Research reveals ‘vital information’ relating to tyre particles…

…and highlights knowledge gaps

The University of Plymouth published government-funded research into what happens to particles released from vehicle tyres back in May. This comparatively recent report – as far as academic research is concerned – was launched with no-small media fanfare amid claims that tyres “could be a significant and previously largely unrecorded source of microplastics in the marine environment”. Be that as it may, the research also highlighted a number of significant knowledge gaps in this field of research.

The Plymouth study is one of the first worldwide to identify tyre particles as a major and additional source of microplastics. Scientists have previously discovered microplastics, originating from microbeads in cosmetics and the degradation of larger items such as carrier bags and plastic bottles, in marine environments globally – from the deep seas to the Arctic.

Following the UK government’s ban on rinse off microbeads, which is one of the toughest in the world, the Defra-funded study led by the University of Plymouth revealed new information that will improve our scientific understanding of how tiny particles from source including synthetic fibres from clothing, maritime gear and, yes, tyres enter the ocean.

This project will be used to guide future research already underway on marine plastic pollution and the impact of human activities on the marine environment amidst efforts to reduce plastic consumption. These include the 5p plastic bag charge – which has led to 15 billion fewer bags distributed – and plans to end the sale of plastic straws and stirrers and plastic-stemmed cotton buds later this year.

The study reportedly shows the tyre particles can be transported directly to the ocean through the atmosphere, or carried by rainwater into rivers and sewers, where they can pass through the water treatment process. Researchers estimate this could place around 100million m² of the UK’s river network – and more than 50million m² of estuarine and coastal waters – at risk of contamination by tyre particles.

Its findings also highlight some of the optimal places for intervention, for example, that fitting filters to washing machines could be less effective than changing fabric designs to reduce fibre loss, with another study at the University having recently shown that normal wear and tear when wearing clothes is just as significant a source of microplastic pollution as release from laundering.

The study was directed by Professor Richard Thompson OBE, Head of the International Marine Litter Research Unit, and Plymouth researchers Dr Imogen Napper and Florence Parker-Jurd. It also involved Dr Geoff Abbott from the School of Natural and Environmental Sciences at Newcastle University (who developed a breakthrough method using mass spectrometry to detect tyre-derived microplastics in the environment), Dr Stephanie Wright from Kings College London, and Simon Hann from Eunomia Research & Consulting Ltd.

Professor Thompson oversaw Defra’s first research project on microplastics and their impact on marine life nearly a decade ago. It was this, and some of his team’s subsequent work, that led to the UK’s pioneering ban on microbeads in rinse-off personal care products coming into force in 2018.

Tyre wear, microplastics and the “many unknowns”

Commenting at the time of the launch, Professor Richard Thompson OBE said:
“This study gives us a real insight into the importance of tyre wear as a source of microplastics. However there are still many unknowns, and compared to other forms of microplastics we know relatively little about tyre wear particles. So it is important to continue to take steps to reduce emissions of better understood sources like fibres from textiles and the fragmentation of larger items.

“What this study also does is provide further evidence of the complex problems posed by microplastic pollution. We have looked at three pathways and shown that all of them are substantive pathways to the environment. As we work to understand their potential distribution and impacts it is important to also work together with industry and policy makers to identify potential solutions which may include changes in behaviour, changes in product design and waste management.”

Domestic Marine Minister, Rebecca Pow added: “Reducing plastic pollution in the ocean is one of the greatest environmental challenges that we face. This study will help us face that challenge by identifying areas for future research, such as changes to roadside drainage and textile design. The UK is at the forefront of a global fight against the scourge of plastics. In addition to the pioneering ban on microbeads and the 5p plastic bag charge, plans are also in place to end the sale of plastic straws, stirrers and plastic-stemmed cotton buds.”
Defra is continuing to invest in research on waste management, ocean littering and microplastics to support the delivery of the government’s 25 Year Environment Plan and the United Nations Sustainable Development Goals. This research will help identify evidence gaps and recommendations to tackle marine plastic pollution, both in the UK and globally.

Tyre wear is not a new discovery, friction between the tyre and the road is an inherent part of grip – something that is inseparable from safe motoring. Increased interest in the nature and dispersion of tyre and road wear particles is informing scientific understanding but large gaps remain. But while there are many unknowns, here is a summary of what we do know. Firstly, market observers and industry organisations report that the particles released are each an approximately 50/50 mix of tyre and road materials.

Such particles are mostly relatively large (with a median size between 80 and 100 micron, 10 times larger than PM10) and dense (estimated to be about 1.8 g/cm3). As a result of their size and density, most tyre and road wear particles quickly fall out of the atmosphere. Only 1 per cent by mass remain airborne. Unlike many microplastics, most tyre and road wear particles sink in water. Only a few per cent of the material released reaches the estuaries. As yet, no tyre and road wear particles have been detected in the oceans.

Along with government and other stakeholders, the tyre industry is pursuing further research into the nature and dispersion of tyre and road wear particles and to identify mitigation opportunities. In addition, the industry is pursuing a tyre abrasion resistance test to reliably inform consumer choice and support possible future minimum performance standards.

Research undertaken by Kings College London and others around the world has shown that less than 1 per cent of fine particulate matter (PM10 and below) found in city centre ambient air sampling is due to tyre and road wear particles. The latest research by Plymouth University confirms that most tyre and road wear material falls out of the atmosphere within 50 metres of the road.

However, the researchers highlight a gap in scientific knowledge regarding the release of fine particulate matter from high-speed driving on motorways and dual-carriageways. They go on to suggest that air-borne transmission of ultra-fine particles could be a significant pathway for tyre and road wear particles to reach the sea. This despite acknowledging that to date, marine sampling studies have not reported the presence of tyre and road wear particles in the oceans.

There are many sources of fine particulate matter in the atmosphere. Defra’s Clean Air Strategy identifies that around 15 per cent of UK particulate matter comes from naturally occurring sources, up to a third from neighbouring countries and around half from UK human-made sources.

Defra indicates that of these UK human-made sources of fine particles, 38 per cent are due to the domestic burning of fossil fuels and wood, 16 per cent arise from industrial combustion and 12 per cent are due to road transport. So, considering all the particulate matter in the atmosphere, exhaust and non-exhaust emissions from road transport in the UK contribute together about 6 per cent of the total.

Microplastics in water

Recent peer-reviewed research indicates that only a few per cent of tyre and road wear material reaches estuaries via water courses and discharges. The research highlights the capacity of sediment in rivers and watercourses to capture tyre and road wear particles. To date, marine sampling studies have not reported the presence of tyre and road wear particles in the oceans.

The Plymouth research highlights concerns, already raised elsewhere, regarding the impact of Combined Sewerage Overflows on the release of tyre and road wear particles into the aquatic environment. In times of heavy rainfall, the capacity of water treatment plants is exceeded and, after initial settlement, the excess flow is released directly into rivers or the sea without further treatment. Other recent research has shown that in normal operation water treatment plants are very effective at removing microplastics from incoming effluent.

However, as dramatic as the initial headlines appear, to cut a long story short, the researchers at Plymouth repeat earlier call suggesting further work is needed to understand the pathways and fate of tyre and road wear particles in the environment.