It's a disturbing thought: At this very moment, tiny crumbs of plastic are trickling through our bodies, a parade of unwelcome houseguests ready to take up residence in some tissue or organ.
A wave of new studies has come out recently, and each one seems to paint an ever more vivid picture of how microplastics — and their smaller counterparts, nanoplastics — have infiltrated the deepest corners of our anatomy. The lungs, liver and heart, guts and brain, even the testicles and placenta — nothing seems to be spared.
The outpouring of research has brought enormous visibility to how these fragments permeate our daily lives. Long studied in oceans, waterways and marine life, researchers have now shifted focus to human health.
A decade ago, Heather Leslie could scarcely find anyone to fund her work in this area.
"It seemed like nobody wanted to touch it," says Leslie, a microplastics researcher in the Netherlands whose team was the first to detect these particles in the human bloodstream several years ago.
As the work has gained momentum, so have questions about the damage microplastics could be doing inside of us. Researchers tend to be wary about making pronouncements because the field is still in a "pioneering phase," as Leslie put it.
And yet there are undoubtedly concerns. Some of the strongest evidence comes from lab studies using animals as well as what's already known about the damaging effects of chemicals added to plastics. A review of the data published Wednesday concludes that microplastics are "suspected" to harm human reproductive, digestive and respiratory health, with a possible link to colon and lung cancer.
"This is a signal that we should be acting now," says Tracey Woodruff, a senior author on the study who directs the Program on Reproductive Health & the Environment at the University of California, San Francisco.
Susanne Brander, an ecotoxicologist at Oregon State University, says it's not helpful to "elicit a gigantic state of alarm," but she agrees that we already know enough about the health risks to push for substantive changes, including a global agreement to curb the rising production of plastics.
As they push to solve the puzzle of microplastics, here are six questions scientists are trying to answer.
1. Which microplastics are actually in our bodies?
Our soil, drinking water and food supply, the air we breathe, all carry microplastics, defined as any plastic particle as small as 1 nanometer and as large as 5 millimeters. Some have built up in the environment over many years, while others arrive daily, as they shed from tires, our clothing, food packaging, personal care products and more.
"Plastics are probably one of the largest exposures we experience as humans and it's been ongoing most of our lives and our parents' lives," says Douglas Walker, a professor of environmental health at Emory University, Rollins School of Public Health.
These particles are endlessly varied, making them challenging to study. Their size and shape, the type of polymer and the chemical makeup can all have consequences for how they accumulate in our bodies and the potential health effects.
Even one microplastic represents an "analytical nightmare," says Leslie. "So it's really hard to make statements over such a broad range of contaminants."
Walker says scientists are still sorting out how to accurately measure microplastics in humans.
The particles that tend to be studied in lab experiments often don't reflect the weathered debris that ends up inside us; labs use a variety of approaches, making it hard to tie together the findings from different studies; and despite advances in technology, detecting the tiniest pieces remains technically difficult.
"I would imagine we're underestimating nanoplastics across the board, including in human tissues," says Brander.
Matthew Campen, whose team has found plastic in a variety of organs, says these can resemble "shard-like, stabby things," which, in some cases, are smaller than a virus.
"You realize, wait a minute, you could fit a lot of these inside even a single cell," says Campen, who researches environmental health and toxicology at the University of New Mexico.
Scientists can isolate microplastics, pull them out of human tissue and take pictures, but seeing where they are inside the body remains a real "struggle," he says.
Despite all the caveats, Campen says the new studies showing microplastics in tissue should be viewed as "linchpins" that will push the field forward. "We need an all-hands-on-deck approach," he says.
2. Once inside of us, do they ever leave?
Imagine being transported to a plastic-free paradise.
How long do you suppose it would take for all those tiny shards to exit your system? Would they ever?
This thought experiment can't be replicated in the real world, since plastic pollution is so ubiquitous.
In essence, Leslie says all of us are being "microdosed" with microplastics around the clock, so there's no way to systematically track how much is coming and going. Even if our bodies are doing an admirable job at clearing out this debris, the constant exposure could make it look like a losing battle.
Research finds microplastics in our stool and urine. And Joana Prata says her review of animal data suggests that, in principle, most of what we ingest or inhale will leave the body during a trip to the bathroom.
"Only a small portion gets absorbed," says Prata, an auxiliary professor at the University Institute of Health Sciences — CESPU in Portugal. "There's still a lot of uncertainty," because the evidence doesn't necessarily reflect the complexities of real-world microplastics.
Campen says you don't see a correlation between age and the concentration of microplastics in human tissue. In other words, it doesn't appear to endlessly accumulate inside us. It's possible that our bodies may reach some sort of "equilibrium" based on how much is around us.
"We don't have enough data," he says, "but [our work] suggests there's a very rapid time to saturation — you do hit a limit and eventually you're clearing it."
A study of Zebrafish (sometimes used in biomedical research) found the uptake of microplastics did plateau at a certain point and levels decreased when the animals weren't being exposed. The problem is the saturation point went up proportionally to how much the animals were exposed to, says Campen.
"That's basically where we are right now," says Campen. "Our environmental exposure keeps going up because we're doing nothing to stop it."
3. What do microplastics do once they get inside our bodies?
Instead of passing through us, some particles move across the thin membrane lining our gut and eventually find their way into the bloodstream.
Size makes a big difference here, Prata says.
"The larger particle will be less likely to cross the biological barrier, but we cannot say that it will never cross," she says. "You can just say it's less probable. "
Brander says there's still debate about the exact size cutoff, in part because that can also depend on the shape of the particle. For example, a long skinny microfiber might be able to sneak through the barrier in our gut more easily than other pieces.
