There are two main types of microplastics. One is primary microplastics, which are small to begin with, like the microbeads sometimes added to personal care products. Then there are secondary microplastics: fragments, fibers and films that break off from larger waste plastics not properly disposed of in the landfill or recycling center.
While cheap to produce and versatile, plastic doesn’t decay in the same way a banana peel or raked leaves do. Plastics are manufactured from oil and natural gas and made of repeating chains of carbon molecules, with extra chemicals added to improve their structure. But those durable polymers don’t decompose into simpler molecules like food waste does, at least not in a timeframe that anywhere near keeps pace with the volume of plastic winding up in the environment.
Instead, once in the environment, the forces of the sun, wind and water weather plastic into smaller pieces that still have the same chemical structure. The half-life of a piece of plastic in the environment can be over 1,000 years.
“If these things are getting into our drinking water sources, we should really care,” said Katie Senft, a staff research associate at UC Davis’ Tahoe Environmental Research Center, “especially if they’re not going anywhere and we don’t know the long-term implications.”
In 2018, Senft began peering into the seemingly pristine waters of Lake Tahoe in search of microplastics. Every month for a year, her team piloted boats across transects of the lake, towing nets with holes small enough to filter coarse ground coffee. The researchers took the contents of the nets back to their lab, where they separated out organic material like pine needles.
Then, they handed over the samples to Jenessa Gjeltema, an associate professor of medicine and epidemiology in the School of Veterinary Medicine, whose team used a laser analysis that detected different kinds of plastic.
“What we found was that Lake Tahoe has one of the highest averages of microplastics per square kilometer of surface area of any freshwater lake in North America that’s been studied so far,” said Senft. “Our average was around 306,000 particles per square kilometer.”
That amount was startling because Lake Tahoe, unlike many other rivers and lakes, does not have any wastewater routed to it (wastewater often contains household sources like fibers from washing machines). This alpine lake’s sources of microplastics are more likely to be waste left behind from recreation, including plastic bottles, fishing gear and floaty toys.
The ocean is another destination for microplastics, which often drift in runoff and wastewater. Researchers have estimated that more than 170 trillion plastic particles, mostly microplastics, are floating in the world’s oceans.
Microplastics in the oceans travel long distances before settling on the seafloor. And the flecks of waste not only pose ingestion risks for marine life; they may also harbor pathogens that have hitched a ride from inland.
Not only can microplastics transport pathogens, but they also pose health risks when inhaled and or ingested.
While there is little research in human populations, a growing body of research in rodents is signaling the potential dangers of microplastics. In a screening of almost 2,000 studies prepared by the California State Policy Evidence Consortium, a collaboration of the University of California, a team of researchers concluded that microplastics are a suspected cancer-causing agent in the digestive system and may pose hazards to the reproductive and respiratory systems as well.
Some plastics are also toxic in indirect ways. Amelie Segarra, an assistant adjunct professor of anatomy, physiology and cell biology in the School of Veterinary Medicine, is studying a chemical derived from a widespread tire additive that poses dangers to already at-risk fish populations.
Starting in the late 1980s, Washington researchers puzzled over spikes in coho salmon deaths following surges in storm runoff entering streams. Finally, in 2021, researchers in Washington figured out a chemical in the stormwater called 6PPD-Q was responsible for the mortality.
Tire manufacturers add 6PPD to nearly all auto tires to increase durability. As vehicles drive, tire particles containing plastic and 6PPD break off. The 6PPD reacts with ozone in the air to form 6PPD-Q. When it rains, the 6PPD-Q is washed away into streams and rivers.
Recently, Segarra tested the risks 6PPD-Q poses on several California fish species of conservation concern. At a concentration of just 100 nanograms per liter, half of the coho salmon in experiments died. The finding is worrisome because similar concentrations have been measured in the San Francisco Bay, said Segarra.
Even other fish species—Chinook salmon, steelhead, Delta smelt, and longfin smelt—that survived the exposure still showed behavioral effects. Some surviving fish were bolder, increasingly swimming in the center of the tank as opposed to staying near the wall, a change in behavior that would leave them vulnerable to predators in the wild.
The threat of microplastics can feel overwhelming in the face of the estimated 242 million metric tons of plastic waste generated every year. But experts say a combination of individual actions and policies can clean up our air and water, reducing risks associated with plastic pollution.
Some governments have started to address microplastic sources. In 2015, California banned microbeads in cosmetics. Several U.S. states have also banned plastic bags, requiring consumers to purchase one if they need it for groceries. France recently mandated that laundry machines include filters capable of capturing the stray filaments that slough off from synthetic clothing.