Human health risk assessment of metals from bio-based microplastics using a bioavailability gastrointestinal digestion model

Bio-based plastics are widely regarded as a promising alternative to conventional plastics; however, assessing their impact on environmental and human health is critically important. This study evaluates the human bioavailability of Ag, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb, and Sn from microplastics (MPs) derived from the most common bio-based materials: polylactic acid (PLA), polyhydroxybutyrate (PHB), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Three scenarios were considered: raw material, material aged in a marine environment, and material subjected to a laboratory-based metal adsorption process. An in vitro gastrointestinal procedure, modified from the Unified Bioaccessibility Method (UBM) by incorporating a dialysis membrane, was employed to simulate intestinal absorption and determine the bioavailable fraction. MPs from both raw and marine-aged materials exhibited neither non-carcinogenic nor carcinogenic risks. However, non-carcinogenic risks were identified in metal-loaded MPs for Cd and Co (in PHAs and PLA) and Sb (in PHAs) in both adults and children, when considering the bioaccessible fraction. In contrast, when bioavailability was used, these risks were limited to Cd and Co. Carcinogenic risks associated with Cr in metal-loaded PHB and PHBV were identified for both adults and children, based solely on bioaccessibility. This study demonstrates that bioavailability provides a more reliable approach than bioaccessibility for evaluating human health risks associated with metals in bio-based MPs, because it accounts not only for the metal fraction that becomes solubilized during the digestive process, but also for the portion that, once solubilized, is subsequently absorbed into the bloodstream. Furthermore, the study highlights that each bio-based plastic type exhibits distinct metal-related behaviour.