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Plastic measures Microplastics now contaminate the food we eat and the air we breathe. The race is on to see if our health is at risk, reports Graham Lawton

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HIS morning I tried to count how many plastic objects are in my house. I got as far as the bottom drawer in my kitchen cabinet – which contained 147 assorted plastic boxes, lids, cups, straws and disposable cutlery – then gave up. I had to get to work. Good job I didn’t get down on my hands and knees with a microscope to look for really small bits of plastic, because I would never have left. By some estimates, the average household generates 6 kilograms of plastic dust every year, around 700 billion fragments known as microplastics. Like snowflakes, every one is different. Every one may also be harmful. They aren’t just indoors. “They are everywhere,” says Dick Vethaak, an environmental toxicologist at the Deltares research institute in Delft, the Netherlands. “In the water, in food, in the air – you are surrounded by a cloud of them. Everything is contaminated.” More are created every day and they will be with us for centuries. Big plastic debris has been on our radar for years. Yet this is just the start of something more insidious. Plastic waste doesn’t biodegrade but it does break down, fragmented by wind, waves and sunlight into ever-smaller pieces. They may be too small to see, but they are still there, worming their way into every nook and cranny of the environment – including our bodies. This, in a nutshell, is the pervasive problem of microplastics. But beyond knowing that they exist and are everywhere, we are woefully ignorant about them and their potential 38 | New Scientist | 7 December 2019

impact on us. That is why the search for answers is taking on a new urgency. It is widely assumed that microplastics are harmful to the environment and ongoing research suggests that this is a fair assumption. But when it comes to human health, we are flying almost blind. “It is only just very recently that we recognised that we are dealing here with a health issue,” says Vethaak. The scale of the plastic waste problem is hard to wrap your head around. In 2017, a team at the University of California published a paper called “Production, use, and fate of all plastics ever made”. It estimated that since plastics were invented, we have manufactured around 8.3 billion tonnes of the stuff. Some 5 billion tonnes of that has been dumped in landfill or discarded into the environment. To put that in perspective, the Great Pyramid of Giza is thought to weigh around 5 million tonnes. Imagine 1000 Great Pyramids made of plastic rubbish and you are getting the picture. And it keeps on coming. Every year around 4 to 12 million tonnes of plastic waste enters the marine environment. Yet it isn’t at all clear how worried we should be about this deluge. Three state-of-the-art reports published earlier this year revealed the depths of our ignorance. The first, by the SAPEA consortium of scientific academies from across Europe, reviewed all the available evidence. On the question of human health, it concluded that “little is known… and what is known is surrounded by considerable uncertainty”. That review fed into an even more

8.3

billion

Tonnes of plastic ever produced

it difficult to predict and explore the health effects,” says Stephanie Wright, who leads microplastics research at the UK Medical Research Council Centre for Environment and Health at King’s College London. That isn’t to say that we are completely in the dark. There is a body of existing work to draw on and scientists are filling in the gaps as fast as they can. Toxicologists agree that, in theory, there are two main routes by which microplastics might get into the human body: ingestion and inhalation. They also agree that, in theory, there are

5

billion

SARAH WILKINS

Tonnes of plastic now in landfill or discarded into the environment

comprehensive report by the European Commission’s Group of Chief Scientific Advisors. It came to a similar, inconclusive, conclusion: “Research on microplastics and their potential threats to humans is in its infancy and is complex – a lot remains uncertain.” The third, by the World Health Organization, which is specifically about the hazards of microplastics in drinking water, described the evidence base as “limited” and “insufficient”. One major challenge is the almost limitless diversity of microplastics. In terms of size, they are thought to span at least seven orders of magnitude, from 5 millimetres down to a nanometre or maybe less – known as nanoplastics. Shape varies enormously too,

from perfect spheres to bobbly lumps, jagged shards and pointy fibres. Then there is their chemical composition. “They consist of hundreds or even thousands of different polymers,” says Vethaak. Microplastics also contain additives such as plasticisers and flame retardants, and have the capacity to absorb contaminants from the environment, like hydrocarbons, pesticides or even metals. To complicate matters further, each particle is surrounded by an “eco-corona” of organic matter and microorganisms. “Almost every particle has its own identity,” says Vethaak. All that diversity makes them fiendishly hard to study. “They are a class of pollutants rather than a type, and this complexity makes

