This article originally appeared at Yale Environment 360.

For two decades, researchers worked to solve a mystery in West Coast streams. Why, when it rained, were large numbers of spawning coho salmon dying? As part of an effort to find out, scientists placed fish in water that contained particles of new and old tires. The salmon died, and the researchers then began testing the hundreds of chemicals that had leached into the water.

A 2020 paper revealed the cause of mortality: a chemical called 6PPD that is added to tires to prevent their cracking and degradation. When 6PPD, which occurs in tire dust, is exposed to ground-level ozone, it’s transformed into multiple other chemicals, including 6PPD-quinone, or 6PPD-q. The compound is acutely toxic to four of 11 tested fish species, including coho salmon.

Mystery solved, but not the problem, for the chemical continues to be used by all major tire manufacturers and is found on roads and in waterways around the world. Although no one has studied the impact of 6PPD-q on human health, it’s also been detected in the urine of children, adults and pregnant women in South China. The pathways and significance of that contamination are so far unknown.

78% of ocean microplastics are synthetic tire rubber, according to one estimate.

Still, there are now calls for regulatory action. Last month, the legal nonprofit Earthjustice, on behalf of the fishing industry, filed a notice of intent to sue tire manufacturers for violating the Endangered Species Act by using 6PPD. And a coalition of Indian tribes recently called on the EPA to ban use of the chemical. "We have witnessed firsthand the devastation to the salmon species we have always relied upon to nourish our people," the Puyallup Tribal Council said in a statement. "We have watched as the species have declined to the point of almost certain extinction if nothing is done to protect them."

The painstaking parsing of 6PPD and 6PPD-q was just the beginning of a global campaign to understand the toxic cocktail of organic chemicals, tiny particles and heavy metals hiding in tires and, to a lesser extent, brakes. While the acute toxicity of 6PPD-q and its source have strong scientific consensus, tire rubber contains more than 400 chemicals and compounds, many of them carcinogenic, and research is only beginning to show how widespread the problems from tire dust may be.

Tire emissions from electric vehicles are 20% higher than those from fossil-fuel vehicles.

While the rubber rings beneath your car may seem benign — one advertising campaign used to feature babies cradled in tires — they are, experts say, a significant source of air, soil and water pollution that may affect humans as well as fish, wildlife and other organisms. That’s a problem because some 2 billion tires globally are sold each year — enough to reach the moon if stacked on their sides — with the market expected to reach 3.4 billion a year by 2030.

Tires are made from about 20 percent natural rubber and 24 percent synthetic rubber, which requires five gallons of petroleum per tire. Hundreds of other ingredients, including steel, fillers and heavy metals — including copper, cadmium, lead and zinc — make up the rest, many of them added to enhance performance, improve durability and reduce the possibility of fires.

Both natural and synthetic rubber break down in the environment, but synthetic fragments last a lot longer. Seventy-eight percent of ocean microplastics are synthetic tire rubber, according to a report by the Pew Charitable Trusts. These fragments are ingested by marine animals — particles have been found in gills and stomachs — and can cause a range of effects, from neurotoxicity to growth retardation and behavioral abnormalities.

Tires release 100 times the amount of volatile organic compounds as a modern tailpipe, says an analyst.

"We found extremely high levels of microplastics in our stormwater," said Rebecca Sutton, an environmental scientist with the San Francisco Estuary Institute who studied runoff. "Our estimated annual discharge of microplastics into San Francisco Bay from stormwater was 7 trillion particles, and half of that was suspected tire particles."

Tire wear particles, or TWP as they are sometimes known, are emitted continually as vehicles travel. They range in size from visible pieces of rubber or plastic to microparticles, and they comprise one of the products’ most significant environmental impacts, according to the British firm Emissions Analytics, which has spent three years studying tire emissions. The company found that a car’s four tires collectively emit 1 trillion ultrafine particles — of less than 100 nanometers — per kilometer driven. These particles, a growing number of experts say, pose a unique health risk: They are so small they can pass through lung tissue into the bloodstream and cross the blood-brain barrier or be breathed in and travel directly to the brain, causing a range of problems.

According to a recent report issued by researchers at Imperial College London, "There is emerging evidence that tyre wear particles and other particulate matter may contribute to a range of negative health impacts including heart, lung, developmental, reproductive and cancer outcomes."

The report says that tires generate 6 million tons of particles a year, globally, of which 200,000 tons end up in oceans. According to Emissions Analytics, cars in the U.S. emit, on average, 5 pounds of tire particles a year, while cars in Europe, where fewer miles are driven, shed 2.5 pounds per year. Moreover, tire emissions from electric vehicles are 20 percent higher than those from fossil-fuel vehicles. EVs weigh more and have greater torque, which wears out tires faster.

Unlike tailpipe exhaust, which has long been studied and regulated, emissions from tires and brakes — which emit significant amounts of metallic particles in addition to organic chemicals — are far harder to measure and control and have therefore escaped regulation. It’s only in the last several years, with the development of new technologies capable of measuring tire emissions and the alarming discovery of 6PPD-q, that the subject is receiving much needed scrutiny.

Recent studies show that the mass of PM 2.5 and PM 10 emissions — which are, along with ozone and ultrafine particles, the world’s primary air pollutants — from tires and brakes far exceeds the mass of emissions from tailpipes, at least in places that have significantly reduced those emissions.

Scientists found that rain gardens could prevent more than 90% of a dangerous tire pollutant from entering streams.

The problem isn’t just rubber in its synthetic and natural form. Government and academic researchers are investigating the transformations produced by tires’ many other ingredients, which could — like 6PPD — form substances more toxic than their parent chemicals as they break down with exposure to sunlight and rain.

"You’ve got a chemical cocktail in these tires that no one really understands and is kept highly confidential by the tire manufacturers," said Nick Molden, CEO of Emissions Analytics. "We struggle to think of another consumer product that is so prevalent in the world, and used