Plasticizer? I hardly know ‘er!

I went to the CleanMed 2024 conference this spring and was surprised to find so very many conference sessions focused on plastics and their harms to health. Microplastics and plasticizers were discussed at length. After hours of superficial discussion of the health harms of plasticizers, I had to admit to myself that I did not know what plasticizers are at all.  So, I did some research.

To discuss plasticizer, we need to start with plastic itself. Some common types of plastics include polyvinyl chloride (PVC), polystyrene, and polyethylene. These are examples of polymers that are derived from crude oil. (A polymer is a long chain of a repeating chemical.) Plain old plastic like this tends to be brittle, lending poorly to applications like soft water bottles, zip lock bags, contact lenses, literally anything. How do we make it softer and more malleable?

Enter plasticizers. Much like salt can be dissolved into water to change the freezing point, plasticizers can be dissolved into molten plastic to change its physical properties. They are not bonded to the polymer itself, but suspended between the many polymer chains (Foroughi-Dahr, et al). This means that even though a plastic product is solid, some plasticizer is still able to leach out into adjacent materials. Plasticizers are bulky organic molecules and tend to be partially polar (Carroll, et al), which is why they dissolve so well in plastic. One of the most common types are called phthalates. Incidentally, hormones like testosterone and estrogen are also bulky organic molecules. A basic concept in chemistry is that “like dissolves like,” so organic molecules are soluble in organic solutions: PVC and other polymers, and all the fats in your body. Not just fat tissue around your midsection, but the membrane of every cell in your body (the lipid bilayer you may have heard about in high school biology). 

One of the “worst” plasticizers still commonly used is called DHEP. DHEP is a particularly bad type of phthalate because it has very long nonpolar chains that make it even more soluble in our tissues than phthalates with short chains. But how does DHEP go from a plastic jug into my body? Well, there are publications showing that DHEP is soluble in our cell membranes (Bad, et al), but we can look at it from an engineering standpoint using the principles of diffusion. I’ll spare you the equations, and use yet another analogy. Take two similar but non-identical items: a kitchen sponge, and a kitchen towel. Dip the sponge in salt water. Dip the kitchen towel in tap water. Now place the sponge on top of the towel and let them sit together for a while. Feeling brave? Do a taste test of the towel. Better yet, do this experiment with food dye instead of salt. What you witnessed is diffusion (and also capillary action but we are ignoring that for the sake of the analogy). The saltwater that started in the sponge is also soluble in the kitchen towel, and when they are put into contact, some of the salt water will slowly migrate down it’s concentration gradient into the towel. To overexplain once again, in this analogy the sponge is plastic, the saltwater is plasticizer, and the kitchen towel is you. Engineering principles tell us that it is inevitable that plasticizers will leach out of plastic and into your food, and into you when you consume microplastics. 

Humans have been using plastics for decades, and obviously nobody is dying within minutes of microwaving their lunch in an old Tupperware. What are the actual health effects and how concerned should we be? Studies as early as 2003 showed that DHEP is an endocrine disruptor in rats. Time for another definition. Your endocrine system includes all the hormones in your body and the glands that produce them. Things like testosterone, estrogen, and cortisol are hormones. Less commonly known hormones include luteinizing hormone (LH), follicle stimulating hormone, thyrotropin-releasing hormone… the list goes on. Together these hormones act to regulate your body through negative feed-back loops. In this study from 2003, they found that DHEP disrupts an important negative feedback loop (Akigbemi, et al). In a normal person (or rat), LH released from the pituitary gland stimulates the male testes to produce testosterone. Increased levels of testosterone in the bloodstream tell the hypothalamus and pituitary to stop making LH, and thus decrease the production of testosterone. Simply put, a person (or rat) with high testosterone should have low levels of LH. In this study, the rats exhibited a broken feedback system; they had elevated testosterone AND elevated LH. Observing the chemical structure of phthalates could probably have told us the same thing. Hormones and phthalates are both bulky organic molecules with moieties lending various levels of hydrophilic interactions. It is well studied that hormones get into our cells by diffusing through the lipid membrane, but for solid reference, this 2022 study shows just how easily and far three common plasticizers do permeate our cell membranes. Once inside, plasticizers may continue to masquerade as hormones and interfere with intracellular signalling to disrupt our bodies’ normal control mechanisms. 

I have not read all of the literature on this topic, and I am no expert. This essay should serve to explain what plasticizers are, and the basics of how they can interact with our bodies. I learned a lot researching the topic and have a greater understanding of why we keep talking about plastic and plastic pollution as a health concern. Am I going to stop using plastics? No. Will I stop microwaving my lunch in a plastic container? Yeah, most of the time. I certainly won’t live in fear of using plastic. Instead, I will use this knowledge to engage in educated conversations about the issue and educate myself as new research emerges in the area.

References: 

Akingbemi, B.T.; Ge, R.; Klinefelter, G.R.; Zirkin, B.R.; Hardy, M.P. Phthalate-induced Leydig cell hyperplasia is associated with multiple endocrine disturbances. Proc. Natl. Acad. Sci. USA 2003, 101, 775–780.

Bad, Y; Li, M.; Xie, Y.; Guo, J. Investigating the Permeation Mechanism of Typical Phthalic Acid Esters (PAEs) and Membrane Response Using Molecular Dynamics Simulations. Membranes 2022, 12(6), 596; https://doi.org/10.3390/membranes12060596

Mohammad Foroughi-Dahr, Navid Mostoufi, Rahmat Sotudeh-Gharebagh, Jamal Chaouki,

Particle Coating in Fluidized Beds, Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, Elsevier, 2017

William F. Carroll, Richard W. Johnson, Sylvia S. Moore, Robert A. Paradis, 5 – Poly(Vinyl Chloride), Editor(s): Myer Kutz, In Plastics Design Library, Applied Plastics Engineering Handbook, William Andrew Publishing, 2011, Pages 61-76