Nur Hazimah Mohamed Nor, Merel Kooi, Noël J. Diepens, and Albert A. Koelmans, 2021. Lifetime Accumulation of microplastic in children and adults. Environmental Science & Technology, 55 (8): 5084-5096. DOI: 10.1021/acs.est.0c07384
Plastic pollution has become a major concern among researchers and the public alike. Large plastic debris often pictured littered across a previously pristine shoreline or in seabirds stomach content is only a part of the problem. Larger plastics are naturally broken down into smaller pieces called microplastics which can be smaller than the width of a human hair. Humans interact with microplastics every day and more and more research is finding microplastics in the air we breathe, the food we eat, and the water we drink. Researchers have even found microplastics in human stool samples, suggesting the particles are commonly consumed by people. Scientists are still trying to determine the implications for human health and in fact, researchers have recently taken a step forward to fine-tune predictive models to describe microplastic concentrations in humans.
Modeling plastic in our bodies
Models are a way to combine information (e.g., concentrations and rates) through a series of mathematical equations to answer a question. In this case, the researchers used two models to investigate microplastics in the gut, body tissue, and stool. The ‘Plastic Model’ uses known concentrations of microplastics in the air, common foods (fish, shellfish, salt), and drinks (water, beer, tap water, bottled water, milk) in an equation to determine how many microplastics people are exposed to everyday. The ‘Chemical Model’ estimated the amount of chemical leaching that is occurring from microplastics into the gut for 4 carcinogens commonly found in plastic. Equations for this model calculated how chemicals seep off plastic particles into the body based on the typical levels of these chemicals on or in plastic particles and in the surrounding tissues. Modeling is an uncertain art as assumptions are made to adjust for unknowns. Additionally, the data are influenced by the equations and chosen rate constants. For example, the rate at which small microplastics are expelled from cells and organ tissues was ‘assumed’ in this study. In essence, models cannot perfectly mimic reality, but are useful tools for predicting outcomes whether it be for climate change or the fate of microplastics in the human body.
How much plastic are we consuming?
The ‘Plastic Model’ estimated the ingestion and elimination of microplastics in the human body. It was determined that children consume about 500 particles a day and up to 50,100 particles can be accumulated in an adult’s lifetime based on the examined food, drink, and air sources for microplastic ingestion. To put this in perspective, the researchers explain that even if the concentration of ingested microplastics were to increase seven orders of magnitude, it would only account for 0.004% of the mass of inorganic particles ingested per day. Besides microplastics, people are exposed to a variety of inorganic, nano- and micro-scopic particles used in food packaging, food processing (e.g., small particles used to enhance flavor), skin application (e.g., sunscreen) and more that were not examined in this study. According to the ‘Chemical Model,’ there is very little chemical leaching from microplastics into the gut. Microplastics spend only a limited time in the gut as the particles are egested in stool relatively quickly (a day or so). There is still much to learn about the potential long-term health effects of microplastic exposure such as how the inhalation of microplastics can affect our lungs. In the meantime, the researchers of this study suggest that microplastic consumption from these specific sources is not a major concern. These models will be adjusted as new findings and data become available to track how many microplastics people are exposed to each day.
I am a PhD candidate at the University of Connecticut-Avery Point exploring the dynamic interactions of microplastics and suspension feeding invertebrates. Through both field and laboratory work, I am working to understand which kinds of microplastics (different shapes, sizes, compositions) oysters, mussels, tunicates, and slippersnails consume and determine which species can be used to monitor microplastic pollution in our coastal waters. When I am not working on my research, I enjoy hiking with my partner and pup, being near or in the water, and spending time with family and friends.