Microplastics and Human Health: Polyethylene
By now, it’s likely you’ve heard of the term “microplastics.” If you don’t know what they are, essentially microplastics are very small (less than five millimeters in size) pieces of plastic that are pervasive throughout our landfills, oceans, air, and even our bodies (National Ocean Service, 2021).
Scientific study and understanding of microplastics is still in its burgeoning stages, but what we do know is that they are everywhere, and they are potentially deadly. One of the major microplastic threats to all biomes and organisms are what are termed “microbeads,” which consist of manufactured polyethylene (PE) plastic (2021).
Importantly, the domains within which our understanding of these plastics impacts our health are growing. Ongoing research shows that microplastics, and PEs in particular, may be causes of new cases of inflammation, oxidative damage, gut biome dysbiosis, and even respiratory issues.
Below, I will first discuss PE in some detail before highlighting some of the potentially severe health impacts life-long exposure may cause.
Polyethylene consists of a long chain of carbon atoms, each carbon joined by two hydrogens — recognizable as C2H4 (Basmage and Hashmi, 2020). The molecular structure of PE can be seen below in Figure 1 (Polyethylene (PE) Plastic, n.d.).
As one of the most widely used plastic products, there are various forms of PE — high density, low density, linear low-density, ultrahigh-molecular-weight, cross-linked, and others — but here I will take a more general approach to the production and subsequent effects of PE.
It is constructed by the combination of PE monomers into a polymer using Zieglar-Natta and Metallocene catalysts, which can be seen below in Figure 2 (Polyethylene (PE) Plastic, n.d.). This chemical reaction, and subsequent catalysis, was first introduced about 50 years ago by Karl Ziegler and Giulio Natta. They were awarded the Nobel Prize for this discovery, in fact, which furthered the commercialized production of polyolefins, one of the largest of produced polymers today.
The enhanced synthesis provided by this finding is due to the existence of single active sites on the metallocene catalysts that allow for improved polymerization (Shamiri, 2014). In other words, this catalyzed capability increased the control of production parameters in material such as monomer distribution, weight, architecture, stereospecificity, and polymer branching (2014).
However, it’s important to note that because of the above-mentioned structural breakthroughs, PE is also non-biodegradable in nature, hence its contribution to the world’s ever-growing collection of plastic waste. In its solid form, PE is nontoxic; however, it can become so if vaporized or liquified.
One important differentiation that is made by the plastics industry is between polyethylene phthalate (a plasticizer) and polyethylene terephthalate (PET), which is ethylene glycol combined with terephthalic acid and more closely resembles the PE discussed above (albeit slightly more transparent and amorphous). A visual representation of PET can be seen in Figure 3 below (Carr, 2020).
As many countries have banned the use of phthalates for their toxic properties, the plastic industry has claimed that terephthalates don’t cause the same problems.
However, while phthalate is not used as a chemical substrate in the production of PET, current research posits that concentrations of phthalates in PET can vary (and have a range of effects) depending on the contents of the product and the environment it’s within (Rustagi et al., 2011).
For example, lower pH products, such as soda, vinegar, and even salad dressing, increase the rate of phthalate leaching into the product itself. Likewise, higher temperatures seem to have a similar effect (2011). So, what does this mean for human health?
Growing bodies of research now link excess phthalate leaching to a variety of negative health outcomes:
- increased adipose storage and insulin resistance
- chronic inflammation
- accelerated oxidative damage
- gut dysbiosis
- respiratory issues
- reproductive issues.
Unfortunately, many of the adverse health impacts of plastic leaching seem to harm infants and children even more so than adults (2011).
Polyethylene Terephthalate
Polyethylene, and specifically polyethylene terephthalate, are some of the most common plastics found in the food we eat, the water we drink, and the air we breathe.
What’s more is that they are so small, that many filters — whether that be air or water — cannot keep all of the particles out. However, the major challenge researchers face today is that despite the ubiquitous presence of these particles, we don’t currently have precise and scalable enough techniques that enable us to isolate microplastics from the environment in such a way that performing comprehensive toxicological studies is possible (Halappanavar and Mallach, 2021).
