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Microplastics’ journey into the gut : human exposure to microplastics and associated chemicals
Summary
This thesis investigates lifetime human exposure to microplastics worldwide and explores how plastic particles act as carriers that transport harmful chemicals into the body after ingestion, known as the vector effect. Using mechanistic models and experimental methods, the work quantifies how much microplastic people consume and how significantly this route contributes to chemical bioaccumulation.
Microplastics are small plastic particles (1‒5000 µm), which are omnipresent in the environment and may likely be ingested by biota, especially humans. There has been an increasing concern on how much microplastics humans are actually exposed to in the world and if the levels can actually cause any effect in us. One of the potential effects of microplastics that have been identified is its ability to transport chemicals into the organism (commonly referred to as the vector effect) when the particles are ingested. Therefore, the fundamental aim of this thesis was to unravel the lifetime exposure of microplastics in humans around the world and the extent of the role of microplastics as a vector for chemical bioaccumulation in humans and other biota. To accomplish these aims, a mechanistic approach was needed that involved developing sophisticated experimental methods, mechanistic models, and modeling tools. The primary focus of this thesis was the transport of microplastics through the gastrointestinal tract and the extent of the chemical vector effect during the gut retention time.This thesis started with understanding the chemical dynamics of microplastics in a simulated gut environment using an in vitro experimental approach. A series of experimental setups to represent three different environmentally relevant scenarios was examined. First, an organism ingests contaminated (i.e., associated with chemicals) microplastics. Second, an organism ingests contaminated microplastics and contaminated food. Third, an organism ingests clean microplastics and contaminated food. Chemical transfer was demonstrated to be biphasic and fully reversible on the microplastics, with fast exchange within hours followed by a slow transfer lasting for weeks to months. Therefore, a biphasic reversible chemical exchange model was constructed and parameterized for organic chemicals on microplastics with the in vitro experimental data. Low density polyethylene (LDPE) showed faster sorption kinetics than polyvinyl chloride (PVC). Additionally, the apparent affinity of chemicals with PVC decreases as chemical hydrophobicity increases, which is contrasting to that found for LDPE. Model parameters from this first study was also later used in a later part of this thesis to understand the chemical vector effect in humans. Overall, this study demonstrated that whether microplastics increase or decrease the chemical body burden of an organism is context dependent.Although the first study in this thesis had provided a fundamental understanding on the chemical dynamics of microplastic-associated organic chemicals in the gut fluid systems, the food component in the experimental setup was an inert pool. In reality, food is constantly digested in the gastrointestinal tract of the biota, which would affect the chemical dynamics. Therefore, a follow-up in vitro gut fluid study was designed to include the food digestion kinetics and further develop the earlier biphasic reversible chemical exchange model. At the same time, the chemical affinities for different gut components (olive oil lipids and micelles) were investigated to evaluate the relative chemical distribution for each component. Our findings showed that as the olive oil lipids were digested by the enzyme, lipase, food-associated chemicals were released and taken up by microplastics. This process would decrease the amount of chemicals available for uptake by the organism when the microplastics are egested. Additionally, the lipid digestion kinetics was rate-limiting and had a small influence in the overall chemicals dynamics on microplastics. Kinetic parameter estimates from the new model in this study can be used to extrapolate chemical behaviour in realistic human gut conditions to evaluate the human health implications of microplastics.Finally, this thesis unravelled the lifetime exposure of microplastics to humans in the final main chapter in which a probabilistic lifetime exposure model toolkit was constructed for children and adults, accounting for intake via eight food types and inhalation (presently known microplastic sources at the point of the thesis). This included the accumulation of microplastics in the body via a gut distribution model which accounted for intestinal absorption, biliary and intestinal excretion. This exposure assessment stands out from previous ones as not only does it quantify the uncertainties surrounding microplastic exposure in humans, it also accounts for the non-alignment of methods used in microplastic research, the heterogeneity of microplastics and the distribution of global intake rates in the world. Therefore, the exposure estimates were reported probabilistically and presented as distributions, accounting for the aforementioned uncertainties and variabilities. The toolkit also included a chemical model, which helps us to understand the potential risk of microplastic due to microplastic-associated chemicals. The previously developed biphasic and reversible chemical exchange model was integrated in the chemical model here. The transfer of four representative chemicals, which are widely known environmental pollutants, from ingested microplastics was simulated under realistic conditions whereby the gut was also exposed to chemicals from dietary intake. Our findings suggest that the chemical contribution from ingested microplastics from the known nine intake media is small. However, our estimates in this study only account for approximately 20% by mass of the total food consumed daily on average. There is still a lack thereof data in other food types such as grains, vegetables, meats and processed food. It is plausible that when these food clusters are included in the future as more studies report microplastic occurrence, the distribution of actual microplastics exposure and intake in humans would widen. This may then result in different implications for the chemical vector effect of microplastics in humans.This thesis was concluded with an evaluation of the weight of evidence for the microplastic chemical vector effect from in vitro gut fluid studies. Recommendations for future in vitro gut fluid studies that aim to elucidate the chemical vector effect are also included. An evaluation of the current developments on models for exposure assessments of microplastic and its associated chemicals was also discussed. In conclusion, the chemical exchange models and probabilistic microplastic human exposure toolkit developed in this thesis can be used in bigger model frameworks in the future such as for ecological and human risk assessments.
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