Microplastics seem to generate potentially harmful molecules when they contact blood, which is thought to be due to their large surface area enabling such reactions. If this effect occurs within our bodies, it could explain why they have been linked to cell damage and other health concerns.
Several recent studies have linked microplastics, which form when plastics degrade in the environment, to issues such as poor cognition and cardiovascular disease. This seems to be related to the production of destructive molecules called reactive oxygen species, says Wei Min at Columbia University in New York. These can then react with different molecules inside cells, contributing to issues like inflammation.
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But the extent to which common plastics could be causing such problems was unclear. They do not always contain oxygen and are therefore not thought to be particularly reactive. “From a pure chemistry point of view, this should be inert,” says Min.
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To find out, Min and his colleagues studied the microparticles of a range of commonly used plastics – including polystyrene, polyethylene and polypropylene – and found that “water is a key player” in their effects.
They used techniques including fluorescence microscopy and electron spin resonance spectroscopy to show that a type of reactive oxygen species called hydroxyl radicals spontaneously forms at the surface between plastic and water, making the latter “not regular water anymore”, says Min.
The team also showed that this effect is linked to electric charges forming at this plastic-water interface. These charges may help strip electrons from OH- ions in water, generating volatile hydroxyl radicals, says Min.
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In experiments, this electric charge attracted biological molecules such as DNA to the surface of microplastics when suspended in water, which appeared to damage both proteins and red blood cells.
The researchers also tested whether hydroxyl radicals were generated when the microparticles were suspended in serum from blood. The effect was weaker, but “still detectable”, says Min. Microplastics have been detected in blood, however, signals of their presence throughout our bodies may be the result of contamination and false positive results.
If microplastics do enter our blood, laboratory studies have linked them to cellular inflammation, DNA damage and death, all of which can be caused by reactive oxygen species.
The findings could help explain how those molecules are being produced, says Min, although he adds that the potential size of the effect is unclear and other mechanisms could be involved. It may also depend on whether microplastics are disposed of by our bodies. “How much plastic is in a regular human being?” he says. “We have no idea right now.”
Hao Chen at the New Jersey Institute of Technology in Newark says the results are “very important”, and although they don’t directly answer how dangerous microplastics might be, they “provide a starting point” to help us understand the interaction between microplastics and cells.
Matthew Campen at the University of New Mexico in Albuquerque describes the results as a “critical building block” for this understanding. The surface area of microplastics is probably key to their effects, he says, pointing out that a half-centimetre ball of plastic could degrade into a quadrillion 50-nanometre microparticles, increasing its surface area 100,000 times.
That’s not necessarily as worrying as it sounds, however. “I encourage people to take a deep breath,” Campen says.
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When the researchers added a surfactant – a chemical that weakens the forces holding the water molecules together – radical generation was dampened. Some lipids act as surfactants in the body. “Microplastics hit your lung and they are surrounded by surfactants immediately,” says Campen. “In most situations that I can imagine in a biological system, they will be surrounded by those lipids. The big concern is [whether] this is getting worse over time, and we really need to get a handle on what the health effects are or could be, so that we can avoid exposures.”
The findings could also help scientists design safer materials, says Min. The researchers found that the most hydrophobic (water-repelling) plastics they tested, such as polyethylene and polypropylene, showed stronger radical-generating activity than more hydrophilic ones, such as PET (polyethylene terephthalate).
Campen notes, however, that the plastics with the lowest activity are also probably the ones that break down more slowly in the environment. “How do you make that trade off?”
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