“Pollution from tyre wear can be 1000 times worse than what comes out of a car’s exhaust.”
It makes a catchy headline – and a startling one at that. But Emissions Analytics found that the particulate emissions from an average family car’s tyres – brand new and correctly inflated – were 5.8g/km. The limit for tailpipe emissions is just 4.9 milligrams over the same distance, or almost 1000 times less.
What’s more, a car’s brake wear also feeds into the problem and most shockingly, non-exhaust emissions (NEEs) are completely unregulated. Feeding into this is the fact that heavier vehicles are more NEE-prone, so there’s the problem of SUVs and their continuing growth in popularity, and yes, even EVs which have to lug heavy batteries around.
When it comes to emissions from EVs, it’s easy to get caught up in all the positives and lose sight of the fact that regardless of how clean they are, there’s still an environmental cost somewhere along the line. Minimising that cost wherever possible is therefore extremely important to ensure that the future of personal transportation lives up to the green reputation that we’ve bestowed upon it.
As such, we wanted to dig beyond the headline and further into Emissions Analytics’ study to find out the facts around particulate emissions straight from the source. Nick Molden, CEO of Emissions Analytics, was happy to give us the low-down.
Discover EV: NEEs: tell us what we’re talking about here…
Nick: “Particulate emissions from tyres are predominantly made up of two different types: coarse and ultra-fine. Coarser particles measure around 10 microns (0.01mm) in size and typically fall to the floor to settle and ultimately be washed away fairly quickly. The problem these particles pose is in them becoming part of the microplastics problem.
“Ultra-fine particles are around 10 nanometres across which puts them in similar size range to tailpipe emissions and means they behave similarly – typically being suspended in the air. Since the mid-2000s, diesel cars (the main contributor of harmful tailpipe particulate emissions) have had particulate filters installed as standard, significantly reducing emissions. This goes unchecked with NEEs.”
Discover EV: So how are these particulates formed and in what way might EVs be worse than a ‘regular’ car?
Nick: “They are mostly produced by thermal processes – i.e. the tyre getting hot during acceleration, deceleration, and especially in cornering. Hard cornering is the biggest culprit by far for producing tyre particulate emissions.
“The biggest factor affecting tyre – and indeed brake – emissions is driving style, just as it is with tailpipe emissions. Heavy throttle, brake and steering inputs have the dramatic consequences, along with vehicle weight and the quality of tyre.”
On that last point there is a huge difference between premium tyres and budget tyres. Emissions Analytics trialled both and found the budget brands to be exponentially worse at polluting the air, with the premium tyre producing many times fewer particulates by mass than the budget tyre.
Discover EV: These emissions are the kind that affects air quality, rather than being greenhouse emissions, then?
Nick: “The predominant issue with regards particulates from NEEs is in air pollution and poorer air quality rather than an inherent issue around greenhouse emissions. However, whether there is a greenhouse issue with regards increased pollutants and microplastics in the polar regions are yet to be fully understood. But the underlying fact is that particulates are bad news.
“Another part of the emissions problem that many people simply don’t realise is that tyres aren’t just natural rubber; they’re predominantly made from petrochemicals and as such are not so dissimilar to diesel emissions. Volatile chemicals react during thermal stress and are given off, with high concentrations of nasty substances such metals and hazardous organic compounds.”
Discover EV: How about situations where tyres aren’t under stress – such as in slow-moving traffic?
Nick: “We found that high-speed driving in a straight line produces relatively low levels of NEEs. Similarly, low speed driving and gentle cornering isn’t particularly problematic. Effectively, so long as a tyre isn’t under heavy load, the particulate production problem isn’t so significant. Specifically for EVs, while they will be slightly worse at speed due to their additional weight, in town they have the benefit of having negligible brake-related emissions due to regenerative braking.
“High speed cornering is by far the worst culprit and this is exacerbated with weight and, as previously mentioned, quality of tyre. With heavier vehicles such as EVs, or indeed hybrids, fitting high quality tyres to counteract the mass is something that should be a major consideration for owners.”
Discover EV: What about the way in which particulates are suspended in the air – how does this change by height and environment?
Nick: “To ascertain particulate behaviour, we undertook fluid dynamics modelling, especially with regards distribution in the atmosphere and when disturbed by passing traffic.
“When studied in an urban environment, we found the way that ultra-fine particles move is comparable to exhaust gasses. Coarser particles will typically stay lower to the ground and in suspension for less time; ultra-fines will disperse more quickly, evenly and higher up. Wider areas allow for greater dispersion and dilution. However, the problem with low-level particulates in urban areas comes into particular focus with regards things like children in buggies sitting at a similar height to the tyres, brakes and indeed exhaust pipes.
“Whilst any particulate can cause respiratory problems, the smaller the size (and greater the concentration), the deeper the penetration into the body occurs.”
It’s important to point out that Emissions Analytics isn’t attempting to demonise EVs for being heavier; their research is simply to ascertain the scale of the existing NEE problem, its various contexts, and how it can be addressed. Where we see it being particularly important for EVs is in the same way we look at upstream emissions and carbon debt from manufacturing – that green credentials are far more than just tailpipe emissions.
The ongoing research is also designed to stimulate conversations among all stakeholders from the buying public, through to legislators – encouraging them to act on the problem. On an individual level this might simply mean splashing out on better tyres; on a legislative level this might end up with better tyre labelling to ensure consumers are informed when making a choice.
Emissions Analytics is continuing its research and will soon be releasing data that directly compares EVs to ICE vehicles and builds on the science already established. Somewhere down the line it would be good to think that an industry body or arm of the government would look at the problem on a more macro scale to discover a truer picture of the national particulate problem, and suggest sensible ways in which it can be addressed.
After all, if tyre wear is harmful to the atmosphere, we might need to rethink our definition of zero emissions vehicles.