Tyres worse polluters than tailpipes

Efficient exhaust pipes and large loads are changing the narrative on air pollution...
14 June 2022

Interview with 

Nick Molden, Emissions Analytics


Tyre tracks on the ground


Bad air sits at the top of the league tables as a leading cause of ill-health internationally. And a lot of air pollution stems from road traffic. Notoriously, it’s the fine particles in exhaust smoke that are responsible. In recent years though, significant pressure has been brought to bear on vehicle manufacturers to clean up their emissions acts, and, at the same time, some areas have introduced emissions control zones, like the one in London, that penalise polluting vehicles and drive them off the streets. But, as these measures take hold - and especially as we switch to electric cars - there’s one form of pollution that’s rapidly becoming the elephant in the room, flying straight under the roadside radar and slipping unnoticed into ultralow emissions zones: the particles that rub off your tyres. These, research suggests, can be just as bad as exhaust smoke. They’re also present in massive amounts, as Nick Molden, the CEO of Emissions Analytics, explains to Chris Smith...

Nick - So I did some calculations and you very quickly realize you were talking about hundreds of thousands of tonnes of material being shed every year across, say, all the vehicles in Europe; in the world, there's about one and a half billion cars. If they are shedding, even relatively small amounts of material, that adds up to very large amount. The regulators have got on top of tailpipe emissions, but it doesn't mean that cars don't have other sources of pollution. Brakes is one, but the one that we think is the biggest and growing is tyres. What is most important for us to think about, and regulators to get their head round? It is now tyres, not the tailpipe!

Chris - Do we have any feel though for the relative nastiness of emissions from tailpipes versus tires, because we've got quite familiar with what these small particles are, how big they are, which are the worst ones, what sorts of impacts they have on human health when we breathe them in. Do we have any data on what comes off of a tire and in what sort of quantity and what its link to disease risk is?

Nick - There's a certain intuitive understanding that or belief rather than understanding that tires are largely made up of natural rubber. And because that's natural, that can't be that bad. Well, it turns out, particularly in car tires and van tyres, this is not true. Actually, natural rubber makes up a pretty small proportion, maybe a quarter maximum. Most of it is synthetic rubber derived from crude oil. And that brings us to a really interesting parallel with fossil fuel vehicles in that we are using derivatives of crude oil to create the tires. And so the pollutants coming off tires are, in a way, you know, cousins of what's coming out of the tailpipe. And so we are seeing a lot of the same common compounds. And one group, particularly to flag is what's called aromatics. Many of them are carcinogenic to humans. If we compare across the hundreds of tyres we've now analysed, that proportion of aromatics varies between about 25% at the low end to 80% at the high end. What that says to me is, one, those are high figures anyway, across the board, but also that they vary hugely from tire to tire.

Chris - The thing is, Nick, the mere fact, they are, there is a bit like me saying, well, I know there's some asbestos in the building that I'm working in. But as long as I don't fiddle with it, I'm quite safe. If the carcinogen stays in the tire or stays on the road surface, that's not in theory, a threat to my health, is it? So have we got evidence that this is coming off the tire and potentially exposing people and therefore increasing their risk of onward disease?

Nick - So there's three directions. These particles take. The smallest ones make up about 11% of the mass and they will hang in the air for quite a while and are subject to human inhalation. They are sufficiently small to transfer into the blood and potentially into the brain. There is now a reasonable amount of evidence that that's not a good thing. But what happens to the other 89%? Our estimates say that ,probably, roughly equally that is split between it going into the water and into the soil, which then can find its way into the food chain. If we just think a bit more about the marine direction, the research is at really early stages at the moment, but the one notable thing that's happened recently in America, they have linked the effect of one compound from tires, which only comes from tires to the deaths of salmon and now to breeds of trout as well. Now I know that's fish, not people, but that gives us sufficient. Cause to think there's enough reason to believe there's a risk that these aromatics will be getting into humans.


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