In March last year, researchers found that among a group of nearly 300 participants, people who had higher concentrations of plastics in deposits of fat in their arteries (arterial plaques) were more likely to experience heart attacks or strokes, and more likely to die as a result, than those in whom plastics were not detected1. Since it was published, the New England Journal of Medicine study has been mentioned more than 6,600 times on social media and more than 800 times in news articles and blogs.
The issue of whether plastics are entering human tissues and what impacts they might have on health is understandably of great interest to scientists, industry and society. Indeed, for the past few years there have been news stories almost every month about peer-reviewed articles that have reported findings of plastic particles in all sorts of human tissues and bodily fluids — including the lungs, heart, penis, placenta and breast milk. And in multiple countries, policymakers are being urged to implement measures to limit people’s exposure to nanoplastics and microplastics.
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Many of the studies conducted so far, however, rely on small sample sizes (typically 20–50 samples) and lack appropriate controls. Modern laboratories are themselves hotspots of nanoplastic and microplastic pollution, and the approaches that are being used to detect plastics make it hard to rule out the possibility of contamination, or prove definitively that plastics are in a sample. Also, many findings are not biologically plausible based on what is known — mainly from nanomedicine — about the movement of tiny particles within the human body.
For an emerging area of research, such problems are unsurprising. But without more rigorous standards, transparency and collaboration — among researchers, policymakers and industrial stakeholders — a cycle of misinformation and ineffective regulation could undermine efforts to protect both human health and the environment.
Plastic, plastic everywhere
Since the term ‘microplastic’ — used to describe plastic particles less than five millimetres long — was introduced in 2004, microplastics have been found not just in the oceans but also in lakes and rivers, soils, food and air. In a study published last year, researchers estimated that people in Denmark inhale about 3,400 microplastic particles every day when indoors2.
Assessing how these tiny particles behave in the human body and whether they accumulate over time requires first identifying and quantifying plastics in blood and tissue samples. To do this, many researchers use pyrolysis gas chromatography mass spectrometry (Py-GCMS), in which high temperatures (around 600–700 °C) break down plastics into smaller organic molecules. The resulting mix of molecules and the amount of each one provides a ‘signature’ that researchers can use to establish whether a certain plastic was in the original sample.
But there are limitations with this approach.
Polyethylene is often the most common plastic that is found in human tissues.Credit: Leonard Ortiz/MediaNews Group/Orange County Register via Getty
Even after samples have been treated, for instance with enzymes, to remove biological material, some residue can remain. And some compounds that indicate the presence of plastics can be produced when non-plastic substances are pyrolysed. Fatty acids, such as triglycerides, can break down into the same compounds as polyethylene3. Polyethylene is often the most commonly reported plastic in human-tissue studies using Py-GCMS4–6, although this is not the case for all studies that have used this method7,8.
Likewise, energy dispersive X-ray spectroscopy, which is often used in combination with scanning electron microscopy, can reveal the presence of carbon-based molecules. But it identifies only what elements are present in a sample, not molecular structures — meaning that it can only suggest the presence of plastics.
Another issue with some of the data generated so far is that they don’t make sense biologically.
In a study9 published last month, researchers examined 91 brains from autopsied bodies and found that plastics made up 0.65% of the brain on average. That’s equivalent to saying that each person had around 4.5 polyethylene bottle caps’ worth of plastic in their brain.
Other studies have reported the presence of large plastic particles, measuring up to 3 mm in length, in human blood samples10. One study that found microplastic particles 5.5–26.4 micrometres in size and synthetic fibres 19–24.5 µm in length in brain tissue, suggested that microplastics that are inhaled through the nose can travel along nerves to the olfactory bulb in the brain11.
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Yet, previous research suggests that particles bigger than 1 µm are probably too large to pass through the lung’s air–blood barrier, and any particle bigger than 10 µm is probably too large to pass through the gut–blood barrier12,13.
Without convincing mechanistic explanations of how larger particles might bypass biological barriers, it is difficult to accept conclusions that particles larger than 10 µm have entered human tissue.
Even when, say, through the use of multiple approaches, it is clear that plastics are present in a sample, there is a high possibility that the sample could have been contaminated with nanoplastics or microplastics at each stage of the research — from sampling and transportation to storage, processing and analysis. Human-tissue samples are often collected in clinical settings in which plastics are commonly used, and intravenous infusion equipment that is used to deliver medication can shed microplastics into a person’s bloodstream. Also, because different researchers use different sampling strategies, materials and analytical approaches, it is hard to compare results across studies. Some laboratories use stainless-steel chambers, for instance, and try to minimize plastic contamination; others use plastic equipment to process their samples.
