Look at the picture above. It shows a piece of plastic on a penny. The diameter of the coin is 1.9 cm. This tiny piece of plastic is called a microplastic, which is defined as a piece of plastic less than 5mm in size (Barnes et al. 2009). More than 51 trillion pieces of microplastic are currently floating all over the ocean (van Sebille et al. 2015). This number is 500 times greater than the number of stars in our galaxy (UNEP Newscentre Feb 23 2017). These tiny plastics were first found in the 1970s, and the word “microplastics”has been often used since a study titled Lost at Sea: Where Is All the Plastic? was published in Science in 2004 (Thompson et al. 2004).
What is the definition of microplastic?
Generally speaking, it is defined as a piece of plastic less than 5mm in size. Some studies categorize it as being less than 2 mm or even less than 1mm. Nonetheless, plastic that is smaller than 5mm is regarded as microplastic in most studies.
Where do these microplastics come from?
Regarding their original sources, they can be divided into two groups (Arthur et al. 2009). The first are the primary microplastics; these have been tiny, smaller than 5 mm, since they were produced. The other are the secondary microplastics; these have decayed and crumbled from larger plastic products (GESAMP 2015, Thompson 2015, RIVM 2014).
Primary microplastics include microbeads, which are contained in personal care products, such as facial wash, toothpastes and body shampoo for cleansing and exfoliating (Fendall & Sewell 2009, Rochman et al. 2015, Leslie 2015). When we use these personal care products containing microbeads, they drain into the sewage system and some of them, not all, slip through the sewage treatment plant and enter the ocean.
Surprisingly, an estiamted 263 tons of polyethylene microbeads drain into the sewage system in the U.S. in a year (Napper et al. 2015). Only a few countries have highly specialized sewage treatment plants which can capture the microbeads. Most countries allow microbeads to slip through their plant and into the ocean (Hammer et al. 2012, Duis & Coors 2016).
Besides personal care products, microbeads used in plastic paints and sandblasting are also a source of primary microplastics (Sundt et al. 2014).
Please take a look at the seashore in your area. Among the large amount of plastic pieces, you will find tiny, round and even pretty looking plastic debris. These are called resin pellets. Resin pellets are the intermediate raw material that is used when plastic products are moulded or processed (Browne et al. 2011). Their size is between 2-6mm, so this is a rather large primary microplastic. Huge amounts of resin pellets are used to produce plastics products. For example, 22,000 pellets are used to produce just a 277g (half pound) of high-density polyethylene, which plastic pails are made of (USEPA 1992). In the U.S., approximately 27 billion kg of resin pellets are produced annually (Elias 2017).
Some of the resin pellets are dropped or spilled during the production or delivery of plastic products. They can be blown away by rain and winds and are finally carried to the ocean. Resin pellets are found in the sea and on seashores all over the world. At a beach in New Zealand, it was reported that more than 100,000 resin pellets were found in every 1 m3 along the seashore (Gregory 1977).
The chips that form during the trimming or processing of plastics are also a source of primary microplastic. These are often managed ineptly and spill out into the environment.
However, most microplastics found in the ocean are secondary microplastics, which are weathered plastic products (Barnes et al. 2009).
Most of the plastic waste drifting in the ocean is exposed to UV light from the sun in addition to extreme heat. It becomes fragile and is decomposed by the effect of photodegradation and thermal-oxidazation (Andrady 2017). Eventually, it crumbles into pieces. Then, it becomes smaller and smaller through physical friction, such as the collision with other pieces of plastic, wave action, rocks and sand, etc. In the case of the plastic that is drifting on the surface of the ocean, the portion above the water is exposed to the sun and heat so that its decomposition is faster than the other portion that is underwater (Halle et al. 2016). For the plastics that have sunk into the deep, cold ocean, where no light penetrates, there is hardly any decomposition (Muthukumar et al. 2011).
Also, plastic on the beach is generally decomposed and turned into microplastic faster than plastic drifting in the sea (Andrady 2011). The reason for this is that the temperature on the beach is higher than in the water. Furthermore, in the water, various kinds of living creatures, such as algae and barnacles, stick to plastic. These block the UV light, keeping it from reaching the plastic and further slowing its decomposition (Weinstein et al. 2016). On the other hand, as nothing sticks to the surface of plastics on the beach, these are fully exposed to UV light and decompose quickly. From this point of view, picking up plastic waste on the beach helps prevent it from turning into microplastic. It is impossible to retrieve it once it has become microplastic.
Secondary microplastics can also form on land. For example, the plastic sheets that cover the roots of vegetables on a farm become tattered and decomposed by being exposed to the sun for a long time. The formed microplastics are blown away by the wind, drained by the rain, and carried to the ocean (Kyrikou & Briassoulis 2007).
The chips from tires can also turn into microplastics: car tires are made from a synthetic polymer and when the cars run on the road, tiny chips form (UNEP & GRID-Arendal 2016). Some reports state that 18% of marine plastic waste is estimated to come from such tire chips (Green Alliance).
When fishing gear, which is generally made of plastic, is discarded into the sea, it crumbles and decomposes and turns into fibrous microplastics. These are called microplastic fibres, and they are also regarded as secondary microplastics. Microplastic fibers are also created when we wash our clothes, particularly when we wash and dry chemical fiber clothes such as a fleece (Fendall & Sewell 2009, Browne et al. 2011). These are blown away by the wind or drained through the sewage system and reached the ocean. According to a study conducted in Paris, 33% of the dust in the air was synthetic fiber. This, astonishingly, is estimated to amount to 3-10 tons of synthetic fiber in 2,500 ㎢ (Dris et al. 2017).
In the U.S., 80% of households use dryers. Therefore, a huge amount of microplastics are entering the air, and this is one of the reasons for microplastic fibers mixing with tap water (Tyree & Morrison).
Some claim that synthetic fiber waste from washers and dryers should be treated as primary microplastics since they enter the ocean via the same route as microbeads, which are primary microplastics (Sundt et al. 2014).
Microplastics ride on the currents and swirl around in every ocean in the world. In the shallow sea, in the deep sea, in the icebergs of the Arctic, on the seashore of a desert island in the middle of the Pacific Ocean that is s far away from a big city as possible, they are found in all of the oceans (Andrady 2017). We can say that microplastics are always there when it come to the sea.
Once microplastics have entered the ocean, there is no way to remove them. Unlike plastic waste, microplastics cannot be retrieved, no matter how much we spend. Regarding primary microplastics, we can track down how much and from where they are made, and the attempt to reduce their use has already begun (GESAMP 2015).
But, as for secondary microplastics, of which there is much more than primary microplastics, there is no way to know where and how much are being made, and we are at a complete loss as to how to deal with them (Andrady 2017).