Young adults today are relatively healthy—they generally drink less, exercise more, and eat healthier than older adults. But while the rates of colorectal cancer are declining overall, they’re steadily rising in people younger than 50.

An estimated 70% of early-onset colorectal cancer cases are sporadic, meaning they’re not linked to inherited genetic conditions or chronic inflammatory conditions, like Crohn’s disease or ulcerative colitis. What’s causing these cancer cases?

To answer that question, researchers are turning to environmental factors. There are likely many overlapping factors at play, but emerging research points to microplastics as a key suspect.

Early-onset cancers started rising in adults born after 1950, around the time mass plastic consumption took off. Between 10 and 40 million tons of plastic pollute the environment annually, and that rate is expected to double by 2040.

Plastics are now ubiquitous. They’re found in kitchenware, clothes, toys, and countless other products that people interact with daily. Over time, those items break down into tiny bits that people can inhale, ingest, or absorb through their skin. Scientists have found microplastics throughout the body, in blood clots, the brain, lungs, breast milk, and more.

Several studies have shown a correlation between microplastic exposure and colorectal cancer risk, but scientists can’t yet explain how microplastics disrupt the gut and spur cancer.

“We’re seeing epidemiological associations. We see the rising incidence of cancer, especially in younger individuals, that kind of coincides with the accumulation of plastic in the human body. And then there are also these laboratory studies that are showing that plastic can have these carcinogenic effects,” said Thejus Jayakrishnan, MD, a gastrointestinal medical oncologist at Dana-Farber Cancer Institute.

How Might Microplastics Contribute to Colorectal Cancer?

The gastrointestinal tract is lined with a protective layer of mucus that plays a crucial role in trapping and expelling pathogens and other foreign particles. As the gut breaks down food into nutrients, they pass through the mucosal membrane to enter the bloodstream and nourish the body.

During the digestive process, bacteria release toxins into the gut. Some foods, like alcohol, red meat, and processed foods, can increase toxin production.

The mucosa helps keep these toxins away from the intestinal walls and the rest of the body. Over time, however, the toxins can start to damage the intestinal walls, explained Frank Frizelle, MBChB, MMedSc, a professor of colorectal surgery at the University of Otago in New Zealand.

Typically, it takes decades for that incremental damage to lead to cancer development.

“If you look at younger people today, they eat less red meat, they eat more vegetables, they drink less alcohol, they smoke less—all the conventional risk factors for bowel cancer. Despite what older people may think, when you look at the hard data, they’re just healthier people,” Frizelle told Verywell.

If the traditional dietary and lifestyle factors that often describe sporadic colorectal cancer in older adults aren’t as common in the younger cohort, something else is likely at play.

Poking Holes in the Protective Barrier

Frizelle leads a research team that uses tissue samples and animal models to tease apart the possible ways microplastics interact with the gut lining.

He explained that the mucus that lines the gut has two layers. The outer layer interacts most with the food and other contents of the gut, while the inner layer is more stable and prevents foreign particles from entering.

His team’s working hypothesis is that microplastics ingested into the hollow part of the gut could act like a dead weight in the mucosal layer. “They might just carry straight through to the bottom, therefore allowing a pathway in for the toxins,” Frizelle said.

It’s a bit like making tiny pinpricks in a condom, he says. Once those holes exist, toxins can leach through and damage the intestines.

“Microplastics themselves don’t cause cancer. If we’re right, which is a big if, then it is the impact of them disrupting the process that’s important,” Frizelle said.

If that model turns out to be wrong, there are plenty of other potential explanations.

The mucus layer is hydrophobic, meaning it attracts fat but repels the fluids that fill the intestine. Plastics are also hydrophobic. Rather than clinging to the watery substances in the hollow part of the gut, they may glom onto fat, including the mucosal lining. This may allow plastics to attach to and pass through the mucosa, potentially opening pathways for toxins to come through.

The gut also has a protective mechanism called the tight junctions. Those are bonds between cells that keep toxins out while letting nutrients through. Scientists think that plastics can disrupt this layer, burrowing between the tight junctions that hold the intestinal cells together.

