Auto tires shed dangerous microplastics and harmful chemicals.
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In the last few years, vehicle tires have emerged as a shockingly prolific producer of microplastics. It probably shouldn’t come as a surprise. Each year, roughly 3 billion new tires are made, consisting of synthetic rubber, which is a plastic polymer, as well as natural rubber, metal, and other materials. And each year, about 800 million of them become waste.
As tires wear down—from contact with the road or the friction of the
brakes—they shed chemical-laden particles, and those chemicals, it turns out,
can find their way into crops.
A new study has shown for the first time that
store-bought lettuce contains chemical tire additives.
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?
As tire particles are shed on roadways, they
are often washed into water catchments by rain. From there, microplastics can
become concentrated in wastewater, where the waste products—biosolids or
as irrigation water—can
be applied to the land.
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
When microplastics are incorporated into
soil, they behave differently from soil particles: They are more easily carried
by wind. In January, Jamie Leonard, a UCLA Ph.D. candidate, found microplastics
in wind-blown sediments from fields amended with biosolids.
“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
Evidence of microplastics’ toxic impacts has
largely been found in marine and freshwater systems, because it’s relatively
easy to measure microplastics in water, says Coffin. In 2020, for instance,
researchers identified 6PPD-quinone, the breakdown product of 6PPD, as the
culprit behind massive salmon
deaths in Washington after storms washed tire particles into
streams.
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
Despite all these efforts, researchers are
not yet able to determine the health threat of the tiniest microplastics.
That’s because it’s not yet possible to detect the smallest, most hazardous
particles. “Below 10 micrometers is when we start to care about [the health
effects of] particles that we’re ingesting—and we can’t detect those in the
environment with standardized methods yet,” Coffin says. While researchers
continue to make progress developing detection methods for water, the
monitoring campaigns are expensive and scientifically challenging, he adds.
“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.
Virginia Gewin is a freelance science
journalist who covers how humans are profoundly altering the environment – from
climate change to biodiversity loss – and undertaking extraordinary endeavors
to preserve nature. Her work has appeared in Nature, Popular Science,
Scientific American, The Atlantic, Bloomberg, bioGraphic, Discover, Science,
Washington Post, Civil Eats, Ensia, Yale e360, Modern Farmer, Portland Monthly
and many others. Read more >
