Road traffic is a major source of particulate matter emissions that pose a significant risk to human health and the environment. It is one of the priority areas for action to improve air quality, and is now the focus of public and scientific attention and the centre of the environmental debate. Over the last twenty years, the issue of reducing exhaust emissions has been brought to the forefront of the international and particularly European scene (introduction of Euro standards, particulate filters, SCR systems to reduce NOx, etc.). Nevertheless, in view of the efforts made to reduce tailpipe emissions, the contribution of non-exhaust emissions has increased and has become predominant in recent years. These non-exhaust emissions are mainly due to brake wear and to particles emitted when tyres come into contact with the road. By 2030, they are expected to increase by 53.5% worldwide, according to the OECD (Organisation for Economic Co-operation and Development). Moreover, epidemiological studies show that the toxicological impact of these particles is far from negligible.

Among non-exhaust emissions, brake particles are predominant, especially in urban areas and near high-traffic roads. Indeed, the wear process that takes place during the contact between the brake pads and the brake disc is responsible for significant emissions of fine and ultrafine particles. They represent up to 55% of PM10 emissions from non-exhaust sources and up to 20% of total PM2.5 emissions from road traffic. These emissions are still poorly known and quantified because of their variability, as many parameters influence their characteristics, such as the diversity of vehicles and brake pads, road characteristics, driving style, etc. They are also poorly studied because the methods used to characterise them are complex. Measurements at the source of brake particles show that their concentration increases significantly with vehicle speed, weight and braking frequency. Indeed, the more the braking system is used, the more the friction of the contact zone on the disc increases. This friction will generate, on the one hand, coarse particles due to mechanical abrasion and, on the other hand, ultrafine particles (with a diameter of less than 100 nanometers) emitted by thermal degradation of the brake pad materials in the event of overheating. Recent studies have confirmed that increasing the temperature of brakes, up to a certain threshold, favours the formation of nanoparticles. Thus, this temperature represents an important issue to be taken into account for the reduction of brake emissions.

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The chemical composition of the brake particles varies depending on whether they come from the wear of the brake disc, the brake pad, or an amalgam of the two. The disc is often made of grey cast iron (high iron content), whereas the brake pads have a more heterogeneous composition (organic, metallic, semi-metallic). The contribution of particles emitted by pad wear seems to be more important than that from the disc. Nevertheless, during braking, the particles formed can either be dispersed directly into the air (so-called primary particles) or be trapped at the contact surface before being released. This second type of particles (called secondary particles), which represents a significant proportion of the particles emitted, undergoes physico-chemical reactions that modify their size (diameter less than 1 µm) and their chemical composition. The latter does not therefore directly reflect the chemical composition of the braking system (pad and/or disc). Moreover, considering the micrometric or even nanometric size of brake particles, they can penetrate the pulmonary alveoli and bronchial tubes, which can lead to cases of inflammation, asthma attacks, chronic bronchitis or, in the most serious cases, to cancer. The chemical composition of brake particles could also reveal their harmful effects on health. Indeed, brake wear generates particles with a high/medium iron content, which also contain carbon, from graphite, and various metals (copper, silicon, barium, zirconium, antimony, etc.). This composition is highly dependent on the composition of the brake pads, which is very complex and varies from one pad to another. Nevertheless, few studies have focused on the composition of the particles emitted in relation to the type of pads tested, and this could explain the diversity of tracers found in the literature. Among the common tracers of braking particles, iron and copper have been shown to be deleterious to brain and heart cells.

In summary, the link between the size of brake particles, their chemical composition and their possible health effects clearly justifies further research in this area, whether it concerns the development of measurement methodologies or technologies/strategies to control these emissions. Thus, in view of the evolution of the vehicle fleet, the situation is now increasingly worrying. The transition to electric vehicles, while reducing CO2 pollution, will not put an end to the emission of brake particles. According to an OECD report published last December, pollution from brake wear will become the main pollution from cars by 2030. In the coming years, the European Union is expected to introduce new solutions including, for example, a particle capture system for brakes (such as the “tamic” system developed by the Tallano company), as well as more durable and less harmful materials for brake pads. Recently, new copper-free brake pads have been developed by Delphi and Feredo as a better friction alternative. However, these pads appear to be more polluting according to a recent study by the Royal Institute of Technology (KTH) in Stockholm.

Finally, the impact on health of brake wear particles is undeniable and cannot be neglected. However, while some countries, such as the United States, have begun to look for solutions to this problem, in France and Europe, the subject is slow to emerge, except in the context of a few calls for projects (H2020, AQACIA-ADEME). Moreover, there are currently no regulations concerning the reduction of fine particle emissions linked to the braking system. The Euro 7 emissions standard, which is still being developed for implementation by 2024/2025, should set a regulatory limit for these particles and would require an evolution of motor vehicles.

Written by Dr. Asma BEJI June 2021

PhD on non-exhaust particles