WASHINGTON — An estimated 1.2 million tons of tire wear particles are released globally each year, according to a 2017 study in the journal Environmental Science & Technology. And inside those particles, a chemical transformation is taking place that researchers identified in 2020. The bottom line: The additive meant to keep tires from degrading on the road is generating a compound that kills coho salmon.
Here is the chemistry. Tire manufacturers add a chemical called 6PPD — N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine — to rubber as an antioxidant and antiozonant.
Its job is to prevent the tire tread from degrading. But as the tread wears, tiny rubber particles break off. Those particles carry 6PPD into the environment.
When that chemical meets ozone — O3, the three-oxygen-atom gas that exists both in the stratosphere and at ground level as a pollutant — it reacts. The product is 6PPD-quinone, or 6PPD-Q.
University of Washington researchers first identified this reaction in 2020. They linked 6PPD-Q directly to acute toxicity in coho salmon. The finding was a surprise to environmental chemists and toxicologists who had been studying tire-wear pollution for years without knowing what specific compound was killing the fish.
The tire industry has been aware of 6PPD’s function for decades. It is a workhorse chemical, added to virtually every tire on the road. The problem is not the additive itself but what happens to it after the tire sheds particles.
Ozone does not just stay in the upper atmosphere; at ground level it is a common pollutant. So the reaction between 6PPD and ozone is happening constantly on roadsides, in stormwater runoff, and in waterways that receive road drainage.
The mechanism
Tire wear particles are now recognized as a major source of microplastic pollution globally. The 1.2-million-ton figure from the 2017 study gives a sense of scale. Those particles do not biodegrade quickly.
They accumulate in soil, in rivers, and eventually in oceans. For years, researchers knew these particles contained chemicals that could leach out, but they did not know which ones were most toxic.
The 2020 University of Washington discovery changed that. By identifying 6PPD-Q as the culprit behind coho salmon die-offs, the researchers gave regulators and the tire industry a specific target. The chemical reaction is straightforward: 6PPD plus ozone yields 6PPD-Q.
The transformation happens on the surface of the tire particle as it sits in the environment. Ozone itself is a gas composed of three oxygen atoms.
In the stratosphere, it forms a protective layer that shields the Earth from ultraviolet radiation. At ground level, it is a different story. Ground-level ozone is a respiratory irritant and a key component of smog.
It is also, as this research shows, a reactant that can turn a tire antioxidant into a potent aquatic toxicant.
The players
The University of Washington team that made the 2020 discovery has continued to study the problem. Their work has drawn attention from state and federal environmental agencies, particularly in the Pacific Northwest where coho salmon runs have been declining. Stormwater runoff from roads has long been suspected as a factor in salmon deaths, but the specific chemical mechanism was unknown until 6PPD-Q was identified.
By the numbers, the problem is large. With an estimated 1.2 million tons of tire wear particles released globally each year, the volume of 6PPD-Q being generated is substantial.
Not every tire particle contains the same amount of 6PPD, and not every particle will react with ozone to the same degree. But the chemical reaction is happening at scale, wherever tires meet pavement. The tire industry is now under pressure to find alternatives. 6PPD has been a standard additive for decades because it works well and is relatively inexpensive.
Replacing it would require reformulating tire rubber compounds, testing new antioxidants for performance and safety, and retooling manufacturing processes. That takes time and money.
What to watch
Regulators in several states are looking at whether to restrict the use of 6PPD in tires. California, Washington, and Oregon have shown interest, given their salmon populations and the documented toxicity of 6PPD-Q. The U.S. Environmental Protection Agency has also begun evaluating the chemical under its existing chemicals review program.
The bottom line is that this is not a hypothetical problem. The reaction between 6PPD and ozone is happening now, on every road in every city.
The 1.2 million tons of tire wear particles released each year are carrying a chemical that, once transformed, kills coho salmon. The science is settled on the mechanism. What remains unsettled is what the industry and regulators will do about it.
