Environmental Question #20 [Microplastic Toxicology]

Courtesy of Reddit user u/buttercup_mauler

Q: I'm an environmental engineer working on my forensic toxicology MS!

I have been interested in microplastics from a toxicology view, but I'm not far enough into my studies to have really delved deep. So far, I've learned mostly about the gen tox stuff like xenobiotic biotransformation, but it is largely focused on drugs, pesticides, that type of stuff.

Do you know what kind of information and studies have been done on the tox side of microplastics? For whatever reason, it's harder for me to wrap my brain around the idea of attempting to metabolize a plastic vs a drug. I am really interested to learn more.

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A: 

Personal Comments:

That's dope! I want to let you in on a little open secret actually. Despite the positive environmental work that I do, you and your peers are my personal heroes. I'm not actually a toxicologist by training, I'm a green chemistry synthesis specialist. My job is to design new products and chemical processes to minimize human and environmental toxicity, so to do my job I rely heavily on the work you and your peers do. If not for the dedicated work of toxicologists, I wouldn't know which chemicals are safe to use and which are off limits for my work. Don't get me wrong, as part of my job I've needed to pick up a very strong background in toxicology over the years, but my work doesn't advance the field of toxicology, it instead depends on the people like you who do. So thank you for your dedication to your studies, I'll be looking forward to referencing your research when you graduate ;)

Microplastic Toxicology:

To answer your questions though, microplastics fall into a strange middle ground toxicologically, because they behave in multiple ways simultaneously. (Disclaimer: I don't know how strong your polymer chemistry background is, so I might over explain this. My goal is to ensure you understand, not to be condescending, so please forgive me if you already know some or all of this.) The central complication of microplastics (i.e. polymers) when it comes to toxicology is in solubility. As polymers are physically ground down, digested, and/or oxidized they will break down to varying degrees into monomers (single molecule units) and oligomers (short polymer chains), and each of these versions behaves differently.

Monomers have a higher functional group density, so they are more likely to dissolve and be absorbed by any given organism. This somewhat high solubility is why so many monomers and small-molecule plastic additives behave as endocrine disruptors. They are chemically similar to hormones, and they have similar solubility to hormones, so they will sometimes find their way to hormone receptors and set those receptors off. Look up the chemical structure of BPA and compare it to the structure of 17 Beta-Estradiol (the main form of estrogen), and you will see they are very similar. It's no wonder BPA can activate estrogen receptors. Many common monomers are biologically active molecules, if you want to get an idea of their toxicological effects, look up a list of common monomers and the toxicological literature about them. You'll find plenty of interesting stuff.

Oligomers are much larger molecules compared to monomers and have lower solubility in water because they are generally quite nonpolar, so they have a lower rate of absorption for most organisms, but this causes its own problems. Oligomers tend to be partially absorbed, then clog things up rather than being metabolized. Stuff like being filtered out by the liver and kidneys and then getting stuck there rather than being successfully purged. The low solubility of oligomers and nanoplastics also often causes them to accumulate in waterways, either being dragged down to the bottom or collecting as a waxy film on top of the water.

Microplastics behave in all of these ways at once, because microplastics themselves are fairly inert physical particles that shed monomers and oligomers as they gradually break down. To understand the toxicological significance of microplastics, you need to remember that they aren't individual molecules like most pesticides are. They're polymer chains, that are constantly being broken down into smaller and smaller units that each have their own somewhat distinct behaviors. Generally, the microplastics themselves don't get metabolized, the oligomers get partially metabolized, and the monomers behave like the drugs you're used to in your classes. There's plenty of literature out there about the toxicological significance of microplastics, but as I'm sure you can tell that's a really big topic, so usually studies focus on a particular type of plastic or on a specific monomer or oligomer. 

The Role of Plasticizers: 

Plasticizers are generally monomers or oligomers that are designed to act as lubricants within plastics, allowing the polymer chains to slide by each other more easily. This means they are usually made of different chemicals than the polymer itself, and they are present in much smaller quantities than the polymer itself. For the purposes of toxicology though, plasticizers and additives leach out of microplastics in the same way that monomers and oligomers do.

It might help to keep in mind that on a microscopic level, microplastics are not hard little plastic balls, they're more like little balls of lint. They have a rough shape, but there are always little bits flaking off from the main ball (monomers and oligomers), and they carry a lot of crud in the crevices that they're constantly shedding too (plasticizers and additives). Also much like a ball of lint, microplastics also pick up random crud in their travels and deposit it along the way.

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