Fate and exposure modeling in regulatory chemical evaluation: new directions from retrospection.

The development and application of fate and exposure modeling has undergone fundamental changes over the last 20 years. This has, in part, been driven by different needs within the regulatory community to address chemicals of concern using different approaches. Here we present a retrospective look at fate and exposure model application over the last two decades keeping an international regulatory perspective and using the Government of Canada's Chemicals Management Plan to illustrate concepts. We discuss the important role fate and exposure modeling has played to help address key data gaps when evaluating the risk of chemicals for both human health and ecological reasons. Yet limitations for more widespread model application within a regulatory context remain. Consequently, we identify specific data gaps and regulatory needs with an eye towards new directions for 21st century chemical evaluation. We suggest that one factor limiting greater model application is the need for increased awareness and agreement of what chemical exposure assessment encompasses within the risk assessment paradigm. This is of particular importance today because of the increased availability of computational and high-throughput data and methods for chemical assessment allowing evaluators to potentially examine exposure from site of release to site of toxic action, thus linking exposure with toxicology. We further suggest there is a need for discussion at a global level to promote the awareness of new tools and approaches available for fate and exposure modeling and suggest that this could be organized using the aggregate exposure pathways concept. Environmental significance The evaluation of chemical impacts is often challenged by the lack of empirically measured multimedia processes that contribute to exposures in human and non-human receptors. Fate and exposure models play a key role in helping decision-makers fill data gaps to better characterize these exposures and thus determine the risk potential of chemicals in the environment. This article takes a retrospective look at the regulatory application of environmental models that have or can be used to assist the characterization of exposures in human and ecological receptors while providing some insights on new directions for modeling within 21st century risk assessment.