Tire toxicity faces fresh scrutiny after salmon die-offs

For decades, concerns about automobile pollution have focused on what comes out of the tailpipe. Now, researchers and regulators say, we need to pay more attention to toxic emissions from tires as vehicles roll down the road.

At the top of the list of worries is a chemical called 6PPD, which is added to rubber tires to help them last longer. When tires wear on pavement, 6PPD is released. It reacts with ozone to become a different chemical, 6PPD-q, which can be extremely toxic—so much so that it has been linked to repeated fish kills in Washington state.

The trouble with tires doesn’t stop there. Tires are made primarily of natural rubber and synthetic rubber, but they contain hundreds of other ingredients, often including steel and heavy metals such as copper, lead, cadmium, and zinc.

As car tires wear, the rubber disappears in particles, both bits that can be seen with the naked eye and microparticles. Testing by a British company, Emissions Analytics, found that a car’s tires emit 1 trillion ultrafine particles per kilometer driven—from 5 to 9 pounds of rubber per internal combustion car per year.

And what’s in those particles is a mystery, because tire ingredients are proprietary.

“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 by virtually everyone, where there is so little known of what is in them.”

Tests by Emissions Analytics have found that tires produce up to 2,000 times as much particle pollution by mass as tailpipes.

These particles end up in water and air and are often ingested. Ultrafine particles, even smaller than PM2.5, are also emitted by tires and can be inhaled and travel directly to the brain. New research suggests tire microparticles should be classified as a pollutant of “high concern.”

In a report issued last year, researchers at Imperial College London said the particles could affect the heart, lungs, and reproductive organs and cause cancer.

People who live or work along roadways, often low-income, are exposed to more of the toxic substances.

Tires are also a major source of microplastics. More than three-quarters of microplastics entering the ocean come from the synthetic rubber in tires, according to a report from the Pew Charitable Trusts and the British company Systemiq.

One outstanding research question is whether 6PPD-q affects people, and what health problems, if any, it could cause. A study published in Environmental Science & Technology Letters found high levels of the chemical in urine samples from a region of South China, with levels highest in pregnant women.