the earth Worrying blanket of microplastics—recorded by scientists virtually everywhere, across the oceans of our planet, high in the clouds above Mount Fuji, buried in human brainsand even in the testicles of our poor damn dogs—might be modestly less apocalyptic than previously thought.
Researchers at the University of Michigan (UM) have identified a surprising and possibly mildly embarrassing error that could be contributing to dramatic overestimates of microplastic content in multiple studies: flecks of debris shed by the standard latex and nitrile gloves that scientists typically wear in the laboratory. According to the new UM study, tiny soap-like salts called stearates coat these gloves as remnants of the manufacturing process, where they can rub off, creating thousands of false positives per square millimeter (or about one-thousandth of a square inch).
The UM team replicated a common test surface for work with microplastics to evaluate how seven different types of disposable lab gloves could cloud the final microplastic count in each case.
“The type of contact we are trying to imitate affects all varieties of microplastics research,” according to a statement from the study’s lead author, Madeline Clough, a recent UM PhD graduate. “If you come into contact with a sample with a gloved hand,” Clough said, “you’re probably imparting these stearates that could overestimate your results.”
A trick of the light
Obviously, a stearate salt is not a plastic. But the microscale geometry of these salts is similar to that of polyethylene, one of the most common forms of plastic that pollutes our environment on a microplastic scale. (Clough’s study was based on the definition of microplastic as a particle between one and 5,000 micrometers, that is, between very small and about 0.2 inches.)
The physical resemblance of stearate to polyethylene at these sizes has been shown to be close enough to fool infrared light-based techniques used to scan samples of microplastic particles.
“Researchers use vibrational spectroscopy to identify microplastics,” Clough explained in an essay for The Conversation, with study co-author Anne McNeil, “which involves measuring how the particle interacts with light to produce what scientists call a chemical fingerprint.”
Vibrational spectroscopy uses non-destructive interactions between energy and photons in a beam of infrared light, an effect called “Raman scattering,” to cause and measure various vibrations, rotations and other movements of the molecules that a given researcher hopes to identify. It can be subtle work. SometimesScience is more art than science. (Many people don’t understand that.)
“That’s why we need chemists and people who understand chemical structure to work in this field,” McNeil, a professor of chemistry, macromolecular science and engineering at UM, said in the university’s statement.
If the glove falls off, it must be retreaded
The researchers rigorously fabricated an identical set of aluminum-coated silicon surfaces and placed them in contact with three types of professional-grade latex gloves, three nitrile gloves of similar quality, and one ultra-high-purity nitrile glove used in circuit manufacturing clean rooms.
“We hypothesized that the strict contaminant-free standards of clean rooms would require particle-free gloves,” the team said. explained in their study, published last month in the Royal Society of Chemistry’s journal RSC Analytical Methods. As expected, they reported, cleanroom gloves produced “fewer false positives per unit area than other glove varieties.”
Latex gloves proved to be noticeably worse than nitrile gloves, overall, with one of those three tests producing approximately 7,000 “false positive” stearate particles per square millimeter. The three standard laboratory nitrile gloves tended to produce false positives in the range of a few thousand per square mm. Clean room gloves, on the other hand, produced a few hundred per mm2 at worst.
Most importantly, however, the researchers don’t believe their findings downplay the reality of microplastic pollution as a very real global problem: “We may be overestimating microplastics,” McNeil noted, “but there shouldn’t be any.”
In addition to recommending cleanroom gloves for future microplastics research or, where possible, no use at all, Clough and McNeil also noted in The Conversation that they had “developed methods to help differentiate the chemical fingerprints” of stearate salts. The team has been refining and publication In this technique, a machine learning statistical analysis of the Raman scattering data called conformal prediction is performed, starting at least 2024.
Their goal, as they wrote in the new paper, is to help themselves and other microplastics researchers “recover older, potentially contaminated data sets.”
“We plan to continue our research into Michigan’s atmospheric microplastic pollution,” Clough and McNeil wrote, “this time without gloves.”