Accordingly, much of our safety reference for these tiny and prevalent materials comes from commercially available standard references, unfortunately, have let a lot of problematic discoveries go unnoticed. So far, multi-cell models comprised of a variety of lung cells and live tissue cultures (most commonly from mice) have provided a sandbox for testing, whereby specialized cells — epithelial and endothelial, alveolar macrophages, and dendritic — have been subject to investigatory exposure to foreign microplastic substances.
While new findings are regularly coming to light, not enough data has yet been validated that warrants introduction to the regulatory decision making process (2021).
Human Respiratory Consequences
One particularly threatening appearance of PE is within the human respiratory system. Similar to the studies mentioned above pertaining to cultured non-human lung tissue, there have been a few studies focused on potential toxicity in cultured human epithelial lung cells (Amato-Lourenço et al., 2020).
Plastic nanoparticles (25–70nm in diameter) were evaluated on the alveolar epithelial A549 tumor cell line, with results revealing that nanoparticle interaction can noticeably, and adversely, impact cell activity. This was seen in the increased activity of inflammatory genes and apoptotic protein expression.
Moreover, this inflammatory activity appeared to cause cytotoxic effects in epithelial BEAS-2B cells by promoting the formation of reactive oxygen species (2020). These species are already present in mitochondria when converted into superoxide ions, and are implicated in numerous pathophysiological conditions.
While much of the research surrounding microplastics is novel and still emerging, the first findings are hinting at more and more potential adverse side effects from our societal reliance on plastic.
As research methods and technologies improve to the point that microplastics can be consistently and accurately isolated and studied from a toxicologically point of view in the context of both their environments as well as human physiology, I have no doubt that we will learn more about the damage being done to our bodies.
Hopefully then we’ll have the insight and wherewithal to change our ways and do something about it.
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References:
National Ocean Service. (2021). What are microplastics? https://oceanservice.noaa.gov/facts/microplastics.html.
Basmage, O.M. and Hashmi, M.S.J. (2020). Plastic Products in Hospitals and Healthcare Systems. ScienceDirect. https://linkinghub.elsevier.com/retrieve/pii/B9780128035818113037.
Polyethylene (PE) Plastic: Properties, Uses & Application. (n.d.). [image]. Omnexus. https://omnexus.specialchem.com/selection-guide/polyethylene-plastic.
Shamiri, A., Chakrabarti, M., Jahan, S., Hussain, M., Kaminsky, W., Aravind, P., & Yehye, W. (2014). The Influence of Ziegler-Natta and Metallocene Catalysts on Polyolefin Structure, Properties, and Processing Ability. Materials, 7(7), 5069–5108. https://doi.org/10.3390/ma7075069.
Carr, C. M., Clarke, D. J., & Dobson, A. D. W. (2020). Microbial Polyethylene Terephthalate Hydrolases: Current and Future Perspectives. [image]. Frontiers in Microbiology, 11. https://doi.org/10.3389/fmicb.2020.571265.
Rustagi, N., Singh, R., & Pradhan, S. (2011). Public health impact of plastics: An overview. Indian Journal of Occupational and Environmental Medicine, 15(3), 100. https://doi.org/10.4103/0019-5278.93198.
Halappanavar, S., & Mallach, G. (2021). Adverse outcome pathways and in vitro toxicology strategies for microplastics hazard testing. Current Opinion in Toxicology, 28, 52–61. https://doi.org/10.1016/j.cotox.2021.09.002.
Amato-Lourenço, L. F., dos Santos Galvão, L., de Weger, L. A., Hiemstra, P. S., Vijver, M. G., & Mauad, T. (2020). An emerging class of air pollutants: Potential effects of microplastics to respiratory human health? Science of The Total Environment, 749, 141676. https://doi.org/10.1016/j.scitotenv.2020.141676.
