Human Health Risk Assessment for Engineered Nanoparticles: from Ranking to Risk Characterization

The ubiquitous application and potential environmental release of engineered nanoparticles (ENPs) has caused human health concerns worldwide. Considering the potential exposure from the usage of ENP containing products through environmental exposure pathways, this thesis developed a systematic human health risk assessment for ENPs. As a novel pollutant, the background knowledge on ENPs was evaluated, and the knowledge gaps were identified for further investigation. Sources of ENPs which can result in potential human exposure were outlined as: the release in the use phase of ENP containing consumer products and potential release into surface water, air, and soil in the natural environment. This thesis conducted a risk ranking for ENPs of human health concern. Silver nanoparticles (AgNP) were identified as having the greatest human health concern as a result of their environmental release and were selected for further risk quantification. As an essential step in a complete risk assessment, a hazard characterization was conducted for AgNPs by evaluating existing animal toxicity studies. Representative dose response relationships were converted to human equivalent doses (HED), showing the oral intake could be the most effective exposure pathway to induce histopathological responses. Potential release of AgNPs into agricultural soil has caused concern as a potential pathway for AgNPs to enter the food chain and resulting in human oral intake. This thesis adopted regional variables in Europe and Ireland to develop a comparative risk assessment for human health risk from AgNPs through crop consumption, including through leafy vegetables, wheat based products, and root vegetables. Results show the negligible risk under the current situation and in future scenarios in both the EU and Irish spatial scopes (the highest mean hazard quotient (HQ) value is 4.47E-04). Irrigation using surface water is a major contributor to the final risk. The irrigation methods are critical to the level of AgNPs internalization in crop edible parts. As the AgNPs released in the surface water were determined to be an important source of risk, the aquatic behavior of AgNPs was identified as a critical knowledge gap and was investigated in this thesis using an experimental approach. The removal efficiency was parameterized by key influential environmental parameters including dissolved organic matters concentration, water hardness, water temperature, and incubation time. Particular high persistence can be observed with DOM mediated secondary formation of AgNPs surrounding primary AgNPs in the HDD of 77 ± 1.2 nm group. The interactive effect between 1.1 mM Ca2+ concentration and 99 mg/L DOM concentration was highlighted which can induce the highest AgNP aquatic persistence in both size groups. Having considered the persistence of AgNPs in the aquatic environment, this allows greater potential for more accurately estimating exposure routes through water sources. Uncertainty exists regarding potential human oral exposure to AgNPs through surface water via drinking water consumption. This thesis thus developed a quantitative risk assessment using a probabilistic approach to model the human health risk for adults and children resulting from drinking water consumption. The model assesses AgNPs fate from nine different source water scenarios considering river, lake, and transitional water combined with different levels of water hardness as source water for a DWTP. Traditional processes in the DWTP exhibited effective removal of AgNPs from the inflow, and resulted in effective control of the final human health risk. The final risk shows negligible hazardous impacts resulting from AgNPs ingestion through drinking water consumption for both age groups (the highest mean HQ value is 4.04E-02). The results on sources of human health risks resulting from AgNPs can help future monitoring and regulation development targeting the safe application of AgNPs.