Dispersants are commonly applied during oil spill mitigation efforts; however, these industrial chemicals may present risks to aquatic organisms individually and when mixed with oil. Fourteen dispersants are listed on the U.S. Environmental Protection Agency (U.S. EPA) National Oil and Hazardous Substances Pollution Contingency Plan (NCP). Availability of environmental effects information for such agents is limited, and individual components of dispersants are largely proprietary. Probabilistic hazard assessment approaches including Chemical Toxicity Distributions (CTDs) may be useful as an initial step toward prioritizing environmental hazards from the use of dispersants. In the present study, we applied the CTD approach to two acute toxicity datasets: NCP (the contingency plan dataset) and DHOS (a subset of NCP listed dispersants reevaluated subsequent to the Deepwater Horizon oil spill). These datasets contained median lethal concentration (LC50) values for dispersants alone and dispersant:oil mixtures, in two standard marine test species, Menidia beryllina and Mysidopsis bahia. These CTDs suggest that dispersants alone are generally less toxic than oil. In contrast, most dispersant:oil mixtures are more toxic than oil alone. For the two datasets (treated separately because of differing methodologies), CTDs would predict 95% of dispersant:oil mixtures to have acute toxicity values above 0.32 and 0.76 mg/L for Mysidopsis and 0.33 mg/L and 1.06 mg/L for Menidia (for DHOS and NCP, respectively). These findings demonstrate the utility of CTDs as a means to evaluate the comparative ecotoxicity of dispersants alone and in mixture with different oil types. The approaches presented here also provide valuable tools for prioritizing prospective and retrospective environmental assessments of oil dispersants.
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