Microplastics are the subject of increasing discussion and worry for environmental and health-related reasons. Hardly a day goes by without hearing anything on the topic. Just a few days ago, they have made the news again after tons of microplastic pellets washed ashore in Sri Lanka following a naval accident. But some of us might be wondering: what are microplastics exactly, and why are they a concern? Why are we talking so much about this stuff?
Plastic, a revolutionary and ubiquitous material
Let’s take a step back. Everyone knows about plastic, right? Plastics are synthetic or semi-synthetic materials based on long-chain organic polymers, which are produced by combining chemical monomers, often derived from fossil fuels. The first synthetic plastic, Bakelite, was invented in 1907; it was used mostly as an insulating material in electrical systems and to make those wonderful vintage radios. Since then, dozens of different types of plastics have been produced, such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene. They are used everywhere: construction, health care, automotive, commodities, home- and personal care, electronic devices, packaging, aviation… it’s actually pretty hard to name an industry that makes no use of plastic.
Polymers represent the matrix of the plastic materials, but these may also contain chemical additives including (but not limited to) pigments, heavy metals, and nanoparticles. For example, some additives are meant to act as flame retardants, to provide resistance to degradation by UV-light or heat, to modify the mechanical properties of the polymer, and to induce antibacterial or anti-static properties. Besides these additives, which are introduced voluntarily into the materials, plastics may contain unreacted monomers and, despite the best efforts, degradation products.
When plastics are micro
So, microplastics. As the name suggests, this word refers to microscopic objects made of plastic. The term itself was coined in 2004, but it wasn’t until 2009 that the upper size limit of 5 mm was proposed to better define these particles. As for a lower limit, this has been quite obscure for a long time, but it could be placed at 1 μm since French researchers proposed, in 2018, that all particles with diameters below this size be termed – more appropriately – nanoplastics.
In 2011, two categories of microplastics were established according to their origin:
• primary microplastics are produced in microscopic dimensions, such as microbeads for personal care products, industrial abrasives, cleaning agents, coatings, and paints;
• secondary microplastics result from the degradation and fragmentation of larger objects taking place in the environment, either during use or after disposal; this is the more abundant category.
Microplastics have been found everywhere in the globe: on the ocean’s surface as well as in deeper waters, soils, air, and biological systems, even in presumably pristine locales, including Arctic Sea ice, the Antarctic, remote mountain ranges, and deep ocean trenches. Because plastic is a man-made material, its presence in ecosystems where it’s not supposed to be and where it can possibly cause harm to living organisms pretty much earns it the definition of polluting agent.
How did this stuff reach so far and out?
Typically, plastics are manufactured and disposed of on soils. Here, they are subjected to exposure to UV light and heat and to abrasion, which can lead to degradation and fragmentation. At this stage, plastic fragments can penetrate the soil with the help of water infiltration. This can take place at landfill sites, but microplastics can also enter the environment via loss from industrial facilities (both at production sites and during shipping) and agricultural gear. Another substantial source of microplastic contamination in soils is represented by the abrasion of vehicle tires and roadway marking paint. From the soil, microplastics can easily reach groundwaters and freshwater bodies under the action of rainfalls.
In some areas, plastic trash is burned under poorly controlled conditions, releasing microplastics and other particulates into the air. Wildfires are another contribution to the phenomenon, as they can engulf homes, businesses, and vehicles that contain large amounts of plastic materials. The fragments are then transported far from the origin location via air currents, reaching the oceans.
The introduction of plastics in surface waters can take place directly via fishing and aquaculture (nets, lines, floats, traps…), intentional disposal from vessels, and debris produced during storms at sea. Then, there are some quite idiotic things that humans have been doing: it’s estimated that, from 1970 to 1990, approximately 1 million waste automobile tires have been sunk in coastal zones, sometimes chipped and placed in mesh bags. However, according to the International Union for the Conservation of Nature, the most abundant source of microplastics in water bodies is represented by the laundry of synthetic textiles, contributing 35% of the world ocean’s microplastic content. Just think of this: the average domestic wash load of acrylic fabric can release approximately 700,000 fibers.
