How microplastics can affect the body and its cells

"Not proven" is not the same as "no effect"

When a scientist says something has not been proven harmful, they are describing a gap in knowledge. It is not an acquittal. With microplastics the gap is also hard to close, for three reasons:

• The decisive experiment can't be done. No ethics board would approve feeding a group of people plastic for ten years to see what happens.
• There is no unexposed comparison group. Everyone takes in plastic, from food, water, air and dust. That makes it hard to compare "with" and "without".
• The effects can take a long time. Diseases such as atherosclerosis, cancer or reduced fertility develop over years or decades, and they have many causes at once.

So a more useful question is: what do we know about how plastic behaves in the body? There is much more to go on there. The research can be divided into three steps, and they have come different distances.

First: the way in, and why size matters

We take in plastic mainly by eating, drinking and breathing in dust and fibres. The larger pieces usually pass through the gut and leave the body. It is the small ones that get further.

The smaller a particle is, the more easily it gets through the body's barriers: the gut wall, the air sacs in the lungs, the walls of blood vessels and finally the cell membrane. Particles under one micrometre are called nanoplastics, and that is where most effects are seen.

A clear example comes from a study in mice. The researchers gave the animals plastic particles of three sizes by mouth. The smallest, around 0.3 micrometres, were in the brain after just two hours. The larger ones did not get there. The same study showed that the particles quickly pick up a layer of the body's own molecules, a so-called corona, which affects how easily they cross the blood–brain barrier.

So far, plastic has been found in human blood, placenta, testicles, liver, kidney and brain, and levels in brain tissue were higher in samples from 2024 than from 2016. That is step one in the figure. The rest of this article is about what can happen once the plastic is there.

1. Oxidative stress: the cell gets too much "rust"

This is the effect that comes up most often in the studies. Cells always produce some reactive oxygen species, a bit like an engine produces exhaust. Normally the body's antioxidants clean them up. When plastic particles are in or around the cell, production goes up and the balance tips.

The result is called oxidative stress: the reactive molecules damage fats in the cell membrane, proteins and genetic material. Review articles describe oxidative stress as a common starting point for many of the other effects below, such as inflammation, cell death and DNA damage.

2. Mitochondria: the cell's power plants falter without the cell dying

A study of human liver and lung cells shows why "no visible damage" does not mean "no effect". The researchers exposed the cells to 80-nanometre nanoplastics. The cells did not die in any large numbers. But under the microscope the particles could be seen inside the cells, and the mitochondria worked less well: they produced more reactive oxygen species, their membrane potential changed and the cell's energy production slowed. The changes appeared even at low concentrations, which the researchers judged could be comparable to levels in the environment.

A test that only counted dead cells would therefore have called the plastic harmless. The change only showed up when the researchers measured how the cells were working. Similar effects on mitochondria have been seen in small lab-grown "mini-brains" made from human stem cells, where nanoplastics also disturbed how the cells developed.

3. Inflammation: the immune system reacts to something it can't break down

The immune system is built to recognise and break down foreign things, such as bacteria and dead cells. Plastic is foreign but cannot be broken down. Researchers have therefore put forward the hypothesis that immune cells can get stuck in ongoing, low-grade inflammation as they try to clear particles that never go away.

The picture is not clear-cut. A large comparison of eight types of plastic in human immune cells found that most did not directly switch on a central inflammation trigger (the NLRP3 inflammasome). But several plastics, including PET and nylon, still increased the release of an inflammatory signalling molecule. Nylon is the same family of materials that most toothbrush bristles are made from.

In animal experiments, nanoplastics caused an inflammatory form of cell death in the liver of mice, and the smallest particles had the largest effect. And in the widely reported heart study from 2024 (more on that below), people with plastic in the plaque in their artery walls also had higher levels of inflammatory markers.

4. The gut: the mucus layer thins and the gut flora changes

The gut is where plastic first meets the body in larger amounts, and it has its own defences: a layer of mucus and a tight row of cells that decide what gets into the blood. In one experiment, mice were given microplastics for six weeks. They produced less mucus in the gut, the gut barrier was damaged and the make-up of their gut bacteria changed.

A more recent study gave mice plastic continuously at levels the researchers considered environmentally relevant. The plastic built up in the gut, the share of bacteria regarded as beneficial fell, the share of potentially harmful ones rose, and both the gut barrier and the gut's immune function were affected. A weakened barrier can in turn let through more of what should stay in the gut, including more plastic.

5. DNA: damage was seen in about half of the experiments

Can plastic damage our genetic material? A 2023 review brought together experiments in which mammalian cells were exposed to nanoplastics. About half of the results (29 out of 60) showed statistically significant DNA damage, such as breaks in the DNA strand or oxidatively damaged DNA. Immune cells were more sensitive than other cell types, and plastic with a modified surface caused damage more often.

