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Microplastic toxicity

“Microplastic” is a general term defining any piece of plastic less than 5mm. Microplastics are highlighted in the media as a major global pollutant of increasing concern. A majority of research has and continues to focus on understanding microplastic distributions globally by discovering the sources, patterns of transport through the environment and the ultimate fate of microplastic particles. As studies have demonstrated the global nature of microplastic contamination in soil, air , water , and within marine , aquatic and terrestrial food webs, the concerns of environmental health have spurred scientific investigations of microplastic toxicity (Toxicological effects to organisms from microplastic exposure).

Plastics are generally considered inert and traditionally classified as being relatively harmless to humans and the environment. However, practically all plastics represent a concoction of various chemicals beside the actual polymer that defines the plastic. The chemicals that become associated with plastics can be purposely used by industrial manufactures to endow specific qualities to the plastic as additives (i.e. UV-stabilizers, flexibility, rigidness, hydrophobicity, etc..) or the chemicals can become associated with the plastic through sorption from surrounding environments.

Studies confirm the ubiquity of microplastics worldwide from the deep sea to mountain tops to arctic ice  and the atmosphere. Though perhaps more alarming are findings of microplastic ingestion across multiple trophic levels and among the varied biomes where the pollutant has been found. Microplastics have been found to transfer among trophic levels  as a predator consumes prey who in turn had consumed microplastic. Discovering whether or not microplastics indeed are able to transfer bioaccumulative toxicants is a major goal of scientists.

Factors in Microplastic Complexity

Because microplastics can occur in a range of sizes, shapes, and chemical compositions, the possibility for adverse effects to organisms can vary greatly depending on the organism or the type of plastic used and the chemicals that may be associated with the plastic.

Size: Microplastic by definition are smaller than 5 mm but can include nano-particles or smaller. The size range of microplastic particles affects the mobility of the particle through various physical mediums (fluid transport and settlement), as well as determining the bioavailability of the particle to smaller organisms that may form the base of trophic food webs.

Shape: Because of the varied uses of plastic in industry and society, microplastic particles come in a wide variety of shapes from spherical nurdles to fibers from textiles, and fragments from the breakdown of plastic products. The shape of the particle affects the surface area to volume ratio which in turn determines the ability for chemicals to sorb and concentrate on each particle.

Polymer: Plastics are made of linked organic chemicals collectively called polymers. The composition of the linked chemicals vary between polymers and thus produce differences in physical and chemical properties (such as density, strength or reactivity) that are inherent to the plastic regardless of size or shape.

Chemical additives: In addition to the base polymer composition, additional chemicals can be associated with plastics to bestow or enhance qualities including color, toughness, stability and flexibility. Most often adsorbed chemicals are Hydrophobic Organic Chemicals (HOC) including antibiotics, and Persistent Organic Pollutants (POPs), but can also include metals of varying charge.

Adverse Effects

Physical Blockage

Microplastics have been most notably indicated in adverse effects to organisms as a digestional obstacle; where in high quantities or irregular shapes microplastic particles can result in partial or complete blockage of the digestional tract leading to reduced productivity or starvation and death.

Chemical Leaching

It is proven that microplastic and plastic in general has the ability to adsorb and desorb chemicals that have toxicological responses in invertebrates  and vertebrates , but often these studies are performed using high concentrations of microplastics, or high concentrations of chemical sorbed onto the microplastics being administered that may not accurately show processes that occur in nature.

Toxicity modeling

As information is gained through research and experimentation, models can be developed that predict toxic effects due to microplastic presence in the environment. The main types of information needed to do this includes needing to know relative abundances of the microplastic and the ability for the microplastic to adsorb, carry and then desorb a toxic chemical to affect a given organism.

The likelihood for a chemical to adsorb or desorb; move between different phases in the environment (such as between plastics and an organism), can be described through equilibrium partitioning which consists of finding the equilibrium constant K as a ratio of concentrations between two phases at equilibrium.

For chemicals associated with plastics, the equilibrium partitioning equation would look like:

Kpl=[Cpl]/[Cw]

Where Kpl is the equilibrium constant and [Cpl] is the concentration of the chemical within the plastic and [Cw] is the concentration of the chemical in the ambient water.

In a synthesis paper by Koelmans et al, plastics were shown to be a relatively unimportant source for HOCs when compared with other media pathways based on approximate age distributions of plastics worldwide and their desorption rates. The authors however caution that future continued production and pollution of plastic will change the age distribution and thus the equilibrium constant that they used and could change the balance as additional plasticizers and additives are introduced. The same authors also concluded that the danger of bioaccumulation of HOCs from plastics was likewise negligible compared to other natural pathways of ingestion assuming a "realistic" level of microplastic ingestion.

Future Studies

As distribution studies confirm the ubiquity of microplastics across the world, scientists call for increased studies investigating environmentally relevant long term effect studies with multiple species. As the effects of microplastics are better understood in realistic levels, proper regulations and actions can then be recommended to prioritize mitigation and regulation policies.