Microplastics in water aren’t inert and make damaging chemicals on their own

One reason plastics are used so often is that they’re chemically inert, meaning bags, food containers, and cups don’t react with what we put in them. Yet they can still break down into microplastics, which turn up in drinking water, human blood, arteries, and brains. The consequences of that are uncertain.

“Whether microplastics can directly cause human disease is still being researched,” said Wei Min, a chemist at Columbia University. In a new Nature study, Min’s team took a step toward answering this question by demonstrating that microplastics are likely not as chemically inert as we might expect.

Microdroplet chemistry

Min’s lab has spent years working on microdroplet chemistry, studying how water broken into tiny droplets behaves at its boundary with air. These droplets have been reported to generate reactive oxygen species, or ROS, a family of harmful, oxygen-containing molecules. “ROS will essentially bump into proteins, lipids, DNA, and cause damage,” Min said. “You could break a DNA strand into segments. You could mutate DNA; you could misfold proteins.”

Many animal studies have tied microplastic toxicity to oxidative stress, which is caused by ROS. So the team wondered if the boundary between water and plastic might do something similar. “We got inspired by that field,” Min said.

In the first test, the team stirred polystyrene powder into water containing methyl orange, a yellow dye that ROS break apart. Over 10 days, the solution faded almost to colorless, losing nearly 90 percent of the dye. In ethanol or other solvents, the dye stayed intact. Min and his colleagues concluded that whatever chemistry polystyrene was undergoing needed water to happen. So they ran more tests.

Water under stress

First was electron spin resonance, a technique that directly detects radicals—atoms or molecules with an unpaired electron that makes them particularly reactive. The polystyrene powder and water mixture contained hydroxyl radicals, a highly reactive ROS.

Next, the team used a fluorescent sensor that lights up only when it reacts with hydroxyl radicals. “You don’t see it, but once it reacts with local ROS, it turns into a bright color,” Min said. Under a microscope, the glow concentrated on the surfaces of the plastic particles and kept building over time.

The effect was not unique to polystyrene either. Scientists tested six common plastics—polyethylene, polypropylene, polystyrene, PVC, PET, and acrylic—and all of them generated radicals. The most water-repellent plastics, polyethylene and polypropylene, were the most active, while PET and acrylic, which contain chemical groups that attract water, were the least.

“The simplest answer is the most water-repellent ones change the water structure the most,” Min said. In ordinary, or bulk, water, each molecule is held in a network of hydrogen bonds with its neighbors. But a few layers of water molecules pressed against a water-repellent surface lose some of those partners. “Their structure is no longer the regular water structure. They are perturbed, and their energy is also significantly increased,” Min said.

Water always contains a small number of hydroxide ions, and turning one into a hydroxyl radical requires stripping away an electron. In bulk water, that costs a lot of energy. But simulations run by the group showed that, at a polyethylene surface, hydroxide ions short on hydrogen bonds needed far less energy to form radicals. On top of that, the plastic surfaces build up a negative electric charge in water, which further lowered the energy cost.

But Min’s team still had doubts to address before making any conclusive claims.

Ruling out the dirt

Min’s earlier work on microdroplet chemistry has drawn a lot of criticism, with some researchers arguing that the reported reactive chemicals come from contamination or measurement artifacts.

To avoid similar objections, Min and his colleagues used several independent detection methods. They also added a detergent-like surfactant that coats the plastic and disrupts the water layer at its surface. They found that the more surfactant they added, the weaker the plastic’s electric charge became and the fewer radicals it produced. “We only change one parameter while keeping all the rest [the same],” Min said. “We’re using exactly the same plastics, the same water, the same everything.”

Impurities, Min explained, are typically stable molecules with no reason to spontaneously generate radicals. He is careful not to overstate it, though. “To be absolutely clear, we cannot 100 percent rule out the involvement of impurities,” he said. “But our controls and our theoretical computation studies highly suggest that this is intrinsic to the microplastic-water interface.”

Finally, the team tested if this interface chemistry matters for biological molecules. And it most likely does.

Proteins and blood cells

Proteins and DNA can gather on plastic surfaces, and lysozyme, a common test protein, also began clumping into aggregates there. Adding caffeine, which mops up radicals, largely stopped the clumping, pointing to the radicals as the cause.

Mouse red blood cells that touched polystyrene particles broke into fragments, and the damage from each type of plastic tracked how many radicals it produced. The effect also persisted in blood serum.

Min stressed, though, that this indicates that microplastics make people sick by generating ROS. While animal studies often point to ROS as possible roots of oxidative stress and chronic inflammation, ROS can be produced in many ways, including through basic metabolic activity.

Min admitted comparing the magnitude of this effect to the impact of ROS our own cells make is rather tricky.

A local problem

“If you are really at the interface, you could be significantly affected by this effect, whereas if you are just a few microns away, you don’t feel it anymore,” Min said. Exposure is another unknown here. Min also emphasized that no one can yet say how many microplastics an average person takes in each year from food, water, and air.

The plastics in the study were mostly fresh, lab-bought powders, unlike weathered particles we encounter in the environment. In the team’s tests, older particles with more oxidized surfaces produced somewhat weaker signals. “This aging process is very heterogeneous,” Min said. “Some microplastic particles were made 50 years ago, some were made five days ago.” Finally, there is the question of size.

Microplastics are usually defined as particles smaller than five millimeters, while the minimum cutoff varies between 100 and 1,000 nanometers. Min’s team did not test how variation in microplastic particle size affects water-interface chemistry, but the researchers suspect it might play a role. The same mass of plastic broken into nanoparticles has far more surface area, which might make the effect larger. “This may be another reason for nanoplastics to be more dangerous,” Min said.

The researchers argue that if water-repellent surfaces drive the water interface chemistry, plastics could be designed to be less water-repellent, either by changing their molecular structure or including additives. First, though, the team wants to focus on the question their study left open. “We definitely want to see how much this effect is directly contributing to disease or biological problems in animals and humans,” Min said.

Nature, 2026. DOI: 10.1038/s41586-026-11141-y

Jacek Krywko is a freelance science and technology writer who covers space exploration, artificial intelligence research, computer science, and all sorts of engineering wizardry.