Microplastics react with water and can damage cells, study finds - critical summary review

Microplastics are the tiny fragments that break off larger plastic objects as they wear down, and studies have found them spread through virtually every ecosystem on the planet, including the water, the air and food. They have also been detected inside people: according to Meio Bit, a Brazilian technology outlet summarising years of research, these particles have been found accumulating in the brain, in organs and in nearly every functional system of the body, with contamination documented from conception onward.

Until now, the working assumption was that these fragments were a nuisance of a particular kind: foreign bodies that could lodge in tissue or carry other pollutants with them, but that did not themselves react with anything. That assumption is what a new paper published in the journal Nature has just put in question. Researchers at Columbia University and the University of California, Berkeley report that plain water is enough to make the surface of a microplastic particle chemically active, and that the reaction produces molecules known to damage cells.

Why plastic was considered harmless on the inside

One of the reasons plastic became so ubiquitous is that it is chemically stable. It does not readily react with the substances around it, which is exactly why it is used for food packaging, bottles, cups and toys. The same property was assumed to carry over to the fragments: smaller pieces of an inert material should still be inert.

The terms matter here. Microplastics are pieces smaller than 5 millimetres. Nanoplastics are smaller still, measuring between 100 and 1,000 nanometres. Research into their effects has long focused on other chemicals leaching out of the particles, or on their purely physical presence in tissue, treating the plastic itself as chemically neutral. For the larger pieces, that generally holds. The new findings challenge the idea that what is true of plastic at full size is also true of the microscopic debris it breaks into.

What happens when the particle gets small enough

The mechanism the researchers describe turns on surface area. As a piece of plastic fragments, the amount of exposed surface grows enormously in relation to its volume, putting far more material in contact with whatever surrounds it. At that scale, according to the study, the surface stops being a bystander and starts taking part in chemical reactions with water.

"As plastic fragments into smaller and smaller particles, its surface area increases enormously," said Wei Min, professor of chemistry and biomedical engineering at Columbia and one of the study's authors, in a press release. "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."

The authors note that this happens under normal environmental conditions, without sunlight or any of the other agents usually needed to degrade plastic.

Reactive oxygen species, and what they do to cells

The products of that reaction are called reactive oxygen species. One variety in particular stands out: hydroxyl radicals, which can pull electrons away from the proteins and fats that make up cell membranes, disrupting their structure and function. Naixin Qian, a postdoctoral researcher in Min's lab who helped lead the research, described these in comments reported by WIRED, the US technology magazine, as "textbook examples of the kinds of molecules that can damage animal and human tissue".

The body is not defenceless against this. Antioxidants exist precisely to neutralise reactive compounds. But if the reactive molecules greatly outnumber the antioxidants available, that line of defence can be overwhelmed, a situation called oxidative stress, which leads to cell damage. Earlier research had already identified oxidative stress among the cellular responses associated with exposure to microplastics, without establishing where it came from.

The researchers were particularly interested in blood, which is largely water. If contact with that fluid can chemically activate the surface of a particle, then plastic sitting inside the body would pose a risk beyond the pollutants it carries. To test the idea, they exposed red blood cells taken from mice to different types of microplastics. The cells became deformed and ruptured, a phenomenon known as hemolysis. The effect was stronger with the plastics that reacted more with water, which is what pointed to the reaction itself as the cause of the damage. This is a laboratory result in mouse cells, not a demonstration of what happens in a living person.

What the study does not show

The paper does not demonstrate that microplastics in water cause disease in people. What the researchers identified is a chemical mechanism that may help explain damage already observed in other studies.

June-Woo Park, a researcher at the Center for Environmental Safety Research of the Korea Institute of Toxicology, who did not take part in the work, commented on it through the Science Media Centre in Spain. He said earlier studies had already linked reactive oxygen species to the toxic effects of microplastics, and that what distinguishes this one is showing how those molecules are generated. "This study is relevant because it provides scientific evidence on the mechanism by which microplastics generate reactive oxygen species," he said, adding that the authors presented enough data to support their conclusions.

He also pointed to a limitation: the work examined microplastics, particles smaller than 5 millimetres, but did not investigate nanoplastics. According to Park, that matters because nanoplastics are considered an even greater environmental concern, given their size and their capacity to interact with organisms. Understanding these reactions better, he said, could help both in assessing the risks of microplastics and in designing policies to control this kind of pollution.

There is a further open question underneath all of this: how much plastic is actually inside human bodies. That remains under debate, and some of the more striking findings have been challenged. Min's own team reported in 2024 an average of roughly 240,000 particles per litre of bottled water, most of them nanoplastics, a figure 10 to 100 times higher than previous estimates had suggested. Those are the particles the new study did not test.

What is known, and what is still unknown

What is established: plastic at ordinary sizes is chemically stable, which is why it is used for packaging and bottles; as it fragments, its exposed surface area grows enormously; according to the Nature paper, that surface reacts with water under normal environmental conditions and generates reactive oxygen species, including hydroxyl radicals; in the laboratory, mouse red blood cells exposed to microplastics deformed and ruptured, with more damage from the plastics that were more reactive with water; and oxidative stress had already been observed in earlier studies of microplastic exposure, without an explanation for its origin.

What remains open: whether this reaction causes disease in people, which the study does not address; whether other laboratories reproduce the mechanism; whether the same process occurs with nanoplastics, which were not tested here and which Park considers the bigger concern; how much plastic is really accumulating in human tissue, a question still disputed; and whether regulators will start weighing the chemical reactivity of these particles, rather than only their physical presence, when assessing the risks of plastic.

There is no new individual precaution attached to any of this. Reducing exposure to plastic was already the standing advice, and the paper proposes no remedy. What changes is the question being asked of these fragments. For years the research looked at what plastic releases and where it lodges. The work now on the table asks what plastic does to the water it sits in, and whether that is where the damage has been coming from all along.