Nonpolar organic chemicals such as polycyclic aromatic hydrocarbons and chlorobenzenes are expected to act additively when exposed as a mixture. The present study examined the toxicity of fluoranthene (FLU) and pentachlorobenzene (PCBz) individually and in a binary mixture using the whole-body residue as the dose metric. Body residues were based on the toxic equivalent body residue, which included the parent compound plus the organically extractable metabolites for FLU and the parent compound only for PCBz. Using a toxic unit (TU) approach, the binary mixtures of FLU and PCBz following 4- and 10-d water-only exposures acted additively. The lethal residue (LR50) values for mixtures of the compounds for Hyalella azteca were 1.26 (1.19-1.33) TU and 1.27 (1.20-1.34) TU for 4- and 10-d exposures, respectively. For Chironomus dilutus, the 4-d and 10-d values were 0.93 (0.90-0.97) TU and 1.01 (0.96-1.06) TU. Additionally, the total molar sum of PCBz and FLU whole-body residues in a mixture were compared to residues from single compound exposures. For both species tested, the LR50 values based on the total molar sum fell within the range of those determined from the single compound tests; providing additional support for molar additivity for nonpolar narcotic compounds. Assuming that residue-effects data among narcotic compounds (e.g., LR50) are similar, applying the molar sum methodology to narcotic compounds in tissues determined from routine biomonitoring programs and risk specific sampling may be a valuable tool to assess potential effects to biota in the field.
"We recognize that there are myriad studies that could have been selected as case studies, but we have limited the number of studies so that specific, key issues such as establishing doseresponse and causality are emphasized. Case study one: Binary mixture Two freshwater arthropods, Hyalella azteca and Chironomus dilutus (previously named C. tentans), were exposed to mixtures of fluoranthene and pentachlorobenzene in aqueous exposures without changing or measuring the water concentrations (Schuler et al. 2009). The 2 chemicals have similar log octanol–water partition coefficients (K OW , 5.18–5.20) "
[Show abstract][Hide abstract] ABSTRACT: Experimental designs for evaluating complex mixture toxicity in aquatic environments can be highly variable and, if not appropriate, can produce and have produced data that are difficult or impossible to interpret accurately. We build on and synthesize recent critical reviews of mixture toxicity using lessons learned from 4 case studies, ranging from binary to more complex mixtures of primarily polycyclic aromatic hydrocarbons and petroleum hydrocarbons, to provide guidance for evaluating the aquatic toxicity of complex mixtures of organic chemicals. Two fundamental requirements include establishing a dose-response relationship and determining the causative agent (or agents) of any observed toxicity. Meeting these 2 requirements involves ensuring appropriate exposure conditions and measurement endpoints, considering modifying factors (e.g., test conditions, test organism life stages and feeding behavior, chemical transformations, mixture dilutions, sorbing phases), and correctly interpreting dose-response relationships. Specific recommendations are provided.
"Because of the almost infinite number of mixture-combinations, and the many different targeted organisms, every individual case cannot be investigated. It is therefore crucial to have models that can predict mixture effects, which can be based on toxic units or critical body residues (Di Toro and McGrath, 2000; Verbruggen et al., 2008; Schuler et at., 2009). It has been suggested that chemical activity could be used in the assessment of chemicals that elicit their toxicity by interfering with membrane fluidity, a term called baseline toxicity (Ferguson, 1939; Reichenberg and Mayer, 2006). "
[Show abstract][Hide abstract] ABSTRACT: Organisms in marine sediments are usually exposed to mixtures of polycyclic aromatic hydrocarbons (PAHs), whereas risk assessment and management typically focus on the effects of single PAHs. This can lead to an underestimation of risk if the effects of single compounds are additive or synergistic. Because of the virtually infinite number of mixture-combinations, and the many different targeted organisms, it would be advantageous to have a model for the assessment of mixture effects. In this study we tested whether chemical activity, which drives the partitioning of PAHs into organisms, can be used to model the baseline toxicity of mixtures. Experiments were performed with two benthic amphipod species (Orchomonella pinguis and Corophium volutator), using passive dosing to control the external exposure of single PAHs and mixtures of three and four PAHs. The baseline toxicity of individual PAHs at water saturation generally increased with increasing chemical activity of the PAHs. For O. pinguis, the baseline toxicity of PAH mixtures was successfully described by the sum of chemical activities. Some compounds and mixtures showed a delayed expression of toxicity, highlighting the need to adjust the length of the experiment depending on the organism. On the other hand, some of the single compounds had a higher toxicity than expected, possibly due to the toxicity of PAH metabolites. We suggest that chemical activity of mixtures can, and should, be used in addition to toxicity data for single compounds in environmental risk assessment.
[Show abstract][Hide abstract] ABSTRACT: Acute toxicity and metabolism of a new insecticide pyridalyl were examined in the larval midge Chironomus yoshimatsui and adult amphipod Hyalella azteca using (14)C-labeling. The median lethal concentrations (LC(50)) of pyridalyl were determined to be 1.1 (48 hr) and 0.015 (96 hr) mg/l for midges and amphipods, respectively, based on its mean measured concentrations in water. Pyridalyl taken up by these organisms underwent ether cleavage at 3,3-dichloroallyloxy moiety followed by conjugation to a significantly higher extent in C. yoshimatsui. The similar acute median lethal body residues (LR(50)) of pyridalyl estimated for both species from its body residues indicated an insignificant difference in its potential toxicity and therefore, uptake and metabolism behaviors were most likely to play a great role in the apparent toxicity. (C) Pesticide Science Society of Japan
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