Microplastic pollution is growing: What's the impact?

BY MARIA BURKE
Image: Macro photograph of microplastics floating in the ocean

Microplastics pollution is growing, with some studies estimating we each ingest 5g of microplastic particles each week on average^[1]. So, what are the consequences? Maria Burke reports

Microplastics – particles smaller than 5mm – form when plastic degrades. They are also found as microbeads, for example, in face washes or as fine powder in toothpaste.

‘Over the past few decades, microplastics have been found in the ocean, in animals and plants, in tap water and bottled water,’ says Eliseo Castillo of in the University of New Mexico School of Medicine, US. ‘They appear to be everywhere.’

However, research on the health effects of microplastics, especially in mammals, is still limited. Castillo is concerned about their accumulation in the human body. His team focuses on what microplastics are doing to the gastrointestinal tract and to the gut immune system. In a recent study, they exposed mice to microplastics in drinking water at amounts equivalent to quantities humans ingest each week. They found microplastics crossing the intestinal barrier and infiltrating other tissues in the liver, kidney and brain^[2]. They also showed changes in metabolic pathways in the affected tissues.

‘These mice were exposed for four weeks. Now, think about how that equates to humans, exposed from birth to old age,’ Castillo says. ‘Could microplastic exposure exacerbate an underlying condition?’

Jaime Ross’ team at the University of Rhode Island, US, has also found microparticles accumulating in multiple organs, including the brain, in mice exposed to microplastics in drinking water for three weeks^[3]. The mice began to move and behave oddly, showing similar behaviours to dementia in humans. The results were even more pronounced in older animals.

‘To us, this was striking,’ says Ross. ‘These were not high doses of microplastics, but in only a short period of time, we saw these changes.’

Ross and her team are interested in the effects of microplastics as we get older. Are we more susceptible to systemic inflammation? Can our bodies remove them as easily? Do our cells respond differently to these toxins?

‘The detection of microplastics in eg the heart and lungs suggests they are going beyond the digestive system and likely undergoing systemic circulation,’ she continues. ‘The brain blood barrier is very difficult to permeate. It is a protective mechanism against viruses and bacteria, yet these particles were able to get in there. [They were] deep in the brain tissue.’

Numerous studies focus on consuming microplastics by drinking and eating. But University of Birmingham, UK, researchers presented what they claim is the first experimental evidence that additives present in microplastics can leach into human sweat and be absorbed through the skin[4]. The team exposed 3D human skin models over 24 hours to two common forms of microplastics containing polybrominated diphenyl ethers (PBDEs), used as a flame retardant. They found as much as 8% of the chemical could be taken up by the skin, with more hydrated (sweatier) skin absorbing higher levels.

‘Our research shows microplastics play a role as ‘carriers’ of harmful chemicals, which can get into our bloodstream through the skin,’ says lead researcher Ovokeroye Abafe, now at Brunel University, UK. ‘These chemicals are persistent, so with continuous or regular exposure, there will be a gradual accumulation to the point they start to cause harm.’

Marine exposure

Every year, 4.8-12.7m t of plastics flow into the marine environment, meaning 82–358 trillion plastic particles, primarily microplastics, were afloat in 2019^[5]. Numerous studies have shown microplastics are entering marine ecosystems but it’s still not clear what effect they are having on wildlife. Some estimates suggest a filter-feeding blue whale may consume 10m pieces of microplastic and a fish-feeding humpback whale 200,000 pieces/day, based on known concentrations off California^[6]. Most microplastic probably passes through their gut, but some also ends up in tissues. A Duke University, US, study found microscopic plastic particles (ranging from 198µm to 537µm in size) embedded in the fats and lungs of two-thirds of the marine mammals in a sample of 32 from Alaska, California and North Carolina^[7]. Polyester fibres were the most common, followed by polyethylene.

‘They’re not only ingesting plastic and contending with the big pieces in their stomachs, they’re also being internalised,’ says Greg Merrill of Duke University. ‘Some proportion of their mass is now plastic.’

Animals could also be inhaling as well as swallowing microplastics. Miranda Dziobak and a team from the College of Charleston in South Carolina, US, studied 11 wild bottlenose dolphins and found they had at least one microplastic particle in their breath. Both fibres and fragments were present, including several types of plastic polymers, such as polyethylene terephthalate (PET), polyesters, polyamides, polybutylene terephthalate and poly(methyl methacrylate)^[8].

‘We know that microplastics are floating around in the air, so we suspected we would find microplastics in breath,’ says Dziobak. ‘Dolphins have a large lung capacity and take really deep breaths, so we are worried about what these plastics could be doing to their lungs.’

Researchers are also finding microplastics in corals. A study on coral growing in reefs off Thailand, for example, has found microplastics in their surface mucus, tissue, and skeleton. The team from the Center for Ocean Plastic Studies in Thailand found 174 microplastic particles, mostly ranging 101-200μm in size^[9]. The most prevalent plastics were nylon (20%), polyacetylene (14%), and PET (10%).

While the health effects of microplastics on coral and the larger reef community are not yet known, these findings may explain the ‘missing plastic problem’ that has puzzled scientists: about 70% of plastic litter entering the oceans cannot be found. Could coral be acting as a sink?

‘Since coral skeletons remain intact after they die, these deposited microplastics can potentially be preserved for hundreds of years,’ says Suppakarn Jandang of Kyushu University, Japan.

