How Much Plastic Really Reaches Your Cells? New Study Rethinks Toxicity Testing

Microplastics are everywhere: in the air we breathe, the water we drink, and the food we eat. As public concern over these tiny plastic fragments has grown, laboratories around the world have rushed to test how toxic different types of microplastics might be to human cells. But a new study from researchers at the Netherlands Organization for Applied Scientific Research (TNO) and Maastricht University argues that many of these tests may be built on a flawed foundation. The problem, they say, lies in a deceptively simple question: how much plastic actually reaches the cells being studied?

The research, published in the open-access journal Microplastics and Nanoplastics, focuses on a technical but crucial distinction in laboratory toxicology: the difference between the administered dose and the delivered dose. The administered dose is simply the amount of plastic a scientist pours into a cell culture dish. The delivered dose is the fraction of that plastic that actually settles onto, or otherwise contacts, the cells over the course of an experiment. When particles are suspended in liquid culture medium, they do not all reach the cell layer at the same speed, and some may never fully deposit at all. Ignoring this distinction, the researchers show, can distort toxicity rankings and lead to conclusions that are wrong by a factor of two or more.

To investigate the problem systematically, the team, led by M. Kloukinioti and colleagues including co-senior authors H. Braakhuis and L. A. Parker, used fluorescent aminated polystyrene microspheres of two very different sizes: one micrometer and ten micrometers in diameter. Polystyrene is one of the most widely used plastics in consumer products and is also the standard material for laboratory test particles, making it a sensible model for the broader microplastics problem. The fluorescent labels allowed the researchers to track exactly where the particles went over time, simply by measuring how much fluorescence appeared at the bottom of the wells in a cell-free system.

The experimental results were striking. Over a standard 24-hour exposure period, neither particle size achieved complete deposition. In other words, even after a full day of settling, a significant portion of the plastic added to the dish never made contact with the surface where cells would normally grow. This finding alone challenges a widespread assumption in in vitro toxicology: that the amount of material pipetted into a well is a reasonable proxy for the amount the cells experience. For small particles suspended in viscous biological media, reality is considerably more complicated.

The researchers then turned to two computational models that attempt to predict how particles settle in liquid. The first is Stokes’ Law, the classic nineteenth-century physics equation that describes the sedimentation of spherical particles in a fluid, taking into account the particle’s radius and density, the density of the fluid, and its viscosity. The second is the RiskGONE model, a more recent dosimetry tool developed within a European nanosafety project, which incorporates additional parameters such as particle size distributions and can account for more complex settling behavior. Both models require detailed input about the culture system, including the density and viscosity of the medium and the physical characteristics of the particles themselves, which the team measured carefully using techniques such as static light scattering.

What emerged was a surprising split verdict. The sedimentation behavior of the smaller, one-micrometer particles was accurately predicted by the RiskGONE model, while the settling of the larger, ten-micrometer particles aligned closely with the classical Stokes’ Law. This size-dependent divergence matters because it means there is no single, one-size-fits-all formula for calculating delivered dose in microplastics experiments. A researcher who applies the wrong model to the wrong particle size could introduce systematic errors into their dose calculations, and those errors would propagate directly into their toxicity conclusions.

The most consequential part of the study came when the team connected their dosimetry findings to actual biological measurements. In parallel with the cell-free sedimentation experiments, the researchers exposed THP-1 cells, a widely used human immune cell line derived from monocytes, to the same polystyrene particles under identical conditions of medium composition, plate setup, and exposure volume. They then assessed cell viability using standard assays and compared the hazard estimates obtained when calculations were based on the administered dose versus the delivered dose.

The result was unambiguous: relying solely on the administered dose led to an underestimation of the microplastics’ hazard potential by approximately a factor of two. Because fewer particles reached the cells than had been pipetted into the wells, the true toxicity per delivered particle was roughly twice as high as the administered-dose calculation suggested. In practical terms, a plastic particle that appears relatively harmless when judged by the amount poured into the dish may in fact be considerably more damaging to the cells it actually contacts. For regulators and risk assessors trying to rank different plastics by hazard, such a systematic bias could shuffle the entire leaderboard.

This finding has implications that extend well beyond polystyrene spheres. Efforts to compare the toxicity of different polymer types, including polypropylene, polyvinyl chloride, polyethylene terephthalate, and polyamide, are accelerating worldwide, and many of these comparisons rest on in vitro data. If dosimetry is not properly accounted for, a plastic that settles quickly might appear more toxic than one that stays suspended, not because it is intrinsically more hazardous but simply because more of it reached the cells. Conversely, a genuinely dangerous polymer could be wrongly exonerated if most of its particles never arrived at the cell layer during the experiment. The study provides a technical roadmap for avoiding these pitfalls, emphasizing that researchers should characterize their particles’ size distribution and effective density, measure their medium’s properties, and apply an appropriate sedimentation model before interpreting any biological endpoint.

The work also highlights a broader lesson about the limits of computational models in new scientific domains. Both Stokes’ Law and the RiskGONE model were validated here against direct experimental measurements, and neither was universally correct. The authors’ approach, testing model predictions against fluorescence-based deposition data before trusting either one, offers a template that other laboratories can follow. As the field of microplastics toxicology matures and moves toward benchmark dose modeling and quantitative risk assessment, the distinction between what is poured in and what lands on the cells may prove to be one of the most important numbers in the entire experiment. The study was conducted as part of the Dutch MOMENTUM research program on microplastics and health, with funding from ZonMw, Health-Holland, and Plastics Europe, though the funding organizations had no influence on the work. The authors note that the article was shared early as a citable, peer-reviewed accepted manuscript, with a final version of record to follow, and the research stands as a reminder that in the fast-moving world of microplastics science, the most important measurements are sometimes the ones nobody thought to make.

Subject of Research: In vitro dosimetry of microplastics and the distinction between administered and delivered dose in cell toxicity testing

Article Title: In vitro dosimetry of microplastics: the importance of understanding the delivered dose in toxicity testing

Article References: Kloukinioti, M., Neijenhuis, F., Nijman, J. E., Kooter, I. M., van de Steeg, E., Braakhuis, H., & Parker, L. A. (2026). In vitro dosimetry of microplastics: the importance of understanding the delivered dose in toxicity testing. Microplastics and Nanoplastics. https://doi.org/10.1186/s43591-026-00234-0

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00234-0

Keywords: microplastics, dosimetry, in vitro toxicology, polystyrene, delivered dose, administered dose, Stokes' Law, RiskGONE model, THP-1 cells, sedimentation, hazard assessment, nanotoxicology