Environmental and human health risks from micro/nano-plastics

Micro/nano-plastics (MN-Ps) constitute a significant form of plastic waste present widely across marine, freshwater, and terrestrial ecosystems and which has emerged as a global environmental challenge. Given the diverse size range and varied properties of MN-Ps, they efficiently permeate the food chain, potentially instigating environmental and human health risks. Recognising and addressing these environmental and human health risks are current global concerns and represent a research area where scientific understanding is currently limited. The overall thesis goal is to assess the environmental and human health risks associated with MN-Ps in food products in Ireland, utilising an established risk assessment framework. The literature review conducted as part of this study lays the foundation for a food safety risk assessment framework to assess the hazard potential of MN-Ps in the marine ecosystem on human health. This study conducts a review of the current state of the art regarding MN-P definition and characterisation, identification, toxicity, and risk assessment methodologies in marine and food systems. The primary pathway of human exposure to MN-Ps has been identified as food ingestion. MN-Ps may pose acute toxicity, (sub) chronic toxicity, carcinogenicity, genotoxicity, and developmental toxicity. MN-P toxicological properties and general quantitative and qualitative analysis methods used in risk assessment are summarised. As part of this study, a semi-quantitative hazard identification model was built to rank MP polymers of potential human health concerns emerging from marine exposure pathways and categorises three exposure probability factors and two hazard impact factors for inclusion in a screening strategy to calculate the final risk scores. The outcome of this study is to inform for better management of the most hazardous polymers, specifically PUR, PVC, PAN, ABS, and PMMA. Dietary HE assessment models on the potential internalisation probability to MN-Ps through vegetable, fruit, grain, and aquatic fish and fish products in Ireland were developed. Linear regression models for MN-P size-concentration relationships and the bioaccumulation factor were developed for HE assessment modelling. The estimated daily intake (EDI) of MPs for Irish adults was found to be in the order of (through) vegetables > fruit > grain (1.62 × 10^3 number/kg body weight/day) and fish and fish products (40.3 number/kg body weight/day). The EDI of MPs through fish and fish products was compared with the no observed adverse effect level with the margin of exposure approach to assess the risk exposure probability, which suggests nearly 70% population is at low risk and 30% at higher risk. The uncertainties of MN-P gene expression were also addressed in this study through a meta-analysis study. In the systematic approach, study characterisation was developed by quantitative analysis to categorise the selected data into DNA/mRNA - positive/negative sign groups. In the meta-analysis, effect sizes were calculated to enhance the robustness of pooled estimates for individual studies. Through subgroup analysis, practical, functional assessment violin plots were generated to determine the significant effect factors of gene expression (MN-P morphology, exposure duration, pathway, and associated toxicity) induced by MN-Ps, leading to a future investigation into the hazard characterisation. Related potential genotoxic responses were reflected on the specific gene endpoints that were classified based on the gene expression analysis, related reference gene and data form. Overall, this thesis developed a comprehensive risk assessment model to evaluate the potential risk from MN-Ps to the environment and human health. The findings will inform the public, food growers, producers and processors, and policymakers on potential exposure to MN-Ps and risk reduction, thus ensuring food safety and encouraging responsible use and disposal of plastic materials.