New study finds brake pad emissions can be more toxic than diesel exhaust

A study by the University of Southampton (UK) has found microscopic particles emitted from certain types of commonly fitted brake pads can be more toxic than those found in diesel vehicle exhaust.

The research shows that a higher concentration of copper in some pads is associated with increased harmful effects on sensitive cells from people’s lungs, as a result of particles being breathed in.

Exposure to pollution generated by cars, vans and lorries has long been linked to an increased risk of lung and heart disease. However, while past attention has mainly concentrated on exhaust emissions, particles are also released into the air from tyre, road and brake pad wear – emissions which are largely unregulated by legislation.

These ‘non-exhaust’ pollution sources are now responsible for the majority of vehicle particulate matter (PM) emissions in the UK and parts of Europe, with brake dust being the main contributor.

Lead author of the study, Dr James Parkin, explains that a move to electric cars is bringing the problem into sharper focus: “People generally associate pollution from cars as being from exhaust pipes and think of electric vehicles as having zero emissions. However, EVs still produce particulate matter due to friction and wear of the road, tyres, and brakes.

“We wanted to understand how different types of chemical composition of pads affect the toxicity of the particles emitted and what this might mean for the health of individuals.” 

The scientists undertook an in-depth study examining the effects on lung health of PM from four different types of brake pad with differing chemical compositions; low metallic, semi-metallic, non-asbestos organic and hybrid-ceramic. The researchers were particularly interested in the smallest particles, of PM2.5 and below (often referred to as fine PM) – 30 times smaller than the diameter of a human hair.

These tiny particles can make their way beyond the upper airways and deeper into the delicate lung air sacs, which enable the exchange of oxygen and carbon dioxide to and from the blood stream. Fine PM from a variety of different sources is associated with over four million premature deaths per year worldwide.

Brake pad particulate matter was collected using specialist equipment. In the laboratory, the Southampton team used samples of cells from the lining of the lung and exposed them to the fine particulate matter to measure its effects, such as oxidative stress, inflammation, or the death of cells.

Results showed that of the four types of brake pads, non-asbestos organic  pads were the most potent in terms of inducing inflammation and other markers of toxicity, and more toxic to human lung cells than diesel exhaust particles. Ceramic pads were the second most toxic. Importantly, both non-asbestos organic and ceramic pads contain high concentrations of copper, and later experiments to remove this copper found the PM became less toxic.

The findings, published in the journal Particle and Fibre Toxicology, suggest that a reduction of copper content in brake pads could help mitigate some of the harmful effects of vehicle particulate matter. Air pollution, including from cars, has been linked to a range of conditions, such as asthma, chronic obstructive pulmonary disease (COPD), cardiovascular diseases, dementia and idiopathic pulmonary fibrosis (scarring of the lung).

Project supervisor Professor Matthew Loxham comments: “This research has important implications for health and future policy because as we switch from diesel and petrol-powered cars to electric vehicles, non-exhaust particle emissions will remain.  Non-exhaust emissions could increase over time due to electric vehicles being heavier than combustion engine vehicles and creating greater friction.”

The researchers highlight that while electric vehicles emit no exhaust emissions they aren’t emission-free and that health effects from vehicle emissions won’t necessarily be completely removed once the fleet is fully electrified. They suggest that current legislation, which focuses on PM exhaust emissions, may be inadequate to fully mitigate the health effects of vehicles in the future.

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Notes to Editors

1. The paper Copper‑enriched automotive brake wear particles perturb human alveolar cellular homeostasis is published in the journal Particle and Fibre Toxicology, DOI: 10.1186/s12989-024-00617-2: https://doi.org/10.1186/s12989-024-00617-2
    
2. For interviews or an embargoed copy of the paper contact Peter Franklin, Media Relations, University of Southampton. press@soton.ac.uk +44 23 8059 3212.
    
3. For more about the Faculty of Medicine at the University of Southampton visit: https://www.southampton.ac.uk/study/subjects/medicine
    
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