Plastics were supposed to be inert. A new study shows they aren't — and explains why.

For decades, the reason plastics were considered safe enough to wrap food, bottle water, and make children's toys was simple: they were chemically inert. They weren't supposed to react with anything. A study published in the journal Nature has now dismantled that assumption — and in doing so, has given scientists a plausible chemical explanation for why microplastics might be harming living things.

Researchers from Columbia University and the University of California, Berkeley, found that when plastic fragments into micro- and nanoparticles, its surface area increases dramatically relative to its volume. That newly exposed surface, it turns out, is not chemically passive. When microplastics come into contact with water — including human blood, which is largely water — they spontaneously generate highly reactive molecules called hydroxyl free radicals, which are capable of damaging DNA, proteins, and cells.

As plastic fragments into smaller and smaller particles, its surface area increases enormously. Then microplastics change the water that surrounds them, generating high-energy, reactive radicals that very likely contribute to biological damage that we see in animals and potentially in humans as a result of microplastic exposure.— Wei Min, Columbia professor of chemistry and biomedical engineering

The mechanism is counterintuitive. Plastics are hydrophobic — they repel water. But it is precisely that repulsion that drives the chemistry. When water molecules are forced into contact with a hydrophobic plastic surface, their molecular structure is reorganized. Hydroxide ions near that interface lose an electron more easily, converting into hydroxyl radicals — among the most reactive and toxic molecular species known, and the target of the entire antioxidant industry.

Our research found that when microplastics interact with water, as they do when they're in bottled water, or when they're absorbed into our bloodstream, they spontaneously generate high-energy reactive radicals. These are textbook examples of the kinds of molecules that can damage animal and human tissue.— Naixin Qian, postdoctoral researcher, Columbia University

The Columbia team tested six common commercial plastics in powdered, lab-grade form and used multiple analytical techniques to confirm free-radical formation. When they introduced lysozyme — a protein found in human tears, saliva, mucus, and milk — it began clumping in a way the researchers described as potentially unhealthy. Protein clumping in the brain is a known driver of neurodegenerative disease. Mouse-derived red blood cells exposed to the process were warped and fragmented. Adding caffeine, an antioxidant, halted the protein-clumping effect in the lab, though the researchers noted its protective effects differ inside the body.

Collaborators at UC Berkeley, led by theoretical chemist Teresa Head-Gordon, ran calculations and simulations to explain the underlying physics: hydroxide ions promoted to the water layers immediately adjacent to the hydrophobic plastic surface can more easily shed an electron, producing the hydroxyl radicals observed in the experiments.

Water is a key active player here, rather than just a spectator. It's integral to making these microplastics reactive.— Wei Min, Columbia professor of chemistry and biomedical engineering

The finding matters because it addresses a gap that has long troubled researchers: studies published in prominent journals including the New England Journal of Medicine, Nature Medicine, and Nature Health have reported links between micro- and nanoplastic exposure and heart attack, stroke, dementia, and brain tumors. But because plastics were assumed to be inert, no one could explain the mechanism. This study proposes one.

The sources of microplastics in daily life are extensive. According to Columbia researchers, they include microwave heating of food in plastic containers, sunlight exposure of bottled water during storage, IV injections in healthcare settings, and even the simple act of opening and closing a plastic water bottle, which abrades its cap. Industrial processes — including plastics manufacturing and laundry — discharge microplastics into aquatic environments.

The study arrives at a moment of significant federal attention. The U.S. Department of Health and Human Services launched a $144 million program to address microplastics in the human body, and the U.S. Environmental Protection Agency added microplastics to a federal list of contaminants that may pose a concern for drinking water, the first time they have appeared on that list, according to Columbia University.

The researchers also point to a potential path forward: because the reactivity originates at the plastic surface, redesigning the surface chemistry of common plastics could, in principle, reduce the toxic outcomes. Head-Gordon's team is exploring that avenue.

Yet the Nature study lands alongside a separate and uncomfortable reckoning in the field. Even as scientists work to understand what microplastics do inside the body, a growing number of researchers are questioning whether earlier studies reliably detected microplastics in human tissue at all.

High-profile studies claiming to have found micro- and nanoplastics in the brain, testes, placentas, carotid arteries, and blood have been challenged by other scientists who say the results are likely the product of contamination and false positives. The Guardian identified seven such studies that have been formally challenged in their respective journals, while a recent analysis listed 18 studies that had not accounted for the fact that some human tissue can produce signals easily confused with those of common plastics.

The brain microplastic paper is a joke. Fat is known to make false-positives for polyethylene. The brain has approximately 60% fat.— Dr. Dušan Materić, Helmholtz Centre for Environmental Research, Germany

The study Materić criticized — published in a top-tier journal in February and widely covered — reported a rising trend in micro- and nanoplastics in brain tissue from postmortems carried out between 1997 and 2024. By November, a formal challenge had been published in the same journal, citing limited contamination controls and a lack of validation steps. Materić and his colleagues suggested that rising obesity levels — since fat tissue can produce false positives for polyethylene — could be an alternative explanation for the apparent upward trend.

Materi ć said he has serious doubts about 'more than half of the very high impact papers' reporting microplastics in biological tissue, according to the Guardian. A chemist formerly at the Dow Chemical Company, Roger Kuhlman, called the accumulation of doubts 'a major finding,' saying: 'This is really forcing us to re-evaluate everything we think we know about microplastics in the body. Which, it turns out, is really not very much.'

The senior author of the brain study, Prof. Matthew Campen, acknowledged the criticisms but pushed back on their framing, telling the Guardian that most of the criticism 'has been conjectural and not buffeted by actual data,' and that his team is focused on generating better methods rather than relitigating past work. Other challenged studies — including those reporting microplastics in blood and arteries — have similarly been contested by their authors.

The stakes of getting this right extend beyond science. Researchers told the Guardian that faulty evidence on microplastic levels in humans could lead to misguided regulations — and could hand lobbyists for the plastics industry a tool to dismiss legitimate concerns as unfounded.

What the field is left with is a picture that is both more alarming and more uncertain than headlines of recent years suggested. The new Nature study provides a credible chemical mechanism by which microplastics could cause harm — but the question of how much plastic is actually present in human tissue, and in what concentrations, remains genuinely unresolved. Analytical techniques are improving, researchers say, but the body of evidence built so far must be read with more caution than it has been.

Why it matters — If microplastics are both chemically active in the body and as widespread as exposure data suggests, the assumption of plastic safety that underpins food packaging, medical equipment, and consumer products worldwide needs to be reconsidered — but so does the quality of the evidence used to make that case.

Sources differ on Reliability of the brain microplastics study (postmortem tissue, 1997–2024): Faces serious methodological challenges including limited contamination controls and lack of validation; results likely reflect false positives from brain fat tissue. (theguardian.com (critics, including Dr. Materić)) vs Criticism is conjectural and not supported by actual data; the field is in an early period and methods are being refined. (theguardian.com (Prof. Matthew Campen, senior author))

Sources differ on Reliability of study claiming 10,000 nanoplastic particles per liter of bottled water: Called 'fundamentally unreliable' by critics. (theguardian.com) vs Scientists behind the study disputed that characterization. (theguardian.com)

Reported by washingtonpost.com, news.climate.columbia.edu, sciencealert.com, thelancet.com, jamanetwork.com, theguardian.com