It’s somehow more than 3.5 years since my original post on this topic, so it’s worth a revisit to see how the state of knowledge has changed. As it happens, my revisit was completely accidental. I went looking for data on something else on .gov.uk, and found two new reports on this.

Before I get into those, let me just outline the background. Pollution from tyres, and brakes, and the road surface has received increased focus in recent years. The cynic might suggest that this is because the other sources of pollution in cities – the “classical” pollutants – are better controlled, particularly as regards the tailpipe emissions of modern vehicles. (CO₂ does not count as a real pollutant.) Either way, and especially if plans to get rid of ICE vehicles come to pass, then Non-Exhaust Emissions (NEEs) are going to make up an increasingly high proportion of the total.

The original post on this topic mentioned a company called Emissions Analytics, and their report on how serious tyre wear was for air quality, i.e. that it was worse than vehicle exhausts. One thing that I skated over at the time was the production of Ultra-Fine Particles (UFPs). These are smaller than the infamous PM2.5, so small in fact that they can easily be absorbed by the body, and after that, well, who knows. It could be that they do no damage, but it seems a risky bet.

The Reports

The first is

Review of PM2.5 reduction technologies for on-road transport: horizon scanning report

and is by Ricardo, available here.

The second is

Road wear and dust resuspension

and is by Ricardo and AECOM, available here.

Both were surreptitiously published on 30^th December, 2025. (Yes, that is me being cynical.)

From the literature review of the first:

Road transport statistics published by the Department for Transport (DfT), based on data from the National Atmospheric Emissions Inventory (NAEI), show that in 2021 52% of PM2.5 emissions from road transport came from tyre and brake wear, and a further 24% of emissions from road abrasion.

Now, my maths is not what it was, but that means only 24% remaining for emissions from tailpipes, right? The agreed best solution is to “reduce traffic volumes.” [Take that tinfoil hat off at the back.]

Since electric vehicles use heavy batteries, their total weight is likely to be 190 kg-430 kg (or 15-20%) greater than a diesel or petrol vehicle of similar size, and so unabated NEE could exceed the total PM emissions of diesel or petrol vehicles.

[Their emphasis.]

Regarding tyres, they note a “magic” triangle, within which manufacturers try to optimise wet grip, longevity and fuel economy. (Maybe an “iron” triangle would have been better.) A study showed an average of 100 mg / km of lost rubber per tyre. Ricardo mention a company called ENSO who are trying to develop biodegradable tyres with equivalent performance to those based on crude oil.

They do not, as far as I can see, mention UFPs.

The second report has this to say on the breakdown of NEEs:

In 2023, road transport emissions of PM2.5 in the UK were estimated at 16% from exhaust, 59% from brake and tyre abrasion, and 23% from road abrasion.

[The data is two years newer than from the other report, which was published on the same day.]

Four factors impinge on road wear and dust resuspension: Vehicles, Vehicle dynamics, Road characteristics, and, you might have guessed, but if you didn’t, I’ll tell you, climate. Under the first, is noted the following:

Vehicle torque has been cited as a cause of greater tyre wear for electrified vehicles and may also affect road wear.

Under the fourth:

Climate change, with more extreme temperatures and increased rainfall, may result in more roads being damaged and an increase in wear.

[I rather heard this as coming through gritted teeth, as if the authors felt compelled to mention it, but had no real desire to.]

Regarding road wear, they reiterate the 4^th power law which is often cited for the relationship between vehicle mass and the damage done to road surfaces. This means that the wear caused by cars is “negligible.”

The impact of vehicles under 7.5 tonnes [in a Scottish study] was found to be negligible compared to HDVs which cause 99% of road wear…

This is entirely obvious given the 4^th power law, which the report says actually varies between 3^rd and 6^th (!) power. On this basis, a laden HGV would produce several thousand times more wear than a car, whether that be BEV or ICEV. So we should be careful about claiming that BEVs damage the road more than ICEVs, although that is true. The same is true for electric lorries:

The findings suggest that electric heavy-duty vehicles could increase road wear by 20%…

Don’t worry, the latest stupid lot will soon be replaced by a new and different stupid lot.

However, it is worth noting (and this should also be quite obvious) that dust resuspension is far less affected by vehicle mass.

This report didn’t seem to mention UFPs, either. The new WHO Air Quality Guidelines do mention UFPs, although no “safe” limit is set, as it is for the other pollutants, including PM10 and PM2.5 (if you were to put UFPs in the same terms, they would be PM0.1):

…quantitative knowledge of UFP is needed, since focusing only on PM2.5 may result in overlooking the impact of UFP and there is no evidence that mitigating particle mass only (PM10, PM2.5), as the existing air quality measures do, will necessarily lead to a reduction in UFP.

You’re probably thinking that you’re not afraid of PM2.5, and you’re not going to start getting afraid of PM0.1 either, just because someone just noticed that they exist. They existed before, and we didn’t fear them. Ignorance is bliss. And one of the faults laid at the door of climate science is that it is continually discovering things that we are now supposed to be scared of. Well, I don’t know how scared we should be of UFPs, but probably more scared than we are of the larger particles. (UFPs have been linked to Alzheimer’s. Then again, so has everything, right?) As to exactly what UFPs are chemically, well, that’s a difficult question, because it varies enormously.

As to Emissions Analytics, whose report sparked my initial interest in this topic (thanks to the Guardian not reporting on it), they are still going. Their head honcho published a book, I think last year, with an academic from Oxford, proposing a vehicle tax system based on vehicle mass * miles travelled. I don’t know exactly what was proposed, but it sounds rather linear to me, and therefore does not really answer what we might naively consider the original purpose of vehicle tax, to deploy it proportionally to vehicles to pay for the wear and tear on the road they cause. The proposal is interesting, but of course has a number of problems. To name a few: the 4^th power law means that large vehicles should be charged very large multiples of the vehicle tax charged on cars, but that would destroy commerce; the load carried obviously adds a lot to the damage done, but there is no way to apply the tax other than to unladen mass; there will be even more motivation for vehicle owners to deploy mileage blockers, and if you think ANPR cameras could sub in for unreliable odometers, then we also have a major problem with unreadable plates.

Pollution control is often a balloon-squeeze. You try to squeeze pollutant X, and out pops a bulge of pollutant Y (c.f. the diesel play). It would be ironic if an attempt to squeeze CO₂ ended up with an increase of UFPs, which may not be that terrifying, but are certainly more dangerous than CO₂.

I’m going to stop there, but there is much on this topic to read, if you are so inclined.

/message ends

Links

The book’s authors can be heard discussing their proposal on Youtube here. (309 views, one of them mine.)

Geoff on mileage blockers here. (148k views, one of them mine.)

A story about the Wild West of camera-defeating number plates is here. (Daily Mail link, sorry if you see ads.)

A 2025 paper by Bondorf et al showing how the production of UFPs by tyres is strongly determined by acceleration and speed is here.