Recent research reveals that microplastic particles (MPs), already known for their widespread environmental presence, may pose serious health risks as vectors for plastic additive chemicals (PACs). Published in Microplastics and Nanoplastics, the study by Todd Gouin and Michael J. Whelan offers a comprehensive analysis of how MPs contribute to chemical exposure in humans and wildlife, presenting a nuanced look at their role as conduits for potentially toxic substances.
Microplastics: The Silent Carriers of Chemical Additives
Microplastics, defined as plastic fragments under 5 mm in size, originate from the degradation of larger plastic items during various stages of use and disposal. With MPs being nearly omnipresent in the environment—from oceans to the food chain—concerns have escalated about their potential to carry PACs into organisms. PACs include substances like bisphenol-A (BPA), DEHP, and other industrial chemicals used for their structural and functional benefits in plastics.
The study highlights that PACs can leach from MPs once ingested, transferring to an organism's gut fluid where they may enter the bloodstream. This is particularly troubling as PACs are often not chemically bound to the plastic polymer, facilitating their release during digestion.
A Deep Dive into the Bioaccumulation Model
To understand the exposure risks, Gouin and Whelan modified the ACC-HUMANSTEADY bioaccumulation model. Traditionally employed to simulate chemical accumulation in food webs, this model was adapted to include MPs as an additional pathway. The study examined the ingestion of MPs by humans, fish, and livestock, modelling various exposure routes, from direct ingestion to uptake through food chains.
The researchers discovered that the relative contribution of MPs to PAC exposure is influenced by factors like particle size and ingestion rates. Specifically, MPs with diameters as small as 1 μm presented significant risks when ingestion rates reached or exceeded 10 mg per day. These scenarios suggested that PACs leaching from MPs could account for a meaningful proportion of daily chemical intake, especially for highly hydrophobic compounds with a high octanol-water partition coefficient (log KOW > 4).
Exploring the Exposure Scenarios
The study proposed three key exposure scenarios to gauge the impact of MPs on human and animal PAC exposure:
- Direct MP Ingestion by Humans Only: This scenario simulated individuals consuming MPs directly, with results showing that for highly volatile and hydrophobic PACs, MPs could constitute the primary exposure pathway.
- MP Ingestion by Humans and Animals: By including animals like fish and cows in the model, the researchers mapped out the amplification of PAC exposure through the food chain. The results indicated that bioaccumulation in animal tissues could shift the primary exposure pathway from direct MP ingestion to dietary sources.
- Environmental Exposure Without MPs: This control scenario measured the baseline exposure through contaminated water, air, and food, providing a contrast to understand the specific contribution of MPs.
In these scenarios, hydrophobic chemicals showed an increased likelihood of bioaccumulation in food chains, reinforcing concerns that MPs could amplify PAC exposure when small and ingested at higher rates.
Risk Assessment of Notable Chemicals
The study applied its findings to four widely scrutinised PACs: bisphenol-A (BPA), DEHP, PBDE-209 (a brominated flame retardant), and UV-328 (a UV inhibitor). Using the modified model, the authors assessed potential human exposure under both worst-case and typical scenarios. The data suggested that PBDE-209 and DEHP posed the highest potential risks when MP ingestion reached or surpassed 0.5 mg per day. For these chemicals, margins of exposure (MOEs) fell below safety thresholds, indicating a potential health concern.
BPA, despite its presence in plastics, exhibited a lower relative risk due to its lower affinity for plastic particles and limited concentration in MPs. This finding highlighted the complexity of assessing chemical risks, showing that not all PACs behave uniformly when bound to MPs.
Insights Into Chemical Transfer Mechanisms
The study delved into how PACs move from MPs into an organism's body. It used a two-film resistance model to calculate the diffusive transfer of chemicals, considering factors like particle size, surface area, and the chemical's solubility. This model illustrated that smaller MPs have a higher surface area-to-volume ratio, facilitating greater chemical leaching compared to larger particles.
The researchers emphasised that while the highest risk is associated with the ingestion of the smallest particles (1 μm), uncertainties persist regarding the real-world ingestion rates and the concentration of PACs within MPs. For instance, ingestion estimates for MPs vary widely, with daily intake rates ranging from micrograms to potentially 500 mg in extreme cases. Such variability underscores the need for more comprehensive data to improve the accuracy of exposure assessments.
Challenges and Research Gaps
Despite the valuable insights, Gouin and Whelan pointed out significant knowledge gaps that need addressing. The variability in MP ingestion rates, particle size distributions, and the PAC content in different polymer types all contribute to substantial uncertainties in evaluating real-world risks. Future studies should aim to better characterise the types and concentrations of MPs present in the environment, their specific PAC load, and ingestion rates among various populations and wildlife.
The study also urged the development of more physiologically relevant data, particularly for smaller MPs under 10 μm, which are more likely to penetrate biological barriers and contribute to systemic exposure.
Implications for Public Health and Regulation
The findings emphasise the importance of a comprehensive, multi-pathway approach when assessing chemical exposure risks. Regulatory bodies may need to consider MPs in chemical risk assessments, particularly for chemicals with a high potential for leaching and bioaccumulation. The use of holistic models like the modified ACC-HUMANSTEADY allows for better understanding of MPs' role relative to other exposure sources.
