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Tire-derived microplastics are a growing source of plastic pollution and a target of the United Nations International Plastic Treaty negotiations. Further, concern is growing about the hundreds of chemicals, up to 15 percent of the weight of the tire, that are shed into the environment via tire microplastics. “It is the additives that are the toxic compounds,” says Thilo Hofmann, an environmental scientist at the University of Vienna.

While scientists agree that tire particles contribute significantly to microplastic emissions in the environment, the numbers are difficult to quantify. Recent studies have found tire particles made up to 30 percent of microplastics in Germany, roughly 54 percent in China, 61 to 79 percent in Sweden, and a whopping 94 percent in Switzerland.

Researchers have already demonstrated that some crops, including lettuce and fruits, can take up microplastics, possibly putting human health at risk. But a new study has shown for the first time that store-bought lettuce contains chemical tire additives. It is an unexpected finding, according to study co-author Anya Sherman, a doctoral student working with Hofmann at the University of Vienna.

Sherman and colleagues found one or more of the 16 tire additives they looked for in 20 of 28 lettuce samples. The concentrations of tire additives in leafy vegetables were low overall, but two compounds were most common: benzothiazole, used to strengthen rubber, was detected in 12 of the 28 samples; and 6PPD, used to prevent its oxidation, was found in seven.

It’s hard to know the exact source of the pollutants. Leaching from tire-wear particles is a major source of benzothiazoles in the environment, but the compound is used in other applications, including agrochemicals and consumer products. Likewise, 6PPD can be found in sporting equipment and recreation facilities.

Sherman’s methodology, meanwhile, couldn’t target all of the tire additives, and therefore can’t provide the total chemical load in lettuces. “We don’t know the total chemical burden; that’s left out of the conversation,” she says. “Some compounds are toxic or mutagenic at trace levels.” Even less is known about the toxicity of the mixture of chemicals.

Still, the study highlights the increased dangers from our industrialized world. Scientists have documented microplastics in human breast milk, semen, placentas, and blood. These tiny particles can accumulate in organs including the lungs, heart, and brain. Microplastics can have a range of health impacts: They can cause oxidative stress, disrupt metabolism, interfere with gut microflora, disrupt immune systems, and alter reproductive health. Perhaps the biggest concern is cardiovascular distress caused by microplastics.

In March, scientists revealed that people who had microplastics in their carotid arteries had a four-fold higher risk of heart attack or stroke. Perhaps not surprisingly, researchers are urgently trying to determine the degree of microplastic risk from ingestion versus inhalation.

To that end, Sherman’s lettuce findings were a surprise in another regard: How did these chemicals get into lettuce fields in the first place? Of the three most likely suspects—biosolids, atmospheric deposition, and recycled irrigation water—none has emerged as the most likely offender.

Biosolids to Blame?

Sherman analyzed lettuce grown in four countries with very different policies for biosolids or recycled irrigation water—the two most direct avenues by which tire plastics could concentrate in farm fields. Switzerland, for example, has banned biosolids applications; Spain and Italy had the highest and lowest application rate, respectively, of biosolids; and Israel relies heavily on recycled irrigation water. But there was no discernable pattern related to waste application policies, suggesting that these particles may be more ubiquitous than anticipated.

“There are so many different pathways by which contaminants can reach fields,” Sherman says. “We are nowhere close to understanding the full picture yet.”

Amid nutrient scarcities, many countries around the world, including the U.S., are dramatically increasing the application of biosolids to farmlands. But because pollutants can concentrate in biosolids, some scientists are concerned that soil biosolid applications could exceed the high concentrations have been found in marine environments. “The solutions are an attempt to be sustainable, but they could be introducing more contaminants to the agricultural environment,” Sherman says.

Roughly 56 percent of biosolids are applied to the land in California and across the U.S.—but state and county policies are sharply divided on their use. “The percentage of biosolids application varies by state,” says Scott Coffin, a research scientist at California’s Office of Environmental Health Hazard Assessment. Some states are near 0 percent; others are near 80 percent.

