Plastic particles smaller than five millimetres are now found throughout the human body. Whether they cause disease at the levels people are actually exposed to has not been established, and the gap between those two statements is the whole subject.

Microplastic particles. The category spans fragments, fibres and beads across a wide size range, and the smallest, the nanoplastics, are the ones most likely to cross biological barriers.
Microplastic particles. The category spans fragments, fibres and beads across a wide size range, and the smallest, the nanoplastics, are the ones most likely to cross biological barriers.Credit: Clandon haverford (CC BY-SA 4.0).

Microplastics are conventionally defined as particles under five millimetres, and nanoplastics as those below one micrometre. The distinction matters because size largely determines whether a particle can cross the gut wall, enter the bloodstream or pass into tissue.

Detection in human tissue is no longer in doubt. Particles have been reported in blood, lung tissue, liver, kidney, placenta, breast milk, semen and stool.

Two findings drew particular attention. A 2024 study in the New England Journal of Medicine examined plaque removed from the carotid arteries of just over two hundred and fifty patients and found micro or nanoplastics in about fifty eight per cent of samples; over roughly the following three years, patients whose plaque contained them had a higher rate of heart attack, stroke and death.

Work on brain tissue reported concentrations far higher than in liver or kidney, with a median figure around seven to thirty times greater, and a subsequent study found particles in nearly every brain sample examined including healthy tissue.

Bottled water, one of the better characterised exposure routes. Quantifying exposure is difficult in part because sampling equipment and laboratory plastics can contaminate the measurement.
Bottled water, one of the better characterised exposure routes. Quantifying exposure is difficult in part because sampling equipment and laboratory plastics can contaminate the measurement.Credit: Walter J. Pilsak, Waldsassen, Germany (CC BY-SA 3.0).

A 2026 state-of-the-evidence review concluded that no causal relationship has been established between microplastic uptake and any specific health outcome in human populations. Human evidence indicates associations with markers of inflammation, oxidative stress and endocrine alteration, and does not establish causation with clinical disease.

Several difficulties stand in the way, and they are methodological rather than a matter of interest or funding.

Contamination is the central problem. Plastic is present in laboratory equipment, sample containers, air and clothing, so a study must demonstrate that what it detected in tissue did not arrive during collection or analysis. Procedural blanks are essential and are not always reported adequately.

Measurement is not standardised. Different techniques, including spectroscopy and pyrolysis mass spectrometry, have different detection limits and different failure modes, and results are reported in incompatible units, sometimes particle counts and sometimes mass. The very high brain concentrations have been questioned on exactly this basis.

The carotid plaque finding, which is the strongest clinical association available, is observational. People with plastic in their plaque may differ from those without it in ways that also affect cardiovascular risk, and the study establishes an association in a selected surgical population rather than a causal effect.

Animal studies generally use doses and particle types far above realistic human exposure, and often use uniform polystyrene spheres rather than the weathered, irregular, chemically mixed particles people encounter.

Three routes of harm are proposed, and each is biologically reasonable and unproven at human exposure levels.

Physical presence of particles may provoke a persistent inflammatory response, in the way that other durable particulates do.

Additives may leach. Plastics contain plasticisers, flame retardants, stabilisers and bisphenols, several of which have documented biological activity, and some of these have far better evidence of harm than the particles themselves. This is an important distinction, because a health effect attributed to microplastics may be an effect of a chemical the plastic carries.

Particles may act as vectors, adsorbing environmental contaminants or supporting microbial films and transporting them.

Exposure is universal and ongoing, through food, water, air and dust, with airborne inhalation increasingly considered a major route.

Microplastic particles recovered from river sediments. Environmental concentrations are rising and plastic production continues to increase, which sets the direction of exposure regardless of what the health evidence eventually shows.
Microplastic particles recovered from river sediments. Environmental concentrations are rising and plastic production continues to increase, which sets the direction of exposure regardless of what the health evidence eventually shows.Credit: Martin Wagner et al. (CC BY 4.0).

Concentrations in the environment are rising, and plastic production continues to increase.

Nothing about the current state of evidence supports the conclusion that microplastics are harmless. Absence of demonstrated causation is not demonstration of safety, and the field is young: most human health work postdates 2018.

This is an unusually clean example of a subject where the honest answer is uncomfortable in both directions. The confident claim that microplastics are damaging human health outruns the evidence. The confident claim that they are safe outruns it further, since almost no study has looked for effects over the timescales that would matter.

It also illustrates a general problem in environmental health. Universal exposure removes the unexposed comparison group that epidemiology depends on, and a hazard everyone is subject to is much harder to detect than one that divides a population.