Microplastics: An Overview

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Author: Dr. Joe Ackerman

As you may already be aware, we are making microplastics (MP) and they are distributing themselves everywhere. The source is from us, from human activity, just living our normal life, because our normal life includes so much plastic. Plastic wears down into smaller and smaller pieces and these bits move far and wide. We have spent the last century making better and better plastics, so that now they last a very long time, and they are so cheap to produce that they are replacing traditional materials such as wood, paper and steel. All of our activities produce MPs: strands from clothing, curtains, or carpet; minute pieces from normal wear of cooking utensils; dust from car tires while breaking hard or turning corners; gradual wear on vinyl flooring; or shredded fragments from outdoor plastic litter eroding in the sun and wind. The size of microplastic range from easy to see 5 mm chunks, to effectively invisible microns (µm) and nanometers (nm). And they are everywhere.

These tiny pieces are turning up in every corner of the planet: Alpine and Arctic snow, in every ocean and lake, each sediment and soil, in every bottle of water and foodstuff, in fact, every place we care to measure, we find them. So just as we have surrounded ourselves with plastic, we have inadvertently surrounded ourselves with microplastic: they are part of our daily air intake and diet and possibly present in our organs and vascular system. Yet we don’t know very much about them and thus we don’t know if we should be worried. Should we be worried?

Smaller than 500 µm, microplastics look like organisms, algae or bacterial flocks and are consumed by zooplankton and worms. Smaller still, they are the same size as bacteria (<10 µm) or even viruses (<0.3 µ m) and we don’t know what they do when consumed by organisms.

Joe Ackerman

Size of Microplastics Matters

Plastic is supposed to be inert and non-reactive, or at least that is what we have assumed up till now. If we follow a shred of microplastic on it journey of dispersion, it will likely end up in the water. Half the polymer resin types we use have a density similar to water, so they either float or are suspended in the water column. There they are sometimes mistaken for food by aquatic organisms. In the size range of < 5 mm, they are eaten by fish and filter feeders and can cause choking, blockages or just fill the intestines, causing pseudo satiation and starving the host. Smaller than 0.5 mm (500 µm) they look like organisms, algae or bacterial flocks and are consumed by zooplankton and worms. Smaller still, they are the same size as bacteria (<10 µm) or even viruses (<0.3 µ m) and we don’t know what they do when consumed by organisms. The bulk is passed out quickly in most cases, but in the nm size range their lipophilic properties allow transport across membranes and their presence has been detected in the bloodstream and tissue structure of their hosts.

The risk of microplastics as a vector for contamination from the environment to organism is low because fugacity constants (tendency for a substance to gravitate to one medium or another) favor fat to plastic.

Joe Ackerman

When plastic starts associating with the organelles in your cells and embedded in your muscle tissue, you probably want to know what effect it is having. Is it occurring randomly there, or does it accumulate? Do the plasticizers, stabilizers, colors etc. leach out over time? Does the lipophilic nature of the polymer attract and concentrate persistent organic pollutants already in the environment? Do these particles or these chemicals react/interact with normal cellular processes? This is where the mystery gets stronger, because it is such a new science we have only just begun to learn to filter, separate, analyze and identify these particles, let alone answer these more complex questions.

Worries of a Scientist

From a scientist’s perspective, microplastics present a great challenge. How do you collect something that is mixed with ordinary house dust, soil, or, for aquatic sampling, evenly mixed with the plankton, diatoms and detritus in your net sample. The particles will no doubt be covered with biofilm, perhaps inorganic precipitates, perhaps the surface will be etched or oxidized from weathering or UV attack. This prevents you from comparing it with the virgin material without some kind of digestion or solvent treatment (taking care not to melt or dissolve the plastic in the process). Once you separate out the mineral bits and organic goo, and then decide how fine a filter you are passing the remains through, how do you measure something so small? How do you identify the resin type? How can you be sure that the fragment or strand (dark blue, let’s say) is polyester and not cotton or wool? How do you know the clear flake is low density polyethylene and not skin or scale or chitin?

Existing analytical methods cannot differentiate between a natural polymer and synthetic polymer. This difficulty in differentiation increases with decreasing particle size of the plastics.

Joe Ackerman

You must either run consecutive density tests (float/sink tests, requiring pieces large enough to pick up) or isolate the tiny pieces and run spectrophotometry on them. Spectrophotometry work is basically seeing how the molecular bonds are absorbing different wavelengths and from that you can make guesses if it is a natural polymer (e.g., protein, cellulose, chitin) or a synthetic one (e.g., polypropylene, polystyrene, polyester). As the particle size decreases, the noise to signal ratio increases, lowering certainty of positive identification. Those surface attacks by UV light, biofilm buildup, and mineral precipitation also confuse the absorption spectra and reduce the certainty of resin type when compared with a virgin standard.

Plankton ingesting microplastics.
Lack of Standardized Methods

Microplastics are a new problem and there is much to learn about occurrences, sizes, resin types, sources, transfer from one medium to another and effects. Sampling methods are varied and not yet standardized. Much of the earlier work was done in aquatic environments and floating fibers and particles were by-catch in large (330 µm mesh plankton nets). The same waters will give very different MP counts when resampled with 50 and 10 µm mesh nets. When these methods are standardized, surveys can be compared. Also, most studies record the number of MPs with no conversion to mass. This then prohibits comparison with Health and Safety regulations (for airborne particulates, for example) which are normally recorded in mass/volume. If size is a related to risk to organisms, then samples need to be quantitatively grouped in this way. Digestion and analysis methods need to be standardized. The list goes on.

It appears that the majority of the microplastics that are ingested by an organism are expelled soon after ingestion.

Joe Ackerman
piles of garbage by the shore
Waste plastics are among sources of microplastics. Photo by Lucien Wanda on Pexels.com
Long-Lived Organisms (like us) Must be Monitored

It appears that the majority of MP that are ingested by an organism are expelled soon after ingestion and some research has been conducted suggesting current densities of MP within a water column do not significantly impair growth or development, although risk of doing so increases with density. Meta reviews of literature have noted laboratory work is usually with a narrow range of organisms at concentrations by in excess of any currently found in the environment. This means that in most cases, eliciting a negative impact on an organism in the lab means dosing them with ten thousand to a million times the concentration ever encountered in nature. Also, risk of MP as a vector for contamination from the environment to organism is low because fugacity constants (tendency for a substance to gravitate to one medium or another) favor fat to plastic.  In order for MPs to transfer organic pollutants to a host, the concentration must be exceedingly high. At these levels, the lipophilic compounds will contaminate the fat in the food of the organism and be delivered that way, not via MP. Leaching of additives is also low risk due to the short retention time and life span of the organism. Long-lived organisms (like us) must be monitored.

Most of all, efforts need to be focused on reducing the production and proliferation of plastic in our society.

Joe Ackerman
Conclusion

Microplastics present a problem that has worsened with the increase of plastic use in our lives. There is good evidence that the presence of plastic debris in surface waters is harming innocent creatures because the plastic is similar in appearance to their food. This new science needs improvement and harmonization of sampling and analysis techniques that enable comparison of studies and routine inexpensive monitoring of this emerging problem. Most of all, efforts need to be focused on reducing the production and proliferation of plastic in our society. If not, the extremely high concentrations identified in the lab as harmful to organisms will become a reality in the natural world, with untold impacts on multiple species and across ecosystems.  On the social and legal front, there is a strong need to begin limiting plastic production, because its fate is eventually MP. Densities can only get worse and ill effects already detected will only increase as global plastic production and its presence in our life increases.


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