Whether it's our bloodstream, brain, or lungs, microscopic fragments of plastic seem to turn up every time scientists scour a new corner of the human body.
The male reproductive organs are no exception.
New research published this month finds microplastics can build up in the testicles of humans and dogs — raising more questions about the potential health impacts of these particles.
Animal studies have shown exposure to microplastics can impact sperm quality and male fertility, but scientists are still in the early stages of translating this work to human health.
"Microplastics are everywhere," says Dr. John Yu, a toxicologist in the College of Nursing at the University of New Mexico and lead author of the study. "The quantification of those microplastics in humans is the first step to understanding its potential adverse effects."
When he set out to do the study, Yu didn't expect microplastics would have penetrated the male reproductive system so extensively, given the tight blood-tissue barrier around those organs. To his surprise, the research team unearthed a wide range and heavy concentration of microplastics in the testicles of about two dozen men and close to 50 dogs.
The results may also be relevant to a well-documented global decline in sperm count and other problems related to male fertility. This trend has been linked to a host of environmental and lifestyle factors, including certain endocrine-disrupting chemicals found in plastics.
The growing numbers of studies like this one are "compelling and should be a wake up call for policymakers," says Tracey Woodruff, director of the Environmental Research and Translation for Health Center at the University of California, San Francisco.
How much and what kind of plastics were in the testicles?
This is the largest study to measure how much of these microplastics that permeate the water, food and even air end up in the most intimate recesses of male reproductive anatomy.
It follows a smaller analysis, published last year by a team in China, that detected microplastics in about half a dozen human testicles and in semen.
For the current study, researchers at the University of New Mexico collected the testicles from autopsies of people ranging in age from 16 to 88 and from nearly 50 dogs after they were neutered at local veterinary clinics.
Dogs can function as "sentinel" animals for disease and harmful chemical exposure because they're so embedded in the human environment, plus canine spermatogenesis is more similar to the human process of producing sperm than lab rats, says Yu.
Instead of trying to count each microplastic particle, the researchers were able to quantify the total amount of plastic by dissolving all the biological tissue and separating out the solids.
About 75% of what remained was plastic.
Polyethylene, or PE, made up a large portion of that. It's the most widely used plastic in the world, showing up in packaging, bags and any number of products.
Matthew Campen, who has examined these tiny particles up close, describes them as "shard-like, stabby bits" because of the way they've become "old and brittle and fragmented."
"What they do in the body, we don't know," says Campen, a professor at the UNM College of Pharmacy and one of the authors of the study, "Obviously, little tiny particles can disrupt the way cells behave."
Polyvinyl chloride — what's in PVC piping — emerged as another prominent culprit and was the second most common in the dog testicles. Vinyl chloride is classified as a carcinogen and long-term exposure, for example in drinking water, can increase the risk of cancer.
What's more, Yu and his team found a correlation between lower sperm count in the dog testicles and the presence of PVC (the analysis couldn't be done on the human samples because of how they had been stored).
There was also an association between greater levels of PVC and decreased weight of the testicles. The same was seen with Polyethylene terephthalate, or PET, another common source of plastic, which recent research suggests may be harmful.
Woodruff says weight is a somewhat "crude" marker for the effects on testicular health, although it's frequently used by regulatory agencies to evaluate the impacts of chemicals.
Implications?
The research comes with many caveats and cannot prove microplastics directly cause problems with male fertility. Nonetheless, Yu says the results are "concerning" and lay the foundation for more targeted studies on the "relationship between microplastic exposure and its potential impact on sperm."
An emerging body of evidence suggests microplastics can have toxic effects on reproductive health.
In a 2022 review of the evidence for the state of California, Woodruff and her colleagues concluded that microplastics were "suspected" to harm sperm quality and testicular health, but she says that may soon tip over from "suspected" to "likely" because more high-quality studies are being published.
"In the history of looking at chemical or environmental health issues, at the beginning you see these indicators of health harms and then those that have some type of evidence behind them just tend to grow," says Woodruff, "I anticipate we're just going to see more health harms from these microplastics."
In the University of New Mexico study, the concentration of microplastics in human testicles was on average three times higher than in dogs.
Campen says there are still many unknowns, like what specific concentration would pose a threat to health, or how that might vary depending on the kind of microplastic or where they accumulate in the body.
"We're just at the tip of the iceberg," says Campen, who has used this same technique to quantify the levels of microplastics in other tissues and organs.
The amount in the testicles is considerably higher than what was discovered in placenta, and second to what they observed in the brain, says Yu.
Exactly how the microplastics are making their way into the testicles requires further study. Campen suspects they could be "hitchhiking" through the gut via tiny fat particles that get metabolized and then fan out across the body.
It's plausible the build-up of microplastics in the testicles could affect reproductive health in any number of ways. Yu says microplastics could physically disrupt spermatogenesis, mess with the barrier between the testicles, or be a vehicle for harmful chemicals.
They could lead to inflammation and cause oxidative stress, which down the road might affect fertility, says Dr. Sarah Krzastek, a urologist at Virginia Commonwealth University.
"It's probably one more piece of the puzzle of things that are contributing to declines in male fertility over the years as these environmental exposures keep accumulating," she says, "We don't know the clinical ramifications of that yet."
Richard Lea, a reproductive biologist at the University of Nottingham, calls the findings "alarming."
"Having something unnatural like that in the testes is not particularly good news for good reproductive health," says Lea.
In his lab, Lea has found that exposure to phthalates, which are chemicals that can leach from plastics, can reduce the ability of sperm to swim and increase the fragmentation of DNA in the sperm head. This is one likely contributor to the decline in sperm quality in household dogs over the last several decades, a trend that mirrors what's seen in humans.
Of course, the testicles are just one part of the male reproductive system.
Lea says there's now research showing these chemical contaminants can affect the hormonal control of reproduction, at different levels in the body, including in the brain.
How to study a substance that is ubiquitous
Dr. Shanna Swan, a reproductive epidemiologist who has documented the global decline in sperm count, says she's concerned about the accumulation of microplastics. But it's not yet clear finding them in the testicles rather than other parts of the body is more worrisome from the standpoint of reproductive health.
For example in her work, she's looked at how prenatal exposure to endocrine-disrupting chemicals like phthalates can affect male reproductive function and "lead to lifetime of reproductive damage."
Swan says a limitation running through many of the recent studies on microplastics is that the samples may be inadvertently exposed to microplastics in the environment and that leads to skewed impressions of what was actually present in the person.
She notes there were similar quality control issues nearly a quarter century ago when scientists first started measuring phthalates in human tissues.
"I think there have to be a lot of caveats saying this is really the beginning," says Swan, a professor at the Icahn School of Medicine at Mount Sinai, "It's suggestive, it's important, and it's preliminary."
The University of New Mexico researchers developed a quality control process to protect the samples from being accidentally exposed to microplastics as much as possible. Campen says there's so much plastic in the human body, the amount that might contaminate the samples is "trivial."
More broadly, though, he acknowledges the field faces some huge challenges moving forward — especially as they try to draw a stronger link between these tiny particles and a decline in reproductive health or disease.
"A lot of the problem is they're so ubiquitous. There are no proper controls anymore. Right? Everybody's exposed," he says.