Scientists have sparked alarm in recent years with reports of microplastics being found everywhere from Antarctic ice to the human brain.
However, researchers have raised an awkward possibility: that many samples have been contaminated by the very laboratory gloves meant to protect them.
If so, much of the “plastic” might not have been pollution at all, but compounds transferred from scientists’ hands.
The study, published by the Royal Society of Chemistry in its journal Analytical Methods, shows that laboratory gloves can shed tiny residues, known as stearate salts, that are easily mistaken for polyethylene, a plastic often found in the environment.
The findings will contribute to growing unease over how reliable some of the most eye-catching claims have been. Over the past decade researchers have reported detecting microplastics, particles smaller than five millimetres, and nanoplastics, smaller than one micrometre, not only in rivers and soil but in blood, urine, breast milk and stool. More recent studies have gone further, claiming to identify plastic particles in organs once thought to be well protected from foreign material.
One study was widely interpreted as implying that some human brains contain as much plastic as you would find in a spoon.
This year, however, scientists told The Times that methodological weaknesses, contamination and false positives may have inflated the problem.
The latest research points to another potential source of error. When laboratory gloves touch a surface, they can leave behind microscopic traces of stearate salts, which are used during manufacture.
Common laboratory tests, such as infrared and Raman spectroscopy, struggle to distinguish stearate salts from plastics as they look almost identical under the specialised lasers and light on which these tests rely.
Professor Anne McNeil and Madeline Clough of the University of Michigan, who carried out the research, wrote on The Conversation website: “Our team found that, even when following established protocols, using certain methods to measure environmental microplastics can potentially contaminate the results.”
They added: “As a result, much of this research may be overestimating the number of microplastics.”
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In their experiments, simple contact with gloves produced “false positives” at rates of about 2,000 particles per square millimetre. In other words, a surface can appear to be covered in plastic pollution when, in fact, it has simply been handled.
Scientists have long known that plastic is virtually everywhere, including the air, on clothing and in lab equipment, making contamination hard to avoid. But gloves, widely used to prevent that contamination, have largely escaped suspicion.
Reports of microplastics inside the human body have fuelled concern about possible health effects, from heart disease to infertility. However, detecting such tiny particles in tissues is difficult, and scientists stress that errors are easy to make. Another source of error has involved compounds found in human fat closely resembling others found in plastics.
As one expert put it in January, researchers need to be more “explicit” about how confident they can be in their results.
McNeil and Clough stressed that their study did not mean microplastics were harmless or rare. Plastic pollution is widespread and well documented. However, the results suggests that some findings may say as much about laboratory methods as they do about the scale of contamination.