Frizelle wonders, too, whether the size of the plastic bits matters. Microplastics are less than 5 millimeters in size, or about the size of a pencil eraser. Nanoplastics, meanwhile, are less than 1,000 nanometers—the size of a bacterium.

Researchers studying lung diseases have established that the smallest particles of toxins in the air—like those inhaled through wildfire smoke—can cause the most damage when they pass through the walls of the tiny blood vessels in the lung and cause inflammation throughout the body.

Some studies suggest that nanoplastics can be more reactive and toxic to the body than larger microplastics, but much more research is needed to confirm the theory.

Disrupting the Balance of the Microbiome

Microplastics may also be driving colorectal cancer by messing with the microbes that live inside the gut.

A healthy gut is teeming with beneficial bacteria and plenty of prebiotic fiber for it to consume. If the gut microbiome has too many harmful microbes, or if those microbes don’t have enough fiber to feed on, they’ll begin to eat away at the mucosal lining.

Over the past several decades, scientists have identified several bacteria that can contribute to cancer growth, such as Fusobacterium nucleatum and E. coli.

When the gut microbiome is in a state of dysbiosis, it can cause chronic inflammation, which can make conditions ripe for cancer development.

“We see dysbiosis associated with cancer. We see some bacteria are carcinogenic. And then we also see this microplastic exposure association with increasing incidence of cancer. When you link all this together, there are clear correlations or associations,” Jayakrishnan said.

Ferrying Bacteria and Carcinogenic Chemicals

Jayakrishnan works with the Young-Onset Colorectal Cancer Center, which is working to understand just how microplastics and microbes interact.

One idea they’re investigating is whether microplastics can harbor cancer-causing bacteria. Scientists have found bits of plastic in the ocean that are coated in communities of microorganisms, including harmful bacteria.

Jayakrishnan said the microplastics in the body may play a similar role, offering a medium for cancer-causing bacteria to grow and disrupt the gut microbiome. If microplastics can push through the mucosal lining, they may even ferry those bacteria to the intestines.

Plastics themselves are chemically inactive. However, they’re often mixed chemicals that can be carcinogenic, said Somdat Mahabir, PhD, MPH, program director in the Environmental Epidemiology Branch of the Epidemiology and Genomics Research Program at the National Cancer Institute’s Division of Cancer Control and Population Sciences.

“Microplastics are not simply small foreign objects in the body,” Mahabir told Verywell in an email. “They come in different sizes, shapes, chemical composition, and added-on chemicals.”

For instance, some plastics contain PFAS, also known as forever chemicals, which have been shown to cause the development of various cancers.

These chemicals may mess with the balance of the gut microbiome. It’s also possible that some of those chemicals damage DNA directly, causing cells to mutate and become cancerous, he said.

The Next Steps for Finding Answers

Scientists have identified several theoretical ways that microplastics could contribute to cancer. But establishing causality is another task.

When researchers sought to establish a link between smoking and lung cancer in the 1950s, they could compare groups of people who had smoked for years to those who had never smoked. This model is harder to replicate with plastic exposure, Jayakrishnan explained.

“The problem is it’s so ubiquitous that it would be very difficult to divide people into groups of those who had been exposed or not. It will also be unethical to expose someone to high concentrations of microplastics and see if there’s a higher risk of cancer and any other health effects,” he said.

Jayakrishnan said his team collects stool, blood, and colonoscopy biopsy samples from people who are diagnosed with cancer to test their microplastic concentration. Testing those samples can help researchers better understand where microplastics appear in the body and how they interact with tissues.

That work could help researchers understand if there are certain microbial or metabolic markers in their genome showing that they are at greater risk for developing cancer, so they can receive earlier treatment or take preventive measures.

Frizelle said that if a model proves promising in tissue samples, the next step is to test whether it holds up in animal studies.

“This has been a lot of trial and error. Research is about 20 failures to one success,” Frizelle said.

He also compared the field with the revelation that smoking caused cancer. Though plastics have been part of the environment for decades, popular interest in their carcinogenic effects is just picking up steam.

“This is new stuff,” Frizelle said. “I think we don’t appreciate the damage microplastics are doing at the moment because we just don’t really have a perception of it. But I suspect it’s a bit like putting sand in the engine—it’s probably doing a lot more damage than we think.”