We all use plastic in our everyday life. I’m currently writing this post on my laptop. What most of us don’t know is how this plastic material is made and what chemicals it might contain.
What is a plastic additive?
Plastics are synthetic polymers made from a chain of repeating molecules, called monomers. Each plastic type, such as polyethylene (used to make plastic bags) and polypropylene (used to make plastic yogurt containers), are different polymers. During manufacturing, monomers are strung together during a process called polymerisation. At this stage, some chemicals are added to give plastic specific properties, such as adding flexibility, durability and/or color. These chemicals are called plastic additives.
The properties, that make plastic a versatile material, are there because of the plastic additives. Plastics gain color from the addition of chemical dyes, flexibility from plasticizers, UV-resistance from antioxidants, and so on. There are many different plastic types, which are used to create many different products, each with their own additive chemicals, making the overall number of plastic additives on the order of thousands of molecules.
It is important to note that these plastic additives are not chemically bound to the polymer, meaning that they can leach from (or leave) the plastic product. This allows the plastic additive to become available to living things in the environment. This may be a problem, if the additive chemical can be toxic.
Why study plastic additives?
Plastic products do find their way into the environment where they undergo degradation into every smaller particles. Plastics smaller than 5 mm in size are called microplastics. Microplastics and larger plastic objects can be eaten by animals, which can result in a false feeling of fullness, suffocation and even death. Such effects can be categorized as physical, because the shape and size of the ingested particles is leading to a consequence. Plastic additives leaching into an animal can have an additional chemical effect. To date, little research has been done on the chemical effects of microplastics because it is complicated to distinguish between physical and chemical effects.
Still, some scientists have demonstrated a chemical effect. In one study, researchers exposed coral reef fish (Pseudochromis fridmani) to chemicals from two different plastic bags. They found that chemicals from one of the bags led to mortality, highlighting that plastic additive composition varies from product to another. In another study, researchers showed that toxicity to water fleas (Daphnia magna) can be caused by both the additives and the particles themselves (i.e., physical and chemical). These studies show more work is needed to understand these effects, and particularly those from additive chemicals.
How do I study additives effects?
I use laboratory and field experiments to answer questions about the role of additives in microplastic toxicity. In the laboratory, I exposed fish (fathead minnow (Pimephales promelas)) to plastic with additives, plastic without additives and additives alone. I used polyethylene, and had it specially made with and without additives. Here, the different conditions allow me to assess whether effects are physical and/or chemical. I looked for any effects related to growth, survival, and gene expression. I am also measuring the bioaccumulation from the additive chemicals.
Field experiments began in Summer 2021 at the Experimental Lake Area. Nine in-lake enclosures were deployed in a boreal lake in Northwestern Ontario, Canada. Each enclosure had yellow perch (Perca flavescens), which were exposed to different and increasing concentrations of a mix of microplastics (polyethylene, polystyrene and polyethylene terephthalate) loaded with plastic additives. We deployed passive samplers in the water that enable us to quantify the leaching of the different additives and also sampled to measure any bioaccumulation (or the uptake of these chemicals into their tissues). Fish will also be assessed for toxicity, including gene expression, fatty acids profiles, growth, and survival allowing us to have a broader understanding of how (and why) microplastics alter fish health.
Overall, both experiments (in the lab and field) will inform how plastic additives leach from microplastics and will inform how plastic additives affect fish. This work fills a critical gap in our understanding about the complexity of plastic pollution – which is more than just a physical particle; it’s also a chemical cocktail.
Written by Ludovic Hermabessiere, postdoc researcher in the Rochman Lab working on plastic additives fate and effects and Chelsea M. Rochman, Assistant Professor at University of Toronto, co-founder of the U of T Trash Team, and Scientific Advisor to the Ocean Conservancy.
This work was undertaken with the financial support of Environment Climate Change Canada through the Increasing Knowledge on Plastic Pollution Initiative.