A Hazard Assessment of Aggregate Exposure to Nonylphenol and Nonylphenol Mono- and Di-ethoxylates in the Aquatic Environment

The most commonly detected environmental metabolites of nonylphenol ethoxylates (NPE) are nonylphenol monoethoxylate (NPE1), nonylphenol diethoxylate (NPE2), nonylphenol ether carboxylates (NPEC), and nonylphenol (NP). Since NPEC have relatively low toxicity, the most relevant NPE metabolites for conducting an aggregate hazard assessment are NP, NPE1, and NPE2. Recent studies support the validity of the U.S. Environmental Protection Agency (2005) chronic water quality criteria (WQC) for NP in freshwater and saltwater environments; thus, these criteria were used as reference values in the aggregate hazard assessment. The toxic equivalency approach was used to assess the aggregate hazard of NP, NPE1, and NPE2. A review of relevant studies indicated that the toxic equivalency factor (TEF) for NPE1 and NPE2 approximated 0.37, which supported the use of the more conservative TEF value of 0.50 used by Environment Canada (2001) for NPE1 and NPE2. The interaction of toxicities between NP, NPE1, and NPE2 is assumed to be additive based on a review of the current literature and the shared mechanism of action of these compounds. The data support previous findings that there is a low likelihood that aggregate NP-equivalent concentrations of NPE and its metabolites in U.S. waters will exceed the national chronic WQC for NP.