Atmospheric Microplastics and Chemicals

“Microplastics are very light,” Leonard says. They also don’t like water, and therefore they are less bound to the soil, which makes them loft into the air at windspeeds far lower than expected for bare soils. As a result, Leonard says, the current dust emission models may underestimate the microplastic component of dust from biosolid-amended soil. It may also help explain why microplastics are able to travel thousands of miles and contribute an estimated 6.6 million U.S. tons of tire particles globally per year, equivalent to approximately 5 percent of airborne ambient particulate matter concentrations.

That includes microplastics from tires, which tend to be overlooked, due to the technological challenges in identifying them. The biggest problem? Black microplastics, including tire wear particles, absorb (rather than reflect) radiation from the instrumentation used to find them.

An alternate approach exists to detect tire microplastics, one that involves heating up a sample to measure its composite chemicals via gas chromatography and mass spectrometry. But few laboratories have this equipment, Coffin says. “That’s why the tire particle aspect of microplastics wasn’t really considered until quite recently; they were just simply not detected.”

Biosolids are complex mixtures of nutrients and pollutants from disparate sources, and they present difficult challenges when trying to separate out microplastics. Scientists have to know which compounds they are searching for, as well as their breakdown products. Given there are thousands of chemicals in tires, it’s literally impossible to trace the environmental fate of all of them.

Furthermore, tire producers do not disclose what additives are used in tires because they’re considered a trade secret. “[Tire additives] are not regulated, which may change in the coming years,” Hofmann says.

Microplastics and Chemicals in Irrigation Water

Given that water is easier to work with than solids, the scientific community has begun to develop a methodology to quantify microplastics in aquatic environments.

“We were strategically using our very limited resources dedicated to microplastics on what we think that we can make the most progress on in the short term; pretty much all of our effort is focused on the marine environment,” Coffin says. Environmental researchers have so far developed hazard thresholds in marine environments, to be adopted by the California State Water Board, to evaluate water body impairment. For a long time, Coffin adds, “the conversation about water has detracted from what’s happening on land.”

Water is also far easier to monitor—and treat—than biosolids, Coffin says. “Treating biosolids is effectively out of the equation,” he says. “Even if we do determine this is a huge problem, we’re basically left trying to find solutions upstream,” he adds, meaning preventing microplastics from getting into biosolids to begin with. There’s also little incentive to challenge the use of biosolids in agriculture, as it’s been touted as an example of sustainable return of nutrients to the soil.

In response to a lawsuit by the Yurok, Port Gamble S’Klallam, and Puyallup tribes, the U.S. Environmental Protection Agency (EPA) is currently reviewing 6PPD as tire makers scramble to come up with alternatives. California’s Department of Toxic Substance Control, which is also part of the state EPA, has a consumer products section that is evaluating safer chemical alternatives to replace 6PPD in tires as well.

Forging Ahead with Research

“We’re not even close to developing standardized methods for detecting microplastics in biosolids or soils or terrestrial samples,” says Susanne Brander, who studies microplastics at Oregon State University in Corvallis.  “Gathering data on [microplastics in] food systems is where [research] needs to go next.”

That research is starting to get underway. Funding to study plastics in agriculture is limited, but Brander says that the USDA is prioritizing microplastics research going forward. Oregon Sen. Jeff Merkley is sponsoring a Research for Healthy Soils Act to fund studies on microplastics in land-applied biosolids.

Although this is a move in the right direction, it sidesteps the main problem. “Those of us who are concerned and have been doing research for a decade are pushing for source reduction and waste management approaches that don’t create more problems,” Brander says. She says the singular focus on 6PPD in recent years risks overlooking the impacts of all the other tire chemicals that are leaching into the environment.

“We know enough to act—that’s the feeling and opinion of most of the other scientists in the [U.N.] global plastics treaty,” Brander says. “We need to push for chemical reduction and a reduction in the production of virgin plastics.”

Reporting for this piece was supported by the Nova Institute for Health.