For decades, coho salmon returning from the Pacific Ocean to the creeks and streams of Puget Sound in Washington state to spawn were dying in large numbers. No one knew why. Scientists working to solve the mystery of the mass deaths noticed they occurred after heavy rains.
Toxicologists suspected pesticides, as the main creek they studied ran through a golf course. But no evidence of pesticides was found. They ruled out disease, lack of oxygen and chemicals such as metals and hydrocarbons.
The first real breakthrough happened when they tested actual runoff collected from a nearby road and exposed test salmon to it. The fish died within hours.
“The harder step was delving into what could be in that stormwater,” said Jenifer McIntyre, an assistant professor of aquatic toxicology at Washington State University, who has spent 15 years searching for what was killing the coho, an important species in the Pacific north-west.
It was when they tested car tyre particles – a poorly understood yet ubiquitous pollutant – that they knew they were on the right track. Using a parmesan grater atop a drill, they carefully shaved tiny fragments of tyre and soaked them in water.
“When we tested the tyres it killed all the fish,” said McIntyre. From there, they were able to identify the culprit: a toxic chemical known as 6PPD-quinone, the product of the preservative 6PPD, which is added to tyres to stop them breaking down. The pioneering study, published in 2020, has been heralded as critical to our understanding of what some describe as a “stealth pollutant”.
Tyre-wear particles – a mixture of tyre fragments, including synthetic rubbers, fillers and softeners and road surface particles – are considered by environmental scientists to be one of the most significant sources of microplastics in the ocean.
Created during acceleration and braking, they are dispersed from road surfaces by rainfall and wind. The main environmental pathway is from road run-off into storm drains, where they empty into rivers and the sea. They are also released from sewage effluent and from the atmosphere, where they can circulate into the ocean and back again. A 2020 study suggested windblown microplastics are an even bigger source of ocean pollution than rivers.
While it is fiendishly difficult to pin down the exact composition of microplastics, there is plenty of research which points to tyre dust making up a significant portion.
In 2017, a global model by the International Union for the Conservation of Nature estimated tyre wear to be the second largest source of primary microplastics in the ocean, at 28%, after synthetic textile fibres, at 35%.
And, in 2019, a report by scientists across Europe concluded abrasion from car tyres was a large source of microplastics and possibly nanoplastics. While there remains a lack of data on risks to the environment and human health, the scientists concluded that if future emissions remain constant or increase “the ecological risks could be widespread within a century”.
One thing is certain. Tyre-wear particles are ubiquitous. The average tyre loses 4kg over its lifetime. About 6m tonnes of tyre particles are emitted annually and have been found everywhere from the deep sea, to the atmosphere, even in the Arctic and the Antarctic.
And it is only going to get worse. Electric cars will lower tailpipe emissions, but tyre wear is projected to rise, due to heavier vehicles and torque (the rotational force of a car engine). The UK’s air quality group warned in 2019 that dust from tyres and car brakes would continue to pollute the air, rivers and ultimately the sea, even when the fleet has gone electric.
In January, McIntyre’s research team published a new study, which found that 6PPD-quinone was “more toxic than previously calculated” to coho and should be categorised as a “very highly toxic” pollutant for aquatic organisms.
Like any detective, McIntyre hopes her team’s work will prompt others to look backwards, at locally extinct aquatic species, to determine whether 6PPD-quinone may have played a role.
Dr Steve Allen, a specialist in atmospheric microplastics at the Ocean Frontier Institute at Dalhousie University in Canada, described the coho salmon study as “landmark”, because it examined the real-world effects of tyre particles.
The study of microplastic is fast moving but still in its infancy. Fewer than 100 scientific papers about them have been published to date, said Allen, all of them in the last decade.
Siobhan Anderson, the co-founder and chief scientific officer of the Tyre Collective, a group of masters students who designed a device to collect microplastics directly from tyres, calls tyre dust “a stealth pollutant” because few people know about it. “There is very little public awareness,” said Anderson, whose organisation is in talks with Volvo and Seft about development of its device.
“Tyre wear is unique in that it can count as microplastic but it is also air pollution because it’s so small,” she said. “Anything that is 10 microns can be inhaled in our lungs and anything that is 2.5 microns has the potential to pass the membrane barrier,” Anderson said.
Tyre dust particles have been found to be smaller than 23 nanometers (0.02 microns).
Edward Kolodziej, an associate professor at the University of Washington and co-author of the coho study, cites two studies from China showing that tyre dust is an important contributor to urban air pollution. “It’s not just the roadway runoff and stormwater getting into the river that kills the fish, there’s also unknown or poorly characterised chemicals present in these things that are ending up in our lungs.”
Kolodziej is concerned about the big data gaps in our knowledge of the effects of thousands of chemicals in the environment. “As a society, we’re literally making 300,000 chemicals, of which 20,000 to 30,000 are the most commonly used,” he said. “Between 90% and 95% of chemicals have had no assessment of what they do in the environment.”
“All these chemicals are proprietary, confidential business information. When you go and buy a product like a tyre, nobody’s being told about what chemicals are in it.”
Frédérique Mongodin, the senior marine litter policy officer of Seas At Risk, said she is very concerned about the “chemical cocktail” from tyres. “Tyre dust is impossible to control. We are pushing the EU to introduce measures at the design stage.”
The $264bn (£194bn)-a-year tyre industry is countering scientific studies on tyre wear and microplastics with research of its own.
The Tire Industry Project (TIP), a body representing 10 tyre manufacturers including Goodyear, Michelin and Pirelli, has commissioned multiple studies over the last decade, concluding that TRWP (tyre and road wear particles) presents no environmental and health risks.
Gavin Whitmore, the communication manager of TIP, disagrees that tyre wear is a major source of ocean microplastics.
“We’re finding something like 2-5% maximum of TRWP is reaching the ocean,” he said, citing a two-part study published in 2018, commissioned by the European Tyre and Rubber Manufacturers Association, which used the Seine watershed as a case study.
Environmental groups, however, have questioned the independence of this research.
Anne Cécile Rémont, the director of the TIP said that after the coho salmon study, the US Tire Manufacturing Association (USTMA) has been involved in discussions with regulators and stakeholder on “potential alternatives” to 6PPD. A proposal in California, where the loss of coho salmon has significantly affected Indigenous communities, would require tyre makers to consider safer alternatives to 6PPD. USTMA has said it supports the proposal.
Asked if the industry was prepared to be more transparent about the chemicals in tyre wear to speed up research, Rémont said the formula was what gives a manufacturer competitive advantage. “Sharing ingredients is very difficult and complicated,” she said but added that the USTMA is developing a “surrogate test material” for researchers.
But experts are calling for more transparency from the tyre companies. It took decades for scientists to narrow down which chemical was causing the mass die-offs of coho salmon in Washington state.
“Very few people, except manufacturers, know what is in the tyres,” said Allen. “There are thousands upon thousands of chemicals. What happens if two of them get together? When it comes to microplastic, we don’t know what a safe level is and we may have already passed it.”