New urgency for answers on the health impacts of microplastics

Every day, adult men in the United States may be eating or breathing in more than 300 bits of plastic, according to a recent estimate.¹ For women, the tally exceeds more than 250 bits. Every year, we are being exposed to tens of thousands of tiny plastic pieces—and until recently, scientists had little idea of the potential consequences.

“Our exposure is fairly total,” says Megan Wolff, PhD, MPH, executive director of the Physician and Scientist Network Addressing Plastics and Health in New Paltz, New York. “We know that microplastics are in the air, the water, the soil, the food, and pretty much everywhere that we look inside the human body, we find them.”

According to current trends, plastic production is doubling by volume every 14 years and creating vast new opportunities for plastics to break into ever-smaller pieces. Microplastics, typically defined as plastics measuring less than 5 mm in at least one dimension, can subsequently break down into nanoplastics, which measure less than 1 µm in size. At that scale, the tiny particles can be swept through the air with other particulate matter such as soot and can travel through blood vessels and infiltrate human cells.

Because plastics are ubiquitous in consumer products and because we spend most of our time indoors in close proximity to them and their breakdown products in concentrated sources such as household dust, Dr Wolff says that humans have far more exposure than the rest of the animal kingdom. Although many of the smallest bits have evaded traditional detection methods, she adds, every improvement in the technology is revealing more of them. The research is still early, but the danger signs are mounting quickly.

So far, the overall evidence of potential harm has been limited mainly to in vitro and animal studies; even so, the existing studies have pointed to multiple areas of concern. Tracey Woodruff, PhD, MPH, director of the Program on Reproductive Health and the Environment at the University of California, San Francisco, recently helped to draft a review of the effects of microplastic exposure on the human respiratory, digestive, and reproductive systems.

Her team’s review included 28 animal studies and three human observational studies published between 2018 and 2024; roughly three fourths of those studies were conducted in China, whereas none were conducted in the United States. “The United States has actually been very slow to invest in this health-related research,” Dr Woodruff notes. The overview, requested by the state of California and subsequently published as a rapid systematic review, concluded that microplastics have a “suspected” role in harming human reproductive, digestive, and respiratory health and a suggested role in increasing the risk of colon and lung cancer.²

The heightened cancer risk, the review suggested, could be mediated through mechanisms such as inflammation and oxidative stress. Even then, Dr Woodruff concedes that the conclusions offer only a small glimpse of the potential impacts.

Alan Workman, MD, an assistant professor of rhinology and skull-based surgery at Harvard Medical School and Massachusetts Eye and Ear in Boston, cautions that the science of measuring microplastics and nanoplastics in the environment and the human body has been “fraught with peril” and is still in its infancy. He joined the effort after reading early papers about the potential inflammatory effects on epithelial surfaces, such as those lining the nasal and sinus passageways. “Are these microplastics getting embedded in our tissues? Are they transiting through the tissues? And so that’s really what got me interested in it,” he says.

Scientists once thought that microplastics and nanoplastics might be relatively inert within human tissue, Dr Workman says. So far, however, his early data on nasal epithelial cells—derived from healthy human donors and grown in culture dishes designed to mimic the surface of nasal airways—have shown some inflammatory effects after exposure to the plastic bits. Those effects include an elevation of cytokine proteins associated with inflammatory pathways. He is now trying to establish the time course of how that inflammation occurs by photographing the cells at the nanometer scale to see where the plastic particles end up.

After conducting in vitro studies, Dr Workman plans to follow up by exposing laboratory mice to microplastics and nanoplastics. The work, he hopes, will help to set a path toward answering the big question about the pollutants: “If they do have these inflammatory effects in the lab tissues, how is that relevant in our daily environment?”

The science is further complicated by the suite of properties, chemicals, and risks associated with different plastics. “In the world of plastics, we know that vinyl chloride is carcinogenic, and we know that styrene is carcinogenic,” Dr Wolff says. Vinyl chloride is a building block of polyvinyl chloride (PVC), which is used for everything from rubber duckies and sewer pipes to medical tubing and blood bags.

Real-world relevance is a critical issue because one of the biggest questions in the field is how microplastics and nanoplastics damage the body. “Is it mechanical damage because of the particle, or is it chemical damage because of additives?” Dr Wolff says. A shard-like particle, for example, could abrade a cell, disturb the membrane, or create inflammation—the kind of damage that Dr Workman is investigating as a potential contributor to cancer. “It makes just as much sense that the chemical contents of that particle would be doing the same thing,” Dr Wolff says.

The potential one–two punch of mechanical and chemical damage may carry even more weight because plastics such as rubber, polyurethane, polycarbonate, and PVC can contain more than 400 distinct “chemicals of concern.”³ The label reflects their ability to be persistent, bioaccumulative, mobile, toxic, or some combination thereof. “As soon as you start looking at plastics, you realize that what you’re really looking at is chemicals and additives,” Dr Wolff says. Dr Warner echoes her point by noting that up to half of PVC plastic by weight can be chemical plasticizers and other additives that often leach from the surface. Those properties mean that the very same material used in hospitals may be increasing the risk to their patients.

In 2024, the PlastChem Report funded by the Research Council of Norway tallied 16,325 compounds known to be either intentionally or unintentionally present in plastics.³ More than 4200 of them, including estrogen-mimicking bisphenol A and the endocrine-disrupting phthalates, have been deemed chemicals of concern. Of those plastic chemicals, more than 3600 are still unregulated around the world. Most of the remaining ones on the list have not been studied at all.

Although some microplastics are manufactured as inclusions in detergents, cosmetics, paints, and other consumer products, the vast majority of microplastics are the result of plastics degrading over time. The weathering process, whether in the ocean, in the soil, or in abundant sunshine with its ultraviolet rays, can break them into ever-smaller pieces, create more grooves and pores to harbor other chemicals, and provide a platform for bacterial biofilms and viruses.

Although some chemical additives can leach out during that weathering process, Dr Wolff notes that they also can attract more pollutants, in turn making the microplastics and nanoplastics much more toxic than their surrounding environment. Microplastics that carry a positive or negative charge, in fact, could act like tiny magnets to attract a corona of harmful toxins. “Thinking about the environment that exists on these pieces of plastics is almost equally as important as what the plastics are doing themselves,” Dr Workman says.

Understanding how microplastics exist in nature and weather over time has become a priority for Dr Warner’s laboratory and others looking to study the impacts of the most environmentally relevant types. The enormous complexity may be daunting, but the exponential growth in the number of plastic particles has only added to their urgent search for answers.