Polystyrene nanoplastics caused different types of damage to the reproductive systems of male and female animals, and this harm extended to the next generation. The study results were published in the journal New Contaminants.
For decades, plastic pollution in the environment has been a fact of life. Exposed to the elements, this plastic breaks down. Now, tiny fragments are being detected in human blood, urine, breast milk, semen, and placental tissue.
A review of 114 studies involving animals and cells revealed that one specific type of plastic—polystyrene—can affect the reproductive systems of males and females differently. The offspring of exposed animals can also be affected.
The review was led by Chunhua Zhang of South China University, along with Liyao Huang and Wanjing Liu.
“Our analysis shows that the reproductive toxicity of polystyrene nanoplastics is not limited to any single organ or biological pathway,” Zhang said. “Data from experimental models indicate impacts on male and female reproductive systems, as well as on embryonic and offspring development. Understanding these mechanisms is crucial for assessing potential health risks and prioritizing future research.”
Plastic fragments smaller than 5 millimeters—roughly the width of a pencil eraser—are known as microplastics.
When their size is reduced to less than 100 nanometers—approximately one-thousandth the thickness of a human hair—they become nanoplastics. At this size, they can penetrate cells and cross certain biological barriers.
Polystyrene is the type of plastic researchers study most frequently, partly because it is widely used in food and cosmetic packaging. It has already been detected in human semen.
Zhang’s team searched four databases and analyzed 285 articles, ultimately selecting 114 that measured the impact of the particles on reproductive function.
In male mice, the particles crossed the barrier separating the testes from the bloodstream. Testosterone levels dropped. Concurrently, sperm count and motility decreased, and abnormally shaped sperm appeared—missing heads, small heads, folded necks, or missing tails.
Oyster sperm exposed to seawater lost 66% of their motility and 38% of their speed. Particles aged under simulated sunlight—mimicking real-world outdoor conditions—suppressed the activity of four genes responsible for reproduction and slowed the development of male gonads in the oysters.
Female mice proved to be more sensitive to these particles than males; this is one reason the review distinguishes between the sexes.
In mice fed these particles over several months, the sacs containing maturing egg cells shrank, and the tissue that produces progesterone after ovulation thinned.
Incidents of egg cell death increased, and reproductive cycles became longer. Blood progesterone levels dropped, fewer embryos implanted, and uterine weight decreased.
Rats exposed to the particles developed ovarian scarring and lost cells that support egg development, resulting in a depleted egg reserve. Zebrafish developed ovulation issues resembling polycystic ovary syndrome in women.
Oysters exposed to the environment for two months produced 38% less spawn, and the remaining spawn was 5% smaller in size.
The study pays particular attention to what happened to animals that were never directly exposed to plastic themselves. Oysters whose parents had been exposed produced 41% fewer larvae, and those larvae grew 18% more slowly.
Roundworm embryos and larvae suffered higher mortality rates. Mice gave birth to fewer live offspring, and those that were born were smaller in stature. Exposed female zebrafish laid significantly fewer eggs, and particles were detected in the organs of the next generation. These young fish exhibited hormonal imbalances. In the group receiving the highest dose, two genes determining sex development, moving in opposite directions.
A common thread runs through this—though it is a correlation rather than a direct cause-and-effect relationship. Polystyrene fragments were found more frequently in the placental tissue of women who had experienced recurrent unexplained miscarriages compared to other women, and the quantity of these fragments correlated with the occurrence of miscarriage.
Other research groups have detected plastic in the bodies of unborn kittens and within the cells of farm animals.
Cells rely on a balance between molecules that strip away electrons and protective mechanisms that absorb them. Disrupting this balance impairs normal tissue function.
Biologists term this state—when it reaches a critical point—”oxidative stress,” and the review identifies it as the primary mechanism underlying almost all the phenomena mentioned above. Polystyrene triggers cells to produce more of these reactive molecules, activating signaling pathways that lead to inflammation and cell death.
In mouse testes, one mechanism amplified inflammatory signals, resulting in reduced sperm quality. In rat ovaries, another mechanism destroyed the cells that support the egg. Hormones are affected via a second pathway: these particles disrupt the communication between the brain and the gonads that regulates sex hormone levels.
Rats exposed to high doses showed reduced levels of testosterone and the pituitary hormone that regulates it, while in zebrafish, estrogen production dropped after the inhibition of the enzyme responsible for its synthesis.
Nanoplastics rarely appear in isolation. They carry other pollutants with them, and it is precisely this combination that the authors highlight for future research. Together, arsenic and polystyrene impaired the testicular barrier function in mice more severely than either substance did on its own.
The same occurred when the particles were combined with dibutyl phthalate—a plasticizer known to cause testicular congestion, damage hormone-producing cells, and reduce the number of developing sperm. In zebrafish, the addition of the painkiller diclofenac reduced hatching rates and led to a higher number of deformed larvae.
Weathering is just as significant as the surrounding environment. Particles aged by exposure to ultraviolet light reduce the reproductive capacity of roundworms and kill more of their embryos and larvae than fresh particles do.
Researchers have not yet established that these particles cause reproductive disorders in humans, and detecting them in an animal does not equate to knowing the dose that animal actually received. Most experiments utilized smooth, spherical, factory-made polystyrene at doses exceeding those a person would encounter in daily life.
Debris weathered on a beach or in a remote canyon has a rough surface, has undergone chemical alteration, and is mixed with whatever substances it picked up along the way. Underlying all of this is the challenge of quantification. Researchers cannot yet measure the amount of this plastic a person carries, so any risk assessment based on this figure remains uncertain.
Plastic has been detected in human reproductive fluids, and its harmful effects have been measured in animals, yet no link has been established between these two facts within the human body. Bridging this gap requires detection methods sensitive enough to count these particles in human tissues, followed by studies on humans rather than cell cultures.
Zhang’s team has prioritized weathered particles and mixtures of pollutants as key factors for the project’s next participants to consider.
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