According to tests, almost 2,000 times more polluting particles are produced by tire wear than by the exhaust gases of modern cars.
Tire particles pollute the air, water and soil and contain a wide range of toxic organic compounds, including known carcinogens, analysts say, suggesting tire pollution could quickly become a major issue for regulators .
Air pollution causes millions of premature deaths worldwide each year. The demand for better filters means that particulate emissions from tailpipes in developed countries are now much lower in new cars, with those in Europe well below the legal limit. However, the increasing weight of cars means that more particles are thrown from the tires as they wear on the road.
The tests also revealed that the tires produce more than a ton of ultrafine particles for every kilometer travelled, that is, particles smaller than 23 nanometers. These are also emitted by exhaust fumes and are of particular health concern, as their size means they can enter organs via the bloodstream. Particles smaller than 23nm are difficult to measure and are not currently regulated in either the EU or the US.
“Tires are rapidly eclipsing the tailpipe as a major source of vehicle emissions,” said Nick Molden, of Emissions Analytics, the leading independent emissions testing firm that carried out the research. “The tailpipes are now so clean for pollutants that if you were starting from scratch, you wouldn’t even bother to regulate them.”
Molden said an early estimate of tire particulate emissions prompted the new work. “We’ve come to a staggering amount of material being released into the environment – 300,000 tonnes of rubber from tires in the UK and US, just cars and vans every year.”
There are currently no regulations on the wear rate of tires or the chemicals they contain. Emissions Analytics has now determined the chemicals present in 250 different types of tires, which are usually made from synthetic rubber, derived from crude oil. “There are hundreds and hundreds of chemicals, many of which are carcinogenic,” Molden said. “When you multiply that by the total wear rate, you get some very staggering numbers as to what is being released.”
The wear rate of different brands of tires varied widely and the content of toxic chemicals varied even more, he said, showing that low-cost changes were feasible to reduce their environmental impact.
“You could do a lot by eliminating the most toxic tires,” he said. “It’s not about stopping people from driving or having to invent new, completely different tyres. If you could weed out the worst half, and maybe line them up with the best in class, you can make a huge difference. But for the moment, there is no regulatory tool, there is no monitoring.
Tire wear tests were carried out on 14 different brands with a Mercedes C-Class driven normally on the road, some tested over their entire lifespan. High-precision scales measured the weight lost from the tires and a sampling system that collects particles behind the tires while driving assessed the mass, number and size of the particles, down to 6 nm. Actual exhaust emissions were measured on four gasoline-powered SUVs, the most popular new cars today, using 2019 and 2020 models.
The used tires produced 36 milligrams of particles per kilometer, which is 1,850 times more than the average of 0.02 mg/km from the exhaust gases. A very aggressive, albeit legal, driving style drove particulate emissions to 5,760 mg/km.
Many more small particles are produced by tires than large ones. This means that while the vast majority of particles by number are small enough to become airborne and contribute to air pollution, they are only 11% of particles by weight. Nevertheless, tires still produce hundreds of times more airborne particles by weight than exhaust gases.
The average weight of all cars has increased. But there has been particular debate over whether battery electric vehicles (BEVs), which are heavier than conventional cars and can have more wheel torque, can lead to the production of more particulates. of tires. Molden said it would depend on driving style, with drivers of mild electric vehicles producing fewer particles than poorly driven fossil-fuel cars, although on average he expects slightly higher tire particles from them. BEVs.
Dr James Tate, from the Institute for Transport Studies at the University of Leeds in the UK, said the tire test results were credible. “But it’s very important to note that BEVs get lighter very quickly,” he said. “By 2024-25, we expect BEVs and [fossil-fuelled] city cars will have comparable weights. Only large, high-end BEVs with high-capacity batteries will weigh more. »
Other recent research has suggested that tire particles are a major source of microplastics polluting the oceans. A specific chemical used in tires has been linked to salmon kills in the United States and California proposed a ban this month.
“The United States is more advanced in its reflection on [the impacts of tyre particles]”, Molden said. “The European Union is behind the times. Overall, it’s only the beginning, but it could be a big problem.