Part 4: You will hear a lecture about tyre-wear particles.
Good afternoon. In this lecture we will look at tyre-wear particulates, an important form of transport pollution that is often overlooked because it does not come from an exhaust pipe. Instead, it is produced by the physical abrasion of tyres as vehicles move. This process happens on every journey, whether a car is petrol, diesel, or electric, so it remains relevant even as fleets become cleaner in terms of tailpipe gases.
To understand the issue, we begin with definition. Tyre-wear particles are fragments released from the tread surface as it rubs against the road. They range from coarse crumbs to fine particles that are difficult to see. In real streets these fragments rarely remain pure rubber. They mix quickly with mineral road dust, forming what many researchers call tyre and road wear particles. This mixing matters because it changes how the particles travel and how they interact with water and sunlight.
Next, consider what tyres are made of. Modern tyres are composite products containing natural and synthetic rubber, oils and resins that control flexibility, and reinforcing fillers. Two common fillers are carbon black and silica, which improve strength and grip. Tyres also contain additives that slow cracking and improve performance in different temperatures. When particles break off, these additives may be present on the particle surface.
Now let’s look at how the particles are generated. The main driver is friction during everyday driving. Abrasion increases during acceleration, braking, and cornering, because forces on the tread rise and the rubber deforms more strongly. The roughness of the road surface also plays a role, as do speed and driving style. Vehicle characteristics matter as well. Greater vehicle mass increases the force pressing the tyre onto the road, so wear rates tend to rise. This is one reason heavy SUVs, and some battery-electric vehicles, can create substantial tyre wear even when their exhaust emissions are low.
After particles are produced, where do they go? Larger fragments settle close to the road, particularly near junctions, roundabouts, and bends where braking is frequent. Smaller particles may become airborne and contribute to fine particulate matter. A significant fraction, however, is washed from the road surface during rainfall. Water carries the material into storm drains and roadside gullies, moving it into streams and rivers. Once in freshwater, particles may settle and accumulate in sediments, especially where the flow is slow and fine material can deposit.
These pathways lead to two main categories of impact. First are physical effects. The smallest particles can be inhaled, and like other fine particulate pollution they are linked with respiratory irritation and increased cardiovascular risk. Second are chemical effects. Because tyres contain additives, chemicals can migrate out of particles over time. In water, some compounds can leach into the surrounding environment, and certain transformation products have been shown to harm aquatic organisms, particularly after the first heavy rain following a dry period when accumulated road material is suddenly flushed downstream.
Measuring tyre-wear particulates is difficult. There is no single outlet to sample, so scientists combine approaches. Roadside air monitoring can identify particle levels and chemical markers, but sources are mixed. Sampling road dust and river deposits shows build-up over time, but it can be hard to attribute material to specific roads. Laboratory wear testing, by contrast, is controlled, allowing researchers to estimate wear rates under set conditions, though it may not match real-world driving.
In terms of solutions, options include designing tyres that abrade less while maintaining safety, smoothing road surfaces, encouraging lower speeds and gentler driving, and improving runoff filtration. The key message is that reducing transport pollution requires attention not only to engines, but also to materials and the systems that carry particles from roads into air and water. Future research needs better monitoring in drainage networks and more reliable chemical markers to reduce uncertainty in inventories. This evidence will help policymakers compare interventions and target budgets effectively for cities.
