In the past few years, microplastics— small pieces of plastic up to 5 millimeters in size—have been getting a lot of attention, but not all studies are created equal.
As scientists, we need to strengthen our research to better understand the problem and its possible solutions.
Inaccurate sampling
Credit: Florida Sea Grant
Microplastics come from a variety of sources and have various shapes, colors, sizes, and chemical make-up.
Microbeads, for example, are tiny spheres put in products such as body washes and toothpaste for abrasion.
Lentil-sized pieces called nurdles are used to manufacture larger plastic products. Other microplastics come from fragmentation of larger pieces of plastic.
Every week new articles on microplastics are published in scientific journals, but not all of them are original or important. Many papers report the number of microplastic particles found in some water body.
This may be of interest to local residents and politicians, but these studies can be inaccurate, and difficult to compare with each other because there are no standardized methods for collection or analysis.
With all of this sampling, a few things have become clear: First, sampling for microplastics with nets misses most of them, since long thin microfibers, which are by far the most abundant type when whole water samples are analyzed, tend to pass through nets. These microfibers are derived primarily from synthetic clothing, which sheds them into wastewater when laundered with washing machines.
Second, not all microplastics float near the surface where the nets are towed; denser plastics sink.
Third, once microplastics are collected, some investigators count them under a microscope, which has a high error rate. Others use sophisticated chemical analytic equipment, which can identify the chemical nature of the plastic.
If we want accurate counts, we should collect whole water samples at different depths, and use chemical analytic equipment.
Attraction and consumption
Another common kind of study documents that some species of fish or plankton or crab eats microplastics. It seems that every animal studied eats them.
It would be of greater interest to find out WHY they eat them – some animals appear attracted by the odor and others by an appearance resembling their natural food.
Most lab studies of animals eating microplastics use tiny spheres, since these are available commercially. However, microspheres are rare in the environment – odd shapes and microfibers are much more common. Research using the types of microplastics that are common in the wild would be more useful.
Although animals eat them, few studies have examined what percentage are eliminated from the animals and how rapidly.
Effects attributed to microplastics may actually be symptoms of inadequate nutrition or a clogged digestive tract. Many may pass through the gut without causing any noticeable effects. The shape may be critical – spheres can probably pass through more easily than odd-shaped pieces, which, with sharp edges, might injure tissues, or fibers, which might clog up the gut.
A recent study found that while a shrimp species defecate spheres, they regurgitate fibers. So, there is more than one way to eliminate plastic that has been eaten.
Future studies of feeding and transfer up the food web should use primarily microfibers, provide real food, and allow time for elimination to occur.
How toxics tag on
Credit: Oregon Sea Grant
Microplastics attract toxic chemicals, such as PCBs and pesticides, from the environment, which bind to the plastics.
The plastics are considered a "vector" for transferring contaminants to animals and up the food chain.
While this transfer of chemicals has been shown in a few studies, it has not often been demonstrated with realistic scenarios such as giving the animals "real" food and time to eliminate the microplastics along with their attached chemicals.
It is important to learn how much of the attached contaminants the gut enzymes can pull off during the time that microplastics are passing through. This will depend on the length and complexity of the digestive system of the species, as well as particular chemicals.
Since microplastics also are found in gills, scientists should investigate the respiratory route of exposure and the uptake of chemicals from gills.
We have learned that microplastics are common in the air and soil, so research on effects on soil and terrestrial organisms should be expanded.
In order to reduce the flow of microplastics from clothing into the environment, studies have investigated which types of washing machines, synthetic fabrics, and washing methods release the fewest microfibers.
At the root of the problem, we need chemical engineers and textile scientists to discover how to modify the manufacture of textiles so that they will shed fewer microfibers.
We've come a long way in the past decade of microplastic research. But to advance the science and recommend meaningful solutions, researchers should focus on quality over quantity and use methods that most closely mimic real world scenarios.
Judith Weis is a Professor Emerita of Biological Sciences at Rutgers University, Newark. Weis recently published a paper in the journal Environmental Research entitled, "Improving microplastic research," which outlines the problems and potential solutions in microplastic studies.