Removal techniques

Not surprisingly, researchers are devising ways to remove microplastics from the environment. Perhaps one of the simplest methods was reported by researchers in China, who found that boiling hard tap water containing 300mg of CaCO₃ (limescale)/L of water for five minutes removed up to 90% of free-floating nano- and microparticles (NMPs)^[10]. Boiling soft water samples (less than 60mg CaCO₃/l) removed around 25% of NMPs.

As the water temperature increases, CaCO₃ forms crystalline structures, which encapsulate plastic particles. The ‘incrustants’ would build up like typical limescale, at which point they could be scrubbed away to remove microparticles. Any remaining incrustants floating in the water could be removed, for example, by pouring it through a coffee filter.

A more complex solution being pursued by a team at Brno University, Czech Republic, relies on smart swimming micromachines about 2.8µm in diameter. They consist of strands of a positively charged polymer linked to magnetic microbeads. When a magnetic field is applied, the functionalised beads self-assemble into rotating planes of different dimensions. They ‘swarm’ together and capture dispersed microplastics and free-swimming bacteria with their polymer arms. By adjusting the number that self-organise into flat clusters, the researchers could alter the swarm’s movement and speed.

In tests, a robot concentration of 7.5mg/ml captured approximately 80% of the bacteria and gradually reduced the number of free microplastics particles^[11]. Afterwards, the robots were decontaminated and reused. The team says ‘dynamic capture’ is more efficient than using static polymeric particles while integrating a cationic polymer onto the superparamagnetic beads enhances the attraction for Gram-negative bacteria and microplastics.

Meanwhile, researchers at the University of Missouri-Columbia, US, have created a liquid-based solution capable of removing almost all microparticles from water in lab tests.

‘Our strategy uses a small amount of designer solvent to absorb plastic particles from a large volume of water,’ says team leader Gary Baker.

The team tested a range of solvents including tetrabutylammonium bromide mixed with decanoic acid, tetraoctylammonium bromide with decanoic acid and thymol with menthol. The solvents extracted on average 98% of the polystyrene beads standing in for microplastics in a single pass across a range of sizes from 100 to 1000nm^[12].

Initially, the water-repelling solvent floats on the water’s surface. Once mixed with water and allowed to reseparate, the solvent floats back to the surface, carrying the nanoplastics within its molecular structure. Baker says future studies will work to scale up the process.

Other approaches have turned to the biological world. Previous research found that insects eat and absorb pure, unrefined microplastics, but only under unrealistic, food-scarce situations. Zoologists at the University of British Columbia, Canada, have now tested mealworms in a more realistic scenario, feeding them ground-up face masks mixed with bran. After 30 days, the team found the mealworms ate about half the microplastics, about 150 particles per insect, and gained weight^[13]. They excreted a small fraction of the microplastics consumed, about four to six particles/mg of waste. Eating microplastics did not appear to affect the insects’ survival and growth.

Microplastics and farming

Microplastics are also finding their way into agricultural soils from plastic mulching films and from sewage sludge applied as fertiliser, which can contain microplastics from synthetic fibres and waste. The concern is that microplastics on farmland will eventually be transported into watercourses through surface water run-off or infiltration to groundwater.

Cardiff University, UK, researchers estimated that between 86-710 trillion microplastic particles are deposited onto European soils annually. Their analysis revealed that each gram of sewage sludge from a treatment plant in South Wales contained up to 24 microplastic particles, roughly 1% of its weight^[14].

‘Our results highlight the magnitude of the problem across European soils and suggest the practice of spreading sludge on agricultural land could potentially make them one of the largest global reservoirs of microplastic pollution,’ says James Lofty of Cardiff University. ‘At present, there is currently no European legislation that limits or controls microplastic input into recycled sewage sludge.’

Many researchers are looking at the impact of microplastics on soil. A recent review found plastics can alter soil texture, structure, bulk density, water aggregate stability, water holding capacity, and rainwater infiltration; and also influence pH levels, electrical conductivity, nutrient cycling and enzyme activity^[15]. This will have far-reaching implications for ecosystem health and agricultural productivity.

Another review by researchers from Australia and Bangladesh concludes microplastics in soil disrupt plant growth, development, and physiological function^[16]. For example, they can interfere with root systems, limiting the plant’s ability to absorb water and nutrients. They also hinder photosynthesis by reducing chlorophyll content, leading to lower biomass accumulation and reduced crop output.

Other effects include oxidative stress, which leaves plants more vulnerable to environmental stressors such as drought or pests. Microplastics can also carry harmful substances, such as heavy metals and persistent organic pollutants, which bind to soil particles and can be absorbed by plants.

Meanwhile, microplastics also impact soil fauna. One study, for example, found reduced growth and reproduction rates in earthworms. Microplastics also affect the earthworm’s ability to burrow, impacting soil aeration, organic matter decomposition and nutrient cycling.

Prolonged plastic contamination may also change the metabolism of microbes, reducing how efficiently they decompose organic matter and recycle nutrients. One result of this is increased emissions of CO₂ and N₂O^[17]. Microbes hydrolyse plastic materials, producing CO₂ in the process. The magnitude of the effect depends on microparticle material, size and concentration, as well as soil type.

Nitrous oxide is mainly released through denitrification where nitrates are gradually reduced. As microparticles alter soil structure, this could potentially increase soil porosity and facilitate N₂O diffusion to oxygen-containing pores, enhancing its release into the atmosphere rather than reducing the nitrates completely to N₂.

References

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