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“This paper examines the importance of considering both potency and mechanism of action of check details different chemicals in complex mixtures, such as crude oil, when analyzing dose–response relationships, particularly when comparing dose–response curves for biological response endpoints for exposures to mixtures with different compositions. Potency

is defined as the probability of a dose having an adverse effect (Ryan, 1993). Changes in potency are most evident when the data in a multiple treatment study fail to follow a single or monotonic dose–response relationship, resulting in two or more discrete dose–response curves. Different mechanisms of toxicity can be implied when the slopes of the dose–response relationships for two exposures to complex hydrocarbon mixtures are different (Hayes, 2007). The absence of a monotonic dose–response relationship is indicative of the need to consider confounding factors including potentially unmeasured toxic compounds associated with the exposure methodology. To examine this issue, we use, as a case study, experiments by Carls et al. (1999) that measured the effects of exposure of Pacific herring (Clupea pallasi) eggs (embryos) to aqueous extracts of crude oil that had undergone different degrees of weathering. This study

provides a good example of the need to consider both potency and toxic mechanism when two distinct dose–response curves are obtained. In this study, Carls et al. (1999) concluded that low concentrations (0.4 μg/L) of dissolved Protease Inhibitor Library total tetracosactide polycyclic aromatic hydrocarbons (TPAHs) from weathered crude oil were toxic to herring embryos and that weathering increased oil toxicity. These conclusions were based on

a single set of un-replicated laboratory experiments. Although we have reviewed this work as well as a similar salmon study by Heintz et al. (1999) elsewhere ( Page et al., 2012), we conducted a further review of this study because of the far reaching implications of the recommendation by Carls et al., 1999 and Carls et al., 2002 that current water quality standards for PAH are not adequate to protect fish early life stages and the assertion that petroleum toxicity increases with weathering. Carls et al. (1999) produced aqueous exposure media by pumping seawater up through vertical cylindrical columns containing gravel that had been coated with crude oil. This oil, which had been artificially weathered by heating overnight at 70 °C, was applied at four oil-on-gravel loading levels (trace, low, middle (mid), and high), plus control (no oil added). Prior to each experiment, gravid adult herring were collected in the field by Johnson et al. (1997) and artificially spawned in the laboratory.