Rolling on the River

What's in a tyre?

Wheels are an integral part of human life. For thousands of years, wheels were made of stone or wood, and it was not until the invention of the car in the late 1800s that the use of air-filled rubber tyres became widespread. Originally, these tyres were made of natural rubber from rubber trees, but as the demand for cars increased in the 1900s, so did the demand for rubber tyres, and in 1909 a German chemist, Fritz Hofman, invented the first commercial synthetic rubber, a plastic polymer made from petroleum. A couple of decades later, the manufacturing of synthetic rubber was industrialised, by the U.S. chemical company DuPont.  

Today, the composition of a tyre is highly complex with more than 200 raw materials being used for tyre production. The main materials are synthetic and natural rubber. In addition, metals, textiles, fillers, oils and a wide range of chemical additives are added during manufacturing to give the tyres desirable properties including enhanced performance, durability, and flexibility. One study compiled a list of more than 800 different tyre-related chemicals. Some of these chemicals are potentially harmful and can leach from the tyres or emitted tyre wear particles into the environment.

Global emissions of tyre wear particles 

Globally, an estimated 1.32 billion cars, trucks, and buses were on our roads in 2016, a figure expected to more than double to 2.8 billion vehicles by 2036. With over 60 million new cars sold annually, this means there are a lot of tyres on the road.

During the lifespan of a tyre, significant quantities of tyre wear particles are generated through abrasion, the process of wearing down or degrading through friction. During its life, a passenger car tyre loses an estimated 15% of its weight to the environment, while this is 18% for a truck tyre.

Globally, it is estimated that an average of 0.8 kilos of tyre wear particles are emitted to the environment per person, corresponding to a total of 6 million tonnes per year. That’s about the same amount as the weight of half a million school buses.

The amount and size of particles released from tyres during driving varies based on several factors. This includes temperature, road surface, composition and structure of the tyre, tyre pressure, wheel adjustment, speed, and driving behaviour. The weight of the vehicle is also an influencing factor. Heavier vehicles, such as buses and trucks increase abrasion of the tyres and emit more tyre wear particles per distance travelled. Electric vehicles are also likely to have higher tyre wear particle emissions as they are, on average, heavier and can accelerate faster than their diesel and petrol counterparts. 

The contribution of tyre wear particles to the total global amount of plastics in the oceans is estimated at 5–10%. As tyre wear particles have a higher density than freshwater and seawater, they are expected to eventually sink and settle in freshwater and marine sediments.  

Across Europe, tyre wear particles entering wastewater treatment systems can be captured, however, most surface runoff goes untreated and can be flushed out via combined sewer overflows during heavy rain. Even the tyre wear particles captured by wastewater treatment plants might eventually find their way back into soils and surface waters, via the use of sewage sludge as fertilizer on farmland. Once emitted, tyre wear particles are very difficult to remove and remain in the environment for a long time.  

Organisms living in oceans and freshwater can be exposed to tyre wear particles through ‘inhalation’ (via the gills) and feeding. This may cause physical and chemical harm, depending on the size of the particles and the chemicals that leach from them. For example, exposure of shrimp and fish larvae to tyre wear particles may cause reduced growth, developmental abnormalities and alter swimming behaviours which could lead to an increased risk of predation and foraging challenges in the wild.  

One chemical in tyres that’s been found to be particularly problematic is 6PPD. This is added to rubber to protect it from degradation by ozone exposure and oxidation, which cause cracks and weaken the rubber over time. 6PPD and particularly its degradation product 6PPD-quinone, has been identified as highly toxic to aquatic life, especially to coho salmon. 

Tyre wear particles may also be problematic in soil, affecting the health of the soil ecosystem and the organisms living there. 

Human health risks 

The impact of tyre wear particles on human health is an increasing cause for concern. Humans can be exposed to tyre wear particles through inhalation and particles entering our food chains. Smaller tyre wear particles released into the air are estimated to account for 3-7% of PM2.5 (particles smaller than 2.5 micrometre). These particles are so small that they can be inhaled, penetrating deep into the lungs where they may cause respiratory and cardiovascular issues. People with pre-existing conditions, such as asthma, may be particularly affected. This suggests that tyre wear particles contribute to the global health burden of air pollution.  

