Evaluating emission rate estimation methods for ecological exposure and safety assessment: a case study for cyclic...

Chemical emission rates and mode-of-release (MOR; distribution of emissions across compartments) are key processes relating to exposures and potential environmental impacts and are often the most uncertain input parameters for mass balance multimedia environmental fate and exposure models. Three independent methods of estimating emission rate and MOR including generic scenarios, expert judgment, and a high-throughput mass-flow tool, are used to calculate exposure concentrations and margins of exposure (MOEs) for select cyclic volatile methyl siloxanes (cVMS): octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6). The same chemical properties and hazard data are used as model input in the methods comparison. The MOR for cVMS has a large impact on estimated exposure levels and MOE for aquatic species; exposure estimates varied by up to seven orders of magnitude, depending solely on MOR assumptions, with even small fractions released to water substantially increasing aquatic exposure. Exposure model predictions using a high-throughput emission estimation tool, use category estimates, and regional production volume information including mechanistic modelling of their release due to their use in personal care products show reasonable agreement with spatially and temporally curated (bio)monitoring data (n = 85). MOEs are ∼10¹ to 10⁸ when calculated for the biomonitoring data and the high-throughput emission estimation and exposure models. Additional methods for addressing uncertainty in emission rates are outlined. The results demonstrate that mechanistic emission modelling can help address uncertainty in chemical emission rate estimates and inform decision-making for chemical safety and sustainability.