Tires producing more particle pollution than tailpipes

Nick Molden of U.K.-based Emissions Analytics says newer vehicles do a much better job of limiting exhaust emissions. So his group looked elsewhere for the source of continuing pollution.

Anyone concerned about air pollution should take a look at the bottom of a vehicle.

The biggest source of pollution may not be the tailpipe.

Experts with the independent company Emissions Analytics say aging tires now typically spit out more particles than exhaust systems.

Nick Molden, founder and chief executive of U.K.-based Emissions Analytics, says newer vehicles do a much better job of limiting exhaust emissions.

His group had to look elsewhere for the source of continuing pollution.

Listen: Nick Molden on how ultra-fine particles produced from braking and accelerating are causing air pollution.


WDET’s Quinn Klinefelter spoke with Nick Molden about particle pollution. Read the interview, edited for clarity, below:

Nick Molden, Emissions Analytics: We got interested in tires. When you do some basic math and realize how much tire mass is being shed into the environment, in the United States, it’s about 200,000 tons every year, just from light-duty vehicles. And no one knows really where it goes or what it does. And we set about trying to understand that question. At the same time, from our tailpipe testing, we could see that the particle mass from tailpipes was getting lower and lower and lower. So the traditional source of pollution is falling rapidly. But this unregulated one [tires] is possibly very large. And, with the transition to battery electric vehicles, getting greater as the vehicles get heavier, we decided to look at that ratio between the two. And the result that came out was even higher than we expected. We concluded that there was about 1,850 times more particle mass from tires than the tailpipe. And that ratio is growing every day.

Quinn Klinefelter, WDET News: There is a difference, is there not, between particle pollution, like what would come from tires, and air pollution you normally would see from tailpipes?

They’re actually more similar than you would think. So the tailpipe now only really emits ultra-fine particles. And by that we mean down to well below 100 nanometers, very fine, invisible to the naked eye. But what turns out to be an urban myth [is that] tire particles are all big chunks of rubber. That’s the bit that turns out not to be true. What we’re seeing is that you get a mixture of bigger particles, you know, 10 microns and bigger, but also these ultra-fines as well, down to below 10 nanometers. The small ones are particularly produced from sharp braking and accelerating, when the tire gets hot. And those ultra-fines are the ones hanging in the air and causing air pollution problems and getting inhaled by humans. And then when you realize that light-duty tires are largely made from synthetic rubber, from crude oil actually, it’s perhaps intuitive that tire-wear emissions are not dissimilar from tailpipe emissions. They come from the same original source.

There are some problems posed by particle pollution that maybe air pollution doesn’t, I would assume. I mean, air pollution hangs in the air, but the particles from tires and the like could get into the soil, perhaps more easily enter the human bloodstream.

By our estimates 11% of the tire mass goes to the air. But 89% goes to some combination of the marine environment and soil. What’s going on on the [U.S.] west coast at the moment, linking the effects of one particular chemical [used in tires] to the problems experienced by the coho salmon and the various trout species. This preservative, called 6PPD, is getting into their reproductive system and leading essentially to them quite rapidly dying out.

What kind of factors are you finding that would increase this particle pollution being produced?

The weight of the vehicle is a crucial aspect to it. A vehicle that’s 1,000 pounds greater in weight will have about 21% more tire-wear emissions. And that 1,000 pounds is about the weight of a battery in a full-battery electric vehicle. So that’s a big driver. There are also the compounds in tires. Lots of electric vehicle manufacturers are producing battery electric vehicle specific tires to handle the weight and torque. And what we’re doing is looking at the chemical composition of those tires to see how they differ from traditional tires and whether they have more of the problematic aromatic compounds, which are often carcinogenic, or less. And we’re finding a huge variation between different brands, makes and models. So we’re concluding that it really depends on the individual tire manufacturers’ recipe as to what the impact on the environment is. But also it’s a bit of a black box [because] no one knows, outside of the company in question, what goes into those tires. And that’s what makes it rather difficult to understand how big this problem is. And part of what we’re doing is trying to open that black box and shine a light and find out what these key chemicals are. The thing which tends to produce more of the very small particles is rapid acceleration and cornering and braking. We are seeing data now coming in from a lot of electric vehicle drivers. And what’s dramatic is that some people have much better reduced tire wear, and others have much worse. And that seems to come down to [if] you use all that lovely torque from the battery electric vehicle. If you do and you drive in a very exciting way, you will get through tires probably quicker than before. And what we’re finding is the increased tire wear from a battery electric vehicle is actually more than the total amount that’s allowed out of the tailpipe. It’s quite a dramatic difference and really points to changes to the regulations that are needed.

At present, are there any regulations that you’re aware of, either in the U.K. or Europe, or in the U.S. in particular, since we’re talking here, that judge such tire particle wear and what should be done or not?

To my knowledge, there are no in-use regulations anywhere in the world around tire wear. Where there is some restriction in certain places, including Europe, is limiting what chemicals you can put into the tire when you manufacture it. There are, I believe, eight chemicals that are limited in Europe. But when we are seeing over 400 compounds in a typical tire, limiting eight of them is only baby steps.

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  • Quinn Klinefelter
    Quinn Klinefelter is a Senior News Editor at 101.9 WDET. In 1996, he was literally on top of the news when he interviewed then-Senator Bob Dole about his presidential campaign and stepped on his feet.