Some of the chemicals present in tyres are known to be toxic to humans, including metals like zinc and lead, and phthalates, phenols and PAHs. A recent study found the potentially harmful chemical 6PPD in the urine of children and adults in China, indicating that chemicals from tyres can enter the human bloodstream.  

Tyre wear particles likely behave similarly to other micro- and nano-plastics in the environment, which means they could potentially enter and accumulate in the food chain. Microplastics have been documented in finfish, shellfish and crustaceans consumed by humans, while edible plants may take up chemicals from tyre wear particles. This causes concern for the potential health risks of exposure to tyre wear particles and their chemicals through the food we eat.  

Collection and removal of tyre wear particles 

Collecting and removing tyre wear particles from the environment poses significant challenges. Cutting-edge approaches to cleaning up microplastics in water, such as advanced filtration systems, and self-cleaning robots, show promise. However, their efficacy and scalability must be validated under real-world circumstances. Nanotechnology-based solutions offer a potentially effective method for mitigating the increasing risks posed by microplastic contamination. However, there are many unknown and potentially high costs involved in these technologies. Therefore, preventing microplastics from entering the environment in the first place will be the most cost-effective and impactful solution. 

The road to solutions 

Preventing the release of tyre wear particles into the environment appears challenging in the short term, primarily due to the absence of a synthetic rubber alternative for tyres. Therefore, mitigation efforts should prioritize making tyre wear particles less toxic while reducing emissions. This includes: 

• Removing and replacing toxic chemicals in tyres 
• Reducing traffic  
• Using alternative transport options such as public transport  
• Increasing the wear-resistance of tyres   
• Utilising open asphalt concrete roads to capture particles from wear and tear processes
• Enhancing the efficiency of sewers and wastewater treatment plants 

There are also multiple measures that consumers can take themselves to limit the release of tyre wear particles when driving their vehicles, using so-called ‘eco driving behaviour’. This includes maintaining a steady speed, and accelerating, breaking and turning less aggressively, which all reduce friction and the release of tyre wear particles. Maintaining the recommended tyre pressure is also important to limit emissions. Finally, existing tyres show high ranges of durability meaning it is possible to limit tyre wear particle emissions by choosing the less eroding tyres. Improved tyre labelling could be a way to inform consumers on the tyre characteristics when purchasing tyres.

Public support for action 

In a recent survey conducted by the University of Vienna as part of the European Source to Seas – Zero Pollution 2030 project, members of the public in five countries across Europe were given a brief introduction to the issue of tyre wear particles and then asked about their level of support for different types of policies to address the issue: 

• Chemical removal: Incentivising manufacturers to remove and replace the most toxic additives in car tyres 
• Public transport: Increasing government funding for accessible public transport to indirectly reduce tyre wear. 
• Labelling: Labelling car tyres based on their eco-friendliness to guide consumers towards environmentally conscious choices. 
• Smoother roads: Investing in smoother road surfaces to minimise tyre abrasion and reduce tyre wear. 
• Large car tax: Imposing an additional environmental tax on larger cars due to increased tyre wear particle emissions. 

The results from the survey indicate relatively strong public support for all policy options, with the highest level of support expressed for removing and replacing toxic chemicals from tyres.  

It is evident that tyre wear particles represent a significant source of microplastics in our environment. As already highlighted in this storymap, a lot can be done to reduce emissions and impacts. Addressing this issue effectively demands heightened awareness, bridging knowledge gaps regarding quantities and impacts, and fostering innovative technical solutions. This requires everyone to work together globally, including consumers, regulators, industry professionals, and researchers.  

Such an integrated approach has been launched in Europe by the Euro 7 legislation which introduces measures to limit pollution from road transport. For the first time, non-exhaust emissions from vehicles including tyre wear particles will be regulated by introducing binding tyre abrasion limits for tyres sold on the EU market. This is an important step towards the EU’s Zero Pollution Action Plan target to reduce microplastic pollution by 30% by 2030.