Airborne plastics — particularly common indoors — can also be inhaled. Larger particles are expected to be filtered out to some extent, perhaps snagged in our nose or coughed up. Only the most "ultrafine" pieces will reach the deepest parts of the lungs where they can enter the circulatory system, says Leslie.
Once absorbed into our bloodstream, microplastics and nanoplastics tend to be quickly coated in proteins and fats, creating a corona, or crown-like appearance.
"And then it's essentially going on a trip around your body," Leslie says.
Their exact itinerary remains unclear. But Brander says research on animals has long shown that microplastics can move all over. The same is probably true for humans.
These foreign travelers seem to find a home in many organs, as well as bodily fluids like breast milk and semen. They can even cross the blood-brain barrier.
"It's very unlikely that they're actually metabolized into anything because these are solid particles," says Walker. "So they would be difficult to break down."
Our immune system can't dispatch these bits of plastic as it would other foreign invaders like bacteria. Immune cells, known as macrophages, will release enzymes that do their best to attack these particles, but the "plastic doesn't mind at all," says Leslie. "It remains intact and becomes like a stubborn opponent for your immune system."
Whether microplastics pile up in certain organs more than others remains a big unknown.
Campen and his team suspect the liver is on the "frontline," doing its best to deal with this debris and push it back into the gut with digestive fluids. Their hypothesis is that smaller plastics like nanoplastics slip through the cracks and are repackaged with fats and circulated throughout the body.
This could mean that microplastics build up in organs with greater energy needs, such as the brain, where Campen's lab has documented higher concentrations than in other organs.
4. Are they harming us?
Scientists don't have definitive answers yet.
Humans encounter many pollutants over our lifetime. And given that researchers are still sorting out the best models for analyzing microplastics, many are cautious not to get ahead of the data.
Still, several recent studies have raised troubling warning signs. They've shown associations — not a direct causal link — between the accumulation of microplastics and health problems in humans.
One that gained attention earlier this year came from Italian researchers who found that people with microplastics in the plaques in their arteries were more likely to have a heart attack, stroke or to die. Some small studies have found higher levels in people with inflammatory bowel disease and liver disease. The review published this week from UCSF also included observational research linking microplastics to reproductive health and chronic sinusitis.
Most of the concern comes from lab studies involving animals or cell lines — scientists observed toxic effects of microplastics on the cellular and molecular levels. The "next puzzle piece" is now to figure out how this research translates to health outcomes for an individual or a whole population, says Leslie.
Brander points to evidence that microplastics lead to oxidative stress, which can damage proteins and genetic materials, and spark inflammation.
"If that's happening in fish and in rodents in experiments that are being peer-reviewed and published, it's probably happening in us too," says Brander. "We just haven't demonstrated it yet."
This body of research suggests fertility problems, neurological diseases, harms to metabolism and the immune system, and changes that signal increased risk of cancer, among other effects.
"These are potential effects because of the mechanisms we know about," says Prata.
You can only extrapolate so much from these studies. For example, the doses given to animals in a lab may be much larger than what humans are ingesting and may not reflect the "wild" microplastics we encounter in our daily lives.
In the absence of clear data, Leslie says it can be tempting to make microplastics the "culprit for every disease."
"I'd like to get to the truth of the matter," she says. "I have the feeling that sometimes we might be blaming plastics for things that they shouldn't be blamed for."
In the UCSF analysis, the suggested link with cancer was mostly based on animal research, but Woodruff says this is "standard" for identifying cancer risks and can still be considered high-quality evidence. "In the field of environmental health, when we have concerning signals, we should be concerned."
5. What about the chemicals in plastics — are they hazardous?
Chemicals added to plastics represent another threat.
Some of these can mess with hormones, affect reproductive health, increase the risk of some cancers and cause metabolic problems like obesity, among other things.
Phthalates and bisphenol A, or BPA, are two of the most well-studied examples.
PFAS, also known as "forever chemicals," are also gaining attention.
But in reality, Brander says there are more than 16,000 chemicals used or found in plastics, about a quarter of which are known to be "hazardous" and many of the others are not well studied.
"There's substantial evidence that many of these chemicals cause disease," says Brander.
Scientists worry microplastics could potentially act "as long-term sources of plastic-related chemicals in your body," says Walker.
Despite the uncertainties, Brander believes there's enough evidence to take collective action to limit our exposure to microplastics.
6. How can we reduce our exposure to microplastics?
Scientists are still investigating the biggest sources of microplastics in our daily lives.
Ideally, the task of reducing your exposure shouldn't fall entirely on the individual. But the fact is major policy changes aren't going to happen overnight.
So here's how you can get started in reducing your own exposure: Eliminate single-use plastic as much as you can, and don't reuse these items. Cut back on foods that come in plastic packaging or cans that have plastic linings. Aim for shorter dishwasher cycles, with fewer plastic items.
"Never cook your food in plastic," says Brander. "The heat will drive those particles and more of those chemicals into your food."
Sadly, this applies to your to-go cup of coffee, which is often lined with polyethylene. When possible, buy cleaning products, both for your household and hygiene, that don't come in plastic containers.
Consider investing in a filter for tap water.
Our clothing can also shed a lot of microplastics, which is why Leslie tries to wear fabrics like wool and linen. When you have a choice, avoid synthetic materials in your rugs and furniture, too. Dust can carry microplastics, which you may breathe in. Research suggests vacuuming can help limit what's in your house.
The concentration of these particles indoors is greater than outdoors, so try to keep windows open and improve ventilation.
Leslie's philosophy? There's only so much you can control, so "don't feel guilty" about using plastics — just try to limit your use when possible.
"You can just experiment with saying no to what you don't want," she says. "The resistance to plastic pollution is in our noncompliance. I'm going to keep on doing just that."
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