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.
Though 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.
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.
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 Trust. 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.
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.”
“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, the CEO of Emissions Analytics. “We struggle to think of another consumer product that is so prevalent in the world, and used by virtually everyone, where there is so little known of what is in them.”