This says nothing about cancer in humans. But it shows that the mechanism exists in the lab, and it most likely works through the oxidative stress in point 1. In one experiment, ordinary PET food containers from a supermarket were ground down into nanoplastics. When human lung cells were exposed to the particles, production of reactive oxygen species rose by about 30 percent, and the number of breaks in the DNA strand increased.

6. The chemicals in plastic: hormone disruptors that can leak out

Plastic is rarely just plastic. It contains additives that make it soft, coloured, flame-resistant or UV-resistant. Many of them are not chemically bound to the plastic and can therefore leak out over time, especially with heat, wear and fat.

Two of the most debated groups are phthalates (plasticisers) and bisphenols. Several phthalates are known hormone disruptors that counteract male sex hormones. This is one of the best-supported areas, because it concerns chemicals that have been studied for decades. In the large European programme HBM4EU, phthalates were measured in the urine of children and teenagers. For about 17 percent, the researchers could not rule out health risks from the combined mixture of phthalates.

A microplastic particle that ends up in a tissue can therefore act as a small depot that releases its additives right there.

7. The Trojan horse: particles can carry other things with them

Plastic surfaces attract other pollutants, such as pesticide residues, heavy metals and medicines. Researchers call this the Trojan horse effect: the particle can carry substances into the body that would not otherwise get there as easily.

So far this has mostly been studied in animals that live in water. In experiments with mussels and small crustaceans, plastic combined with other pollutants caused different and sometimes larger effects, including on DNA, than the substances did on their own. How much this matters in humans is not known. But the mechanism shows that the effect of plastic cannot be judged from the plastic alone.

8. The heart and blood vessels: the strongest signal from humans

This is where we find the study that has come closest to step three. In the New England Journal of Medicine in 2024, researchers examined plaque surgically removed from the carotid artery in 257 patients who were followed for almost three years. In about 58 percent, polyethylene was found in the plaque, and jagged particles could be seen inside the tissue under the microscope.

Those with plastic in their plaque had a heart attack, stroke or died about 4.5 times as often during the follow-up. They also had more inflammation in the artery wall.

Cell and animal studies point to how this could happen: plastic particles damage the innermost layer of the vessel wall, affect blood fats, help so-called foam cells form in the plaque and affect blood clotting. These are the same building blocks that drive atherosclerosis in general.

9. Reproduction: plastic in every testicle examined

A 2024 study measured microplastics in the testicles of 23 men and 47 dogs. Plastic was found in every sample, and levels were higher in the humans than in the dogs. In the dogs, where organ weight and sperm count were also recorded, higher levels of PVC and PET went together with lower testicle weight.

Animal experiments have shown effects on sperm shape and motility, on the hormonal axis between the brain and the sex glands, and on the ovaries, through the same routes as above: oxidative stress, inflammation and cell death. Add the hormone-disrupting additives in point 6, and it becomes clear why researchers are watching reproduction closely.

10. The brain: the barrier is crossed

The brain is protected by the blood–brain barrier, which stops most foreign substances. As we saw above, nanoplastics can get past it, at least in mice. Plastic has been found in human brain tissue as small, shard-like particles, and in a group of deceased people diagnosed with dementia there was even more plastic, especially in the walls of blood vessels and in immune cells.

The association does not tell us what came first. A brain that is already ill may have a weaker barrier and therefore let in more plastic. But together with the findings on mitochondria in lab-grown mini-brains, this is an area where research is now moving quickly.

How the mechanisms fit together

On their own, the studies don't prove much. But the same pattern turns up in completely different systems: cells in test tubes, mice, mussels, lab-grown mini-brains and human tissue. Oxidative stress leads to inflammation. Inflammation and stress damage barriers and DNA. Additives and attached pollutants add their own effects. And the smaller the particle, the further in it gets.

There are also objections that belong in an honest picture:

• Lab doses are often high, and many experiments use round polystyrene beads. The plastic we meet in everyday life is a mix of shapes, materials and aged plastic.
• The measuring methods are new. Measuring plastic in tissue is difficult, and researchers are still discussing how accurate the figures are.
• Data from humans are still scarce, and they show associations, not causes.

None of this makes the mechanisms invalid. It means we know more about how plastic can affect the body than about how much it does so in an ordinary person. That reading of the research is more reasonable than both "it's dangerous" and "it's harmless".

What can you do with this?

You can't avoid all plastic, and you don't need to. But when an exposure is unnecessary and easy to remove, there is little reason to wait for final proof. The usual advice from researchers is to cut down on the sources you control yourself:

• Don't heat food in plastic. Heat increases both particle shedding and the leaching of additives.
• Air out your home and dust. Much of what we breathe in is plastic fibres from textiles indoors.
• Choose tap water over water in plastic bottles. A water filter can reduce the number of particles further.
• Replace plastic that wears against your mouth every day, such as your toothbrush, the brush head of your electric toothbrush and dental floss.

If you want to read more about where plastic has been found and how to interpret the risk, see How dangerous are microplastics?.