Microplastics in the body: risks and unanswered questions

Microplastics have become an indispensable part of our environment. These tiny plastic fragments are even found in the human body. Their exact role there is still not fully understood. An overview of current findings and research gaps.

There are various ways microplastics can enter the human body. The most common route is through food and drinking water. One source of entry is plastic containers, said Eleonore Fröhlich, head of the Core Facility Imaging Department at the Medical University of Graz in Austria, at a Science Media Center press briefing last week. Microplastics can be formed from the material in PET bottles themselves or through abrasion when opening and closing the bottle.

"But many sea creatures and salt also contain comparatively large amounts of microplastics," Fröhlich continues. The oceans are full of microplastics, so fish and other marine animals ingest the tiny particles while breathing and eating. The plastic accumulates in their bodies and can also enter the human body if humans later consume the animals.

There is a lot of plastic floating in the oceans that fish inhale or consume.

Source: Unsplash/Naja Bertolt Jensen

Another route of entry is through the air we breathe. Tire wear, among other things, releases microscopic pieces of plastic into the air, which are then inhaled through the mouth or nose. However, the particles have difficulty penetrating the lungs because they first have to overcome a size barrier. To reach the lungs, the plastic must be smaller than five millimeters, "otherwise it is either filtered out or ends up in the mouth," explained Fröhlich. "This means the lungs protect themselves very effectively because the rest of the epithelium ( tissue that covers all external and internal surfaces of the body, editor's note) is comparatively sensitive."

The absorption of microplastics through the skin can be neglected, says the researcher. The skin has such a good barrier that hardly any particles can penetrate the human body.

Experiments with mice have shown that microplastics can penetrate "practically all organs" and "very quickly," reported Lukas Kenner, deputy director of the Clinical Institute of Pathology at the Medical University of Vienna. "After just two hours, the particles are practically everywhere." Plastic particles have even been found in the blood and heart.

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"We also saw that the plastic penetrates the brain very quickly," said Kenner, "because it can practically cover itself with molecules, for example cholesterol molecules." These molecules enable the plastic to cross the blood-brain barrier—the barrier between the bloodstream and the brain that protects the brain from harmful substances.

Researchers at the University of Murcia in Spain recently discovered microplastics in the female and male reproductive organs. For their study, they examined the semen of 22 men and the follicular fluid of 29 women. They found that microplastic particles were detectable in approximately half of the male samples and more than two-thirds of the female samples.

Every person ingests the equivalent of a debit card's worth of microplastics (about five grams) every week: This claim has been circulating for years. But more and more experts are doubting its veracity. Karsten Grote also considers the statement "a bit exaggerated." "But the bottom line is that we ingest somewhere in the gram range of microplastics per week," said the head of the Experimental Cardiology Working Group at the Department of Cardiology at the University Hospital of Giessen and Marburg.

Most figures on the amount of microplastics in the human body are estimates, and therefore subject to uncertainty.

A study by the University of Toulouse has now concluded that, particularly indoors, such as in homes and cars, an average of around 68,000 tiny plastic particles could enter the respiratory tract of an adult every day – and in children, the figure is around 47,000. The study did not attempt to directly detect microplastics in the human body. Instead, air samples were taken indoors and used these to calculate how many particles are theoretically inhaled through breathing – for example, in everyday life at home or in the car. The researchers used standard values for the respiratory volume of a resting person.

Fröhlich points out that the data are based on measurements from relatively small air volumes and on the analysis of only a small portion of the total sample and may therefore be subject to uncertainty: "Every small measurement error carries over into the future." Furthermore, physical activity can significantly increase actual intake. A broader data basis is necessary for reliable conclusions.

In water, in chewing gum, in cosmetics, even in the air—microplastics can be found almost everywhere. Environmental organizations and experts speak of an invisible danger. But what makes microplastics so dangerous? How do they form? And how can they be avoided? We explain.

The surface of microplastics acts as a "carrier," said Grote. That is, it acts as a carrier for other pathogenic substances – such as polycyclic aromatic hydrocarbons (PAHs), as the German Federal Institute for Risk Assessment (BfR) explains. PAHs are primarily formed during the incomplete combustion of organic materials such as coal, oil, wood, and tobacco, and some are classified as carcinogenic. However, the institute points out: "Whether potentially bound contaminants in the cells can be released again by the particles has not yet been conclusively clarified."

Bacteria are also known to be able to settle on microplastics and thus spread.

Experts are currently unable to estimate the exact health damage microplastics can cause. However, it is known that the particles can release toxins once they enter the body. Especially in the intestines, "if acid or bile acid is present, then potentially bound toxins can be released and are then more readily available for absorption into the intestines or the body than without microplastics," said biochemist Fröhlich.

Microplastics can cause inflammation in the intestines, but also in other cells of the body. "It's not a severe inflammatory process that's taking place there, not comparable to an infection, but rather a smoldering process," cardiologist Grote described it. Such inflammatory reactions are known, among other things, from blood vessels .

Further research is needed to assess the health risks posed by microplastics. "Currently, there is no reliable information on the effects of microplastics on humans," summarizes the BfR. "However, plastic particles are considered to be very unreactive, which is why acute toxicological effects are very unlikely. There is still no reliable information on long-term effects from chronic ingestion. However, it can be assumed that knowledge about microplastics will advance significantly in the coming years, thus enabling a better assessment of health risks in the future."

In cell cultures, Kenner and his colleagues were able to determine that microplastic particles are absorbed by prostate cancer cells. "That was astonishing for us," said the researcher. "Anyone who knows cancer cells knows that they are survivors. They don't normally take up anything they can't use." Similar mechanisms were also observed in lung cancer.

In their study , the researchers were also able to demonstrate that the plastic particles are passed on to the newly formed cells during cell division. "In patients, this might indicate that tumor cells can spread more quickly in the body if they also ingest microplastics," said Kenner. However, whether the study results can be extrapolated to living organisms is currently questionable.

How long microplastics remain in the body is unknown. "At least when we examine patients' urine, we see that plastic is detected in microplastic particles," said Kenner. "That means the body must somehow get rid of the plastic. But how this works and to what extent it is absorbed daily, unfortunately, we cannot yet say." Microplastic residues have been found not only in urine, but also in human feces.

Microplastics are actually just the "tip of the iceberg," said biochemist Fröhlich. Nanoplastics could be even more dangerous to human health. These are extremely small plastic particles, measuring 1 to 1000 nanometers, that can penetrate deep into the body. However, the effects of nanoplastics are even less understood – partly because these particles are not so easily detectable. "Unfortunately, we lack any reasonable detection method for nanoplastics," said Fröhlich.

Pathologist Kenner pointed to so-called O-PTIR microscopy, which could help in the future search for nanoplastics. "This is a method where we can detect very small particles down to 500 nanometers, i.e., already in the nanoparticle range," he said. "We are currently analyzing human material in this area, and we hope to publish something on this soon."