The analysis of microplastics observed in surface waters has shown that the most abundant polymers are polyethylene and polypropylene, which can float owing to their low density. These are commonly employed in the manufacturing of single‐use products. Denser particles are found deeper in the water column (e.g., polyester, polyamide, and acrylics), especially in the form of fibers. These generally derive from textiles, rope, and netting.
Micro plastic, macro danger?
Now, this wouldn’t be such a big deal if it weren’t for one prominent feature of plastic materials: resistance to biodegradation. This means that they can persist in the environment for an extremely long time. For example, a study on textile fibers has shown that, after 243 days, the rate of biodegradation was 76% for cotton, 62% for rayon, 40% for polyester/cotton, and only 4% for polyester. Depending on the polymer, a plastic object could remain in the environment for even up to 500 years.
Thanks to their porosity, plastics are ideal substrates for the adsorption of organic and inorganic materials, microorganisms, algae, and invertebrates in aqueous environments, so much that the term “plastisphere” has been coined to describe this novel habitat. The interaction of organisms with plastics and their additives can lead to various biological effects. Aquatic organisms, from plankton to shellfish to whales, may ingest microplastics particles, either by accident or upon sensory reaction (i.e., the plastic particles are perceived as prey), with possible consequences on their health. The study of marine specimens has shown the presence of microplastics in their digestive tracts. Once ingested, plastics may either be expelled as such or accumulate in the body, causing digestive system blockage or preventing consumption of sufficient quantities of regular food, thereby leading to starvation and death. Other possible effects are tissue damage, behavioral changes, disruption of development and reproduction, and immune response.
Particular concern is raised by nanoplastics: these are small enough to present colloidal properties when found in water, that is, rather than floating or sinking, they remain dispersed through a water column, where aquatic organisms can more easily consume them. In addition, these particles have a suitable size to move through tissues and enter cells in animals.
How does this affect us?
There are several modes of exposure to microplastics for humans, and eating contaminated seafood is only one of those. For example, the consumption of bottled water has been proven to result in the ingestion of 40 times more plastic particles compared to tap water.
Studies have shown that the concentration of microplastics in the air of homes and offices can exceed that measured outdoors 40-fold. This is a consequence of insulating buildings with plastics such as polystyrene. Considering that citizens of developed countries reside indoors for over 90% of their lives, the risk of inhalation appears significant. Another source of microplastic intake is through plastic containers used for food storage and reheating. Everyone has thawed a frozen lunch in a microwave at least once. Well, it appears that plastic containers exposed to such high temperatures can shed huge numbers of microscopic plastic fragments, as do kettles and baby bottles.
The effects on human health are not clear yet; however, the possibilities are not so different from those that concern other animals, as described above. Moreover, some plastic polymers are not so inert as they might appear. For example, the monomer of polystyrene, styrene, is extremely harmful and suspected to be toxic to reproduction.
What are we doing about it?
Some plastic can be recycled, and most developed countries have put in place systems to separate plastic from other trash and deliver it to recycling plants for processing. However, recycling comes with an economical cost and is a slow process, while the use and production of plastic keep increasing each year. For this reason, considerable research effort is being directed toward designing and producing biodegradable plastic materials. Among the most promising materials, we can name starch blends, polylactic acid, and polycaprolactone. Not all of the candidates are entirely of natural origin, though; some are still produced from fossil material.
In conclusion, a valid alternative to the plastic we know is clearly not yet available. What we can do, for the time being, is act to change our lifestyles little by little each day, for example by recycling carefully, refusing to buy products enveloped in excessive packaging, and reusing as many plastic objects as possible – the latter is becoming even easier since some countries are beginning to ban single-use plastic products. Hopefully, science will provide answers and a better outlook in the next few years. Before then, we better start using refillable bottles and ceramic plates. It has worked well for thousands of years, after all.
MM
References
Lim, XiaoZhi. “Microplastics are everywhere—but are they harmful?.” Nature 593, 22-25 (2021)