Formulation of a Target Plastic Model to estimate critical plastic burdens of toxicants.

Plastic pollution has become a widespread problem affecting multiple environmental compartments, with associated chemicals having harmful effects on living organisms. Here, we developed a Target Plastic Model (TPM) to estimate the critical plastic burden of various toxicants in five types of plastics, namely polydimethylsiloxane (PDMS), polyoxymethylene (POM), polyacrylate (PA), low-density polyethylene (LDPE), and polyurethane ester (PU), following the Target Lipid Model (TLM) framework. The critical plastic burdens of baseline (n=115), less-inert (n=73), and reactive (n=75) toxicants ranged from 0.17-51.33, 0.04-26.62, and 1.00 × 10^-6 - 6.78 × 10^-4 mmol/kg of plastic, respectively. While critical plastic burdens were also estimated for other plastic phases, such as polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), ultra-high molecular weight polyethylene (UHMWPE), and high-density polyethylene (HDPE), the findings were less reliable due to a lack of experimental data. Our study showed that PDMS, PA, POM, PE, and PU are similar to biomembranes in mimicking the exchange of chemicals with the water phase. Using the TPM, median lethal concentration (LC50) values for fish exposed to baseline toxicants were predicted, and the results agreed with experimental values, with RMSE ranging from 0.311-0.538 log unit. For less inert chemicals, predictions were within a factor of 5 of experimental values. The TPM's performance was comparable to other widely used models, such as the TLM, ECOSAR, and Abraham Solvation Model. However, like other models, TPM was not effective in predicting the toxicities of reactive toxicants, with RMSE exceeding 1 log unit. TPM can provide valuable insights into the toxicities of chemicals associated with environmental plastic phases, assisting in selecting the best polymeric phase for passive sampling and designing better passive dosing techniques for toxicity experiments. Moreover, TPM can assist in selecting the best plastic phase for developing animal alternatives for toxicity measurement and determining the toxicity of complex mixtures such as those arising during oil spills.