Estimating Screening-Level Organic Chemical Half-Lives in Humans

Relatively few measured data are available for the thousands of chemicals requiring hazard and risk assessment. The whole body, total elimination half-life (<i>HL</i>_T) and the whole body, primary biotransformation half-life (<i>HL</i>_B) are key parameters determining the extent of bioaccumulation, biological concentration, and risk from chemical exposure. A one-compartment pharmacokinetic (1-CoPK) mass balance model was developed to estimate organic chemical <i>HL</i>_B from measured <i>HL</i>_T data in mammals. Approximately 1900 <i>HL</i>s for human adults were collected and reviewed and the 1-CoPK model was parametrized for an adult human to calculate <i>HL</i>_B from <i>HL</i>_T. Measured renal clearance and whole body total clearance data for 306 chemicals were used to calculate empirical <i>HL</i>_B,emp. The <i>HL</i>_B,emp values and other measured data were used to corroborate the 1-CoPK <i>HL</i>_B model calculations. <i>HL</i>s span approximately 7.5 orders of magnitude from 0.05 h for nitroglycerin to 2 × 10⁶ h for 2,3,4,5,2′,3′,5′,6′-octachlorobiphenyl with a median of 7.6 h. The automated Iterative Fragment Selection (IFS) method was applied to develop and evaluate various quantitative structure–activity relationships (QSARs) to predict <i>HL</i>_T and <i>HL</i>_B from chemical structure and two novel QSARs are detailed. The <i>HL</i>_T and <i>HL</i>_B QSARs show similar statistical performance; that is, <i>r</i>² = 0.89, <i>r</i>^2‑ext = 0.72 and 0.73 for training and external validation sets, respectively, and root-mean-square errors for the validation data sets are 0.70 and 0.75, respectively

Exploring the Role of Shelf Sediments in the Arctic Ocean in Determining the Arctic Contamination Potential of Neutral Organic Contaminants

The main objective of this study was to model the contribution of shelf sediments in the Arctic Ocean to the total mass of neutral organic contaminants accumulated in the Arctic environment using a standardized emission scenario for sets of hypothetical chemicals and realistic emission estimates (1930–2100) for polychlorinated biphenyl congener 153 (PCB-153). Shelf sediments in the Arctic Ocean are shown to be important reservoirs for neutral organic chemicals across a wide range of partitioning properties, increasing the total mass in the surface compartments of the Arctic environment by up to 3.5-fold compared to simulations excluding this compartment. The relative change in total mass for hydrophobic organic chemicals with log air–water partition coefficients ≥0 was greater than for chemicals with properties similar to typical POPs. The long-term simulation of PCB-153 generated modeled concentrations in shelf sediments in reasonable agreement with available monitoring data and illustrate that the relative importance of shelf sediments in the Arctic Ocean for influencing surface ocean concentrations (and therefore exposure via the pelagic food web) is most pronounced once primary emissions are exhausted and secondary sources dominate. Additional monitoring and modeling work to better characterize the role of shelf sediments for contaminant fate is recommended

Using Model-Based Screening to Help Discover Unknown Environmental Contaminants

Of the tens of thousands of chemicals in use, only a small fraction have been analyzed in environmental samples. To effectively identify environmental contaminants, methods to prioritize chemicals for analytical method development are required. We used a high-throughput model of chemical emissions, fate, and bioaccumulation to identify chemicals likely to have high concentrations in specific environmental media, and we prioritized these for target analysis. This model-based screening was applied to 215 organosilicon chemicals culled from industrial chemical production statistics. The model-based screening prioritized several recognized organosilicon contaminants and generated hypotheses leading to the selection of three chemicals that have not previously been identified as potential environmental contaminants for target analysis. Trace analytical methods were developed, and the chemicals were analyzed in air, sewage sludge, and sediment. All three substances were found to be environmental contaminants. Phenyl-tris­(trimethylsiloxy)­silane was present in all samples analyzed, with concentrations of ∼50 pg m^–3 in Stockholm air and ∼0.5 ng g^–1 dw in sediment from the Stockholm archipelago. Tris­(trifluoropropyl)­trimethyl-cyclotrisiloxane and tetrakis­(trifluoropropyl)­tetramethyl-cyclotetrasiloxane were found in sediments from Lake Mjøsa at ∼1 ng g^–1 dw. The discovery of three novel environmental contaminants shows that models can be useful for prioritizing chemicals for exploratory assessment