In vitro mobilization of polychlorinated biphenyls and pesticides sorbed to ingested soils and house dusts
Citation:
Starr, J., S. Graham, E. Valentini, H. Shen, W. Li, AND B. Parker. In vitro mobilization of polychlorinated biphenyls and pesticides sorbed to ingested soils and house dusts. Society of Toxicology 63rd Annual Meeting, Salt Lake City, UT, March 10 - 14, 2024.
Impact/Purpose:
Soils and house dusts are sorbents that function as sinks for a large number of regulated organic compounds. Since children consume relatively large amounts of soil and dust, ingestion constitutes an important pathway for children’s exposure to these chemicals. Therefore, to more precisely estimate risk to children, it is important to understand the behavior of sorbed chemicals following ingestion. Following soil and dust ingestion, the bioaccessibility (percent mobilization) of sorbed organic toxicants determines the amount of chemical that is available for uptake into the circulatory system. However, current bioaccessibility estimates used by the Agency assume all organics are 100% desorbed, and this likely results in an overestimation of dose. Further this assumption lacks predictive power as it ignores quantitative differences in the physicochemical properties of various types of soils, dusts, and organics that determine bioaccessibility. Therefore, new predictive methods are needed to reduce uncertainty, in bioaccessibility calculations. The purpose of this research is to produce empirical post ingestion bioaccessibility models for relevant organic compounds and incorporate them into more encompassing exposure/dose models. In this research we have considered the sorbed organics to be the chemical stressor and non-chemicals to be the factors that affect mobilization of the organic following ingestion.
Description:
Background and PurposeSoil and dust are effective sorbents for many organic chemicals and are known to concentrate lipophilic, less volatile, environmental pollutants. For children’s risk assessment, this is important, primarily because children consume relatively large quantities of both soil and dust. Following ingestion of soil/dust sorbed pollutants, desorption (mobilization) is the initial determinant of dose and quantifying the factors underlying mobilization can reduce uncertainty in post soil/dust ingestion risk assessment models. We used a three-compartment (salivary, gastric, and intestinal) in vitro digestive system (based on DIN 19738) with a series of polychlorinated biphenyls (PCBs, log Kow 5.69-8.27) and pesticides (log Kow 2.56-6.82) sorbed to soils (n = 34) and house dusts (n = 32) to determine whether soil and dust organic carbon content and analyte log Kow can be used to predict analyte mobilization. MethodsThe percent organic carbon content of each soil and dust sample was determined, then all samples were screened to measure concentrations of native analytes. Next, fresh 500 mg aliquots were fortified with 100 ng of PCBs or pesticides in methanol. The amended soils and dusts were stored (uncovered, in the dark) for one week to allow solvent evaporation, then processed through the in vitro test system. After each experiment, the sediments and digestive fluids were separated and purified using solid phase and liquid/liquid extraction as appropriate then analyzed using gas chromatography- mass spectrometry or liquid chromatography- tandem mass spectrometry. Percent mobilization of each PCB was calculated: (mass PCB in digestive fluid/(mass PCB in digestive fluid + mass PCB in sediment)) * 100. Because most of the pesticides were partially degraded in the digestive fluid, percent pesticide mobilization was calculated: (1-((residual mass in sediment)/(100 ng + native mass measured during screening))*100. ResultsThe mean percent organic carbon content of the soils (3.2 ± 2.6%) was lower (p < 0.0001) than that of the house dusts (18.3 ± 6.6%) and the difference in carbon was reflected in higher mobilization for each of the analytes (t-test, p < 0.05) in soils. The mean percent mobilization of the PCBs in both soil (65 ± 16.4%) and house dust (38 ± 13.1%) was less than (p < 0.0001) that of the pesticides (soil = 84 ± 14.5%; dust = 58 ± 24.0%) and was likely a result of their generally lower solubilities in the digestive fluid. Individually, the percent mobilization of each analyte was correlated (p < 0.009) with the organic carbon content in both soils and dusts. After controlling for carbon content, simple linear regression showed that percent pesticide mobilization was correlated with their respective log kow (p ≤ 0.0002), while PCB mobilization was not (p ≤ 0.05). This was likely due to group solubility differences between the PCBs and pesticides and suggests a biphasic response curve in both soil and dust where increases in solubility above log kow 6.0 did not result in measurable changes in mobilization. ConclusionThe results indicate that organic carbon content of soil and house dust, can be used in conjunction with analyte log kow to predict post ingestion mobilization; thereby reducing uncertainty in relevant risk assessments. However, above log kow 6.0, variability in mobilization due to differences in octanol water partition coefficients may not be important.
