggested for conducting an exposure assessment of mixtures that may contain numerous chemical components. The stages are: (1) determination of mixture composition and variability, (2) selection of component groups within the mixture and documentation of criteria used for this selection, (3) compilation of relevant property data for each group, (4) assessment of environmental fate of each group, and (5) assessment of environmental and human exposure to each group and to the mixture as a whole. A subsequent step is the assessment of environmental and/or human risk associated with the individual and aggregate exposure to each group. The approach is illustrated by application to gasoline, which is treated as 24 component groups or hydrocarbon blocks. Focusing on stages 2-4, the illustration shows that the groups display widely different environmental fates as a result of their different physicochemical properties, degradation half-lives, and mode-of-entry into the environment. As a result, the relative proportions of groups in each environmental medium (such as air and water) differ greatly from that of the original mixture. It is thus important to treat gasoline and similar mixtures as a number of component groups instead of as a single substance. A generic procedure is suggested in which the model is run for unit emissions of each component group to air, water, and soil. These results are compiled into matrices that can then be conveniently scaled to actual emission rates without rerunning the model. Methods for determining subsequent exposure and risk are also briefly outlined.