If you’ve been to the beach this summer, whether in the UK or abroad, the chances are your charming vista will have been spoiled at some point by a piece of plastic litter: a disused face mask buried in the sand or ring can floating in the shallows.

Such is the scale of the world’s plastic waste problem that microplastics – tiny plastic fragments, particles or fibres – have been detected in the most remote corners of the planet, from Antarctic glaciers to deserts. One survey estimates that somewhere between 15 and 51 trillion particles are floating around in the world’s oceans.

For many years microplastics were largely considered an environmental issue, with scientists mainly concerned about their impact on the ocean’s fragile ecosystem. But their growing omnipresence has steadily increased the risk of human exposure.

Microplastics from waste dumped at sea, and even from our swimming costumes and suncreams, are working their way into the water cycle, and being consumed by the animals and fish that ultimately end up on our plates. Oceanographers have found microplastics in high quantities in shellfish such as mussels and scallops. Last month, research by the University of Portsmouth found microplastic levels in seafood may be underestimated – tests showed that when the plastics are covered in the microbes they attract in the ocean, they are more likely to be ingested by oysters and other edible marine species.

They’re working their way into our bodies in other ways too – in tap and bottled water, or meals microwaved in plastic containers. Tiny plastic fragments have been found to be ubiquitous in the air of many cities. The weathering of car tyres, clothing, paint coatings, and the leakage of pellets and powders from factories all contribute to a fine plastic dust being continuously released into the atmosphere. Based on current surveys, we are likely to be consuming or breathing in anywhere from dozens to more than 100,000 microplastics each day.

Now concern is growing over just what microplastics are doing to our health.

As a professor of public health at Imperial College, London, Frank Kelly has devoted much of his career to studying the impact of air pollution. But over the past five years, Kelly has become increasingly concerned by the threat of microplastics.

“One of the things which worries us is that plastic tends to be very hard to break down,” says Kelly. “In the outdoor environment, it takes decades to fully degrade. So microplastics may accumulate in the body, which probably isn’t good, but we haven’t got any hard evidence at this point that says they’re having an impact on human health. This is what we need to find out.”

Earlier this year, the first concrete evidence that microplastics are lingering in the body was obtained by obstetricians at San Giovanni Calibita Fatebenefratelli Hospital in Rome, who discovered microplastics of different shapes and sizes in placenta samples. At Imperial, Kelly’s research group is now examining lung and intestinal samples from autopsies to see if microplastics can be identified in these tissues as well.

For scientists, this represents the first step towards convincing policymakers that microplastics are a serious health problem.

Most agree that the vast majority of microplastics which get into our body are likely to end up being excreted. The human body is highly capable of filtering out waste. Our immune system contains cells called macrophages, which are specifically designed to gobble up anything potentially harmful. “We evolved in a world of particles,” explains Bart Koelmans, a microplastics researcher at Wageningen University in the Netherlands. “We are creating them and ingesting them all the time.”

However if the doses ingested are particularly high, and the particles are small enough – those 1–10 micrometres or less in size – they could cross the epithelial lining in the lungs and intestines, work their way into the bloodstream, and slowly accumulate in organs like the kidneys and even the brain, a potential risk which has already been demonstrated in experiments with mice. The smallest plastic fragments of all – so-called nanoplastics, less than 0.1 micrometres in size, too small for scientists to even measure with current technologies – could pass directly into our cells.

Early evidence suggests that plastic accumulation may not be at all good for the body. Since 2019, Dick Vethaak, a professor of ecotoxicology at VU University Amsterdam has been leading a series of investigations, looking at the effects of microplastics settling in various human cell types in petri dishes. “We’ve seen inflammatory responses in different tissues, and impaired brain and placental cell function, and impaired airway growth,” he says.

Similar results have been found in other studies which have exposed either human cells or rodents to microplastics, resulting in DNA damage, inflammatory and immune reactions, and neurotoxic effects in brain cells. Kelly also points out that we know that factory workers who consistently inhale large amounts of fine, plastic dust, are more prone to developing lung injuries and cancer.

All these studies have involved extremely high doses of microplastics. Most of us are unlikely to be exposed to such concentrations on a daily basis. As such, scientists are trying to gather concrete data on exactly how many microplastics the average person is consuming each day, how much this varies depending on where you live, and whether continuous exposure can be proven to cause us harm.

To address some of these questions, Kelly is planning to set up the first ever human challenge trials for microplastics. This will involve getting a range of volunteers, from the completely healthy to those with respiratory conditions such as asthma or rhinitis, to inhale different microplastics through their nose.

“By doing that, and then looking at the reaction of the epithelium in the nasal airways, we can start to understand how the body is reacting to these microplastics in comparison to other particles,” he says. “We’ve done this kind of thing in the past, exposing volunteers to the type of diesel exhaust they would experience if they were shopping on Oxford Street, and seeing how normal airways and diseased airways actually responded to that environment.”

Microplastics are not all the same: some may contain pigments or additional chemicals which are far more toxic to the body than others. Kelly says that human challenge trials will enable scientists to identify which microplastics are inert, and which are problematic.

Koelmans is looking to obtain more accurate data on the typical levels of microplastics in our daily diets. So far, scientists have found evidence of these particles in around 20 per cent of the foods one might encounter in a supermarket shop, from fish to honey, but their levels in the remaining 80 per cent – which includes cereals, meat, and vegetables – remains unknown. “We urgently need more solid data on this, if you want regulators to make decisions on the amounts of microplastics which should be allowed in our food,” he says.

There are other possible hazards as well. Some scientists suspect that microplastics could act as transporters for antibiotic resistant bacteria, as well as viruses, making it easier for them to penetrate deep into the body. This remains an area of open investigation.

Governments are beginning to wake up to the urgency of the problem: the EU is financing five new research projects looking at the impact of microplastics on health.

“We need to act quickly,” says Koelmans. “We know we are being broadly exposed to these particles but we need to find out much more about the quantities we’re consuming, and what they’re doing to us.”     

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