three potential hazards: physical, chemical and microbiological. The first comes from the particles getting lodged in organs and tissues and causing damage and inflammation. The second is from toxic chemicals leaching out into the body. These could be the remnants of chemicals that are used to make plastics, additives in the plastics such as flame retardants, and pollutants soaked up by plastics from the environment. The third comes from the pathogenic microbes that appear to grow enthusiastically on the surface of plastic particles. All of which sounds troubling. But the key words are “in theory”. As yet there is almost no evidence of actual health effects, let alone long-term risks. Consider the microplastics we swallow, which is probably the way most get into our bodies. Microplastics have been detected in drinking water and food; bottled water contains up to 106 particles per litre, and beer, sea salt, seafood, honey, sugar and teabags have also been found to be contaminated. Shellfish, which feed by filtering seawater and which we eat whole, including their digestive systems, are a rich source. A portion of mussels typically contains 800 microplastic particles. Microplastics also rain out of the air and onto our food. One estimate suggests that > 7 December 2019 | New Scientist | 39

we know about the behaviour of small particles suggests that uptake across the gut wall is likely to be very limited, with only 1 to 2 per cent of the smallest nanoplastics getting across. That may not seem much, but smaller nanoplastics are more likely to be hazardous. “They can more easily pass the membranes and enter blood circulation and maybe even pass the blood-brain barrier or the placenta,” says Vethaak. “In principle.” The problem is that when you get down to the really tiny particles, we have no idea how many we are exposed to or have in our bodies. As far as we know, plastic breaks down into smaller and smaller pieces so it is a fair bet to assume that they exist at nanoscales. Yet analytical methods aren’t sensitive enough to detect them in the environment on this scale. In fact, according to the SAPEA consortium report, we can’t confirm that nanoplastics even exist. It is a similar story for inhaled plastics. The upper airways are good at clearing out small microplastic particles, says Kooter, but nanoplastics might penetrate deep into the lungs and possibly cross into the bloodstream. Even if they do, the biological consequence 40 | New Scientist | 7 December 2019

All plastics lead to the sea Not all microplastics in the ocean come from the break down of litter. Here are other leading sources, as a percentage of total plastics in the sea 35 30 25 20 15 10 5 0

SOURCE: doi.org/dfbz

Number of microplastics we eat in a year from particles falling on food

Per cent

68,000

isn’t clear. “Do they have an effect on lungs? The honest answer is that there is not much knowledge,” says Fransien van Dijk at the University of Groningen in the Netherlands. Ditto the risk from microbes that colonise plastics. According to Ana Maria de Roda Husman of the Netherlands National Institute for Public Health and the Environment in Bilthoven, microorganisms grow extremely well on microplastic surfaces and are “probably” vectors of bacterial and viral disease, as well as antibiotic resistance genes. Chemical toxicity is also murky. According to an analysis by the European Food Safety Authority, one portion of mussels can deliver small doses of known toxins including polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and bisphenol A (BPA). However, it found that the quantities are negligible compared to what we ingest from other sources. But this is just one analysis. The sheer diversity of compounds that could be dumped in our bodies by microplastics means we have still only scraped the surface. As for biological effects, animal studies have been done but – no surprise – they aren’t very informative. Most involve aquatic organisms and, although they often find toxic effects, the experiments typically use very high concentrations of clean, spherical microplastics that aren’t representative of real-world exposure. Despite these uncertainties – or perhaps because of them – scientific authorities have been quick to damp down speculation of a risk to public health. SAPEA concluded that “we have no evidence of widespread risk to human

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we ingest more than 68,000 particles a year from this source. If there was any doubt that we regularly ingest them, a study last year put that to rest. It looked at samples of human faeces from around the world and found microplastics in all of them. But how much we swallow and to what effect is unknown. The ones in our faeces are actually the least of our worries, says Heather Leslie of Vrije University Amsterdam, because they are simply passing through. What we really need to worry about is whether microplastics damage the lining of the gut, or are absorbed into the bloodstream. According to Ingeborg Kooter of the Netherlands Organisation for Applied Scientific Research (TNO) in Utrecht, what

700

billion

Number of microplastic particles generated in an average house per year health from [micro and nanoplastics] at present.” The World Health Organization noted that humans have ingested microplastics for decades with no signs of serious health effects. Routine monitoring of microplastics in drinking water isn’t necessary, it said. Scare over? Far from it. This relaxed stance surprises and dismays many researchers working in the field. “I’m a critic,” says Vethaak. “It would have been better if they said, ‘Sorry, the data is very limited, we don’t know anything about these very small particles, we need more research’.” “I think we’re making a certain error of logic called the ‘appeal to ignorance’ fallacy,” says Leslie. “Absence of evidence [of harm] is not evidence of absence. The last thing we need now is another large-scale, long-term threat to human or ecological health.” Leslie is one of a group of researchers urgently working to reduce that absence of evidence. Earlier this year, she received funding from ZonMw, the Netherlands Organisation for Health Research and Development, to find out whether microplastics enter the bloodstream. Hers is one of 15 quickturnaround projects funded by ZonMw to the tune of €1.8 million. These will report their results next year and those with interesting findings will be given more money to follow up. The research is an attempt to shed light on five key concerns over micro and nanoplastics and health: risk from food, risk from inhalation, effects on the immune system, whether the particles reach the brain or cross the placenta, and their potential as carriers of pathogens. The projects are all ongoing, but some presented interim results at the Plastic Health Summit in Amsterdam in October. Although preliminary, they don’t make for comforting reading. Take a project led by Nienke Vrisekoop at the University Medical Center Utrecht on how the immune system responds to microplastics. She says previous research has shown that in

Wear and tear of tyres on roads is a leading source of microplastics

want to scare people; some of the data is comforting,” says Vrisekoop. “But some of it is worrying too.” Vethaak accepts the World Health Organization’s line that if there was a serious risk from microplastics we would be seeing it already. But he nonetheless suspects that they are contributing to the epidemic of chronic inflammation that has been linked to diseases such as neuro-degeneration, cardiovascular disease, diabetes and cancer. “We are exposed maybe only to low concentrations, and maybe these particles are not so toxic themselves, but it is lifetime exposure and there is potential that they may accumulate in organs. And we know that they have the potential to cause inflammation,” he says.

GETTY IMAGES

Bad to worse

mice, microplastics can be detected in the liver, kidney and gut, which means they are likely to come into contact with the immune system. To find out what might happen in humans, she added polystyrene microbeads to dishes containing immune cells called neutrophils, which engulf and kill bacteria and other invaders. Some of the microplastics were pure, uncontaminated plastic; the neutrophils just ignored them. But spheres coated with blood plasma – to mimic how they are likely to be encountered by the immune system after circulating in the bloodstream – triggered the neutrophils to attack and engulf them. The neutrophils then died. “We find quite extreme effects,” says Vrisekoop. The implications for human health are worrying, she says, though there are still many unknowns, such as whether the microplastics trigger repeated neutrophil attacks in a chainreaction immune response. “That really requires further research,” she says. Vrisekoop now plans to investigate whether microplastics ingested by mice activate their immune system. She and collaborators at the Amsterdam University Medical Center are also developing an assay to measure microplastic loads in human tissue.

800

Number of microplastic particles in a portion of mussels

“There’s really a lot of work to be done,” she says. In another early finding, toxicologists at Utrecht University discovered that cultured human placenta cells can absorb polystyrene beads that are 200 nanometres across. A third study found that lung tissue incubated with microplastics died. These are preliminary results and hardly add up to a comprehensive case against microplastics. For one thing, the full relevance to human health is still unclear. But many researchers in the field say that there are already good reasons to believe that chronic exposure is detrimental to our health. “I don’t

What, then, can be done? Vrisekoop advises avoiding plastic packaging and urges food manufacturers to do their bit by offering plastic-free alternatives. We could also do more to eliminate some of the sources of microplastic pollution, such as banning single-use items, says Vethaak. The personal care industry has already committed to eliminating microbeads from toiletries and cosmetics next year, which will choke off around 10 per cent of the microplastics entering the sea from rivers. But these are a microdrop in the ocean. Given how much plastic rubbish is already out there, microplastic pollution will continue to rise for the foreseeable future as this waste breaks down. “Even if we stop plastic pollution right now, there will still be increasing levels for decades,” says Vethaak. And existing microplastics won’t vanish, but keep on getting smaller – and potentially more dangerous – as they circulate from water to air to soils and ultimately into our bodies. One thing everybody agrees on is that there are more questions than answers. Vethaak says he sees progress, but there is a long way to go. “I think it will take many years, maybe even a couple of decades, of research before we come up with some hard evidence or a better understanding of what’s happening,” he says. “Is there a risk? I don’t know. But I think we should find out ASAP. I think there is no time to waste.” ❚

Graham Lawton is a staff writer at New Scientist. Follow him on Twitter @GrahamLawton 7 December 2019 | New Scientist | 41