What do we really know about microplastics and human health?
Plastic pollution is a grave and growing concern, but the biggest problems that it poses may just come from its smallest forms. Tiny fragments of the stuff are now strewn across the entire globe and are beginning to show up in different parts of the human body. But what are the health risks associated with ingesting and inhaling this now omnipresent synthetic material? While early studies do offer some cause for concern, they also show there is much still to learn before we truly start to sound the alarm.
Microplastics are smaller pieces of plastic measuring less than 5 mm in size (0.2 in), and can wash into the environment in ready-made form, such as the microbeads used in cosmetic products or the tiny fibers used in synthetic textiles. Alternatively, microplastics can come via the breakdown of larger plastic products in the environment, such as plastic bags or soda bottles that are weathered and corroded by forces like UV light and ocean currents.
Plastic, plastic, everywhere
In recent years, scientists have intensified their focus on the behavior of microplastics, exposing the many ways they move through the environment. This has led to a string of important discoveries as groundbreaking as they are concerning, showing that plastics can be driven into the deep ocean, spread through the air with the wind, drift back to Earth with snowfall in the Arctic and Antarctic, and even turn up near the summit of Mount Everest. Further, we know that disposable coffee cups release trillions of plastic particles into their liquid, and that plastic bottles can shed particles and chemicals into drinking water during use.
Meanwhile, scientists have also dialed up their efforts to understand the presence of microplastics in the human body. This includes a 2018 study that found microplastics in human stool samples all around the world, and a 2020 study that unearthed plastic particles in every human tissue it sampled. More recently, significant discoveries have shown for the first time that microplastics can exist in living lung tissue, and also that they can enter the bloodstream.
"We already knew that microplastic is present in excrements of humans, but it is toxicologically relevant to check what might be circulating in the human blood," Heather Leslie, lead author of that study, explained to New Atlas. "We went from expecting there to be microplastic in human blood to knowing it is there. It’s the first real-world evidence plastic particles are actually absorbed in the human bloodstream."
Knowing that the plastics are there is one thing, understanding the threat that they pose to our health and that of other living organisms is very much another. Given the mounting concentration of plastic in the oceans, the potential impacts on marine species seem like a good place to start, and a solid body of evidence is building around this which paints a disconcerting picture.
Studies have shown that microplastic exposure can cause aneurysms and reproductive changes in fish, impair cognitive function in hermit crabs, lead to swimming abnormalities in shrimp and weaken the grip of mussels and potentially hamper their growth. We've also seen efforts to extrapolate these kinds of findings to humans through studies that use the genetically similar fruit fly as a model, which showed that microplastics can alter gene expression associated with stress response and oxidative damage.
"Much of what we know about microplastics has come from the marine environment research and it is only more recently that human impacts have come under scrutiny," Jeanette Rotchell, principal investigator of microplastic impacts on human health at the University of Hull, told New Atlas.
Toward human impacts
Last December, scientists published research investigating the toxic effects of microplastic exposure on human cells. The first study of its kind, it built on previous work in this area such as research demonstrating that microplastics have the ability to alter the shape of human lung cells. This involved a systematic review of the available literature on the impacts of microplastics in lab-based exposure studies.
Among the toxic effects the team considered were cell death, impacts on the cell membranes, oxidative stress and characteristics akin to allergic reactions. The research showed, according to lead author Evangelos Danopoulos, that we are ingesting microplastics at "levels consistent with harmful effects on cells, which are in many cases the initiating event for health effects." Though clearly not good news, there is an important distinction to be made between what occurs in laboratory experiments, and what we know for sure to be taking place in the human body.
"The limitation in the current scientific evidence is that although we know we are regularly exposed to microplastics, we do not exactly know how they will behave inside our bodies," Danopoulos told New Atlas. "For example, we don't know how much will simply pass through and be excreted and how much will have the ability to cross the barrier of the gastrointestinal system and make their way further in our bodies. More research is needed specifically to target these unknown and crucial elements."
Research in rodents has hinted at these possibilities, such as a study last year showing that microplastics can infiltrate the blood-brain barrier in mice, pointing to potential toxic effects on mammal brains. But by and large, the question of how microplastics behave in the human body, and therefore the health risks they pose, is a great unknown. According to Rotchell, differences in the types of plastics used in these experiments, and those found in the environment, is one of the reasons why.
"In conducting the lab-based human cell and tissue culture type experiments, researchers have tended to use microplastics that are easy to source from supply companies," she told us. "They are 'virgin' in that they have not been weathered or eroded and that changes their physico-chemical properties, in turn changing their level of impact. Researchers tend to use beads and spheres, yet fragments and fibers tend to be more common in the environment and are the types likely to be ingested or inhaled. That difference in shape between realistic microplastics and the supplier bought virgin microplastics changes the impacts reported."
Filling in the blanks
As studies continue to offer a clearer picture of the shape, size and nature of microplastics polluting the environment, scientists can approach investigations on human health impacts with greater certainty. Part of this also includes new information on the concentrations of microplastics we are exposed to as we go about our lives, as demonstrated in Danopoulos' pioneering study, and as Rotchell explained.
"The early lab-based studies also tended to use levels of exposure that were very high relative to what you might be exposed to in the diet and air," she told us. "There's increasingly more data available on the levels and types and characteristics of microplastics in food and drinks and the air, so there's less excuse now for not using more environmentally-relevant levels of exposure."
The chemicals within
While research into the potential toxicity of plastic particles themselves is still in its very early stages, the chemicals they contain are known to carry possible problems of their own. An example of these are the plasticizers that are added to plastics to make them durable and flexible, with BPA perhaps the most famous example. Chemicals of this nature are linked to a range of adverse health effects in humans, with alarming damage to brain cells and elevated cholesterol and heart disease risk among the more recent findings.
"At the moment we know more about the toxicity of chemical additives that leach out of plastic materials," Leslie told us. "Microplastic might be toxic because of these additives or because of the particle getting caught up in biological processes, or a combination of the two."
As it stands
Though these kinds of early, and relatively small-scale studies have shed important light on one of the most pressing environmental issues of our time, it is far too early to draw any concrete conclusions around the health impacts of microplastic exposure.
"It’s hard for people to accept that science hasn’t delivered an answer yet, but we have to wait and see before we jump to conclusions without evidence," said Leslie.
The discoveries in this space so far do lay an important foundation for deeper investigation. To truly ascertain the risks, scientists will need to significantly scale up their studies to understand true degrees of exposure for the average person, and then determine what levels are toxic and what levels are safe. From there, we can look at ways to mitigate the risk. But for Leslie, like many other researchers in this field, the best way to mitigate the risk is by addressing it at the source.
"Clean-up measures and other 'end of pipe solutions' are costly and only come into effect after the pollution has already been spread around and done damage for a while," she told us. "It makes so much more sense to me to design the pollution out of our systems in the first place, so we can avoid having to deal with the mess."