By Isabel Rowbotham, Co-Editor in Chief

Published October 11th, 2021

Car particulate, or non-exhaust emissions, comes from brake, tyres and road dust. This dangerous form of pollution contains heavy metals which are distributed into the atmosphere, potentially posing a risk to pedestrians and other traffic users.

Transportation exhaust emissions in the form of fine particulate matter (PM) and its effects on the environment have become well known to the public as a considerable source of air pollution. Emissions can be categorised as exhaust derived, which also includes gasoline derived compounds such as carbon monoxide (CO) and oxides of nitrogen (NOx). Non-exhaust emissions are typically dust particles from the wear and tear of brakes, tyres and road surfaces.

Research shows that non-exhaust emissions contribute to 50–75% of PM less than 10 micrometres (PM₁₀)emissions, which are larger particles, and 15–40% of PM less than 2.5 micrometres (PM_2.5) in the USA and China, and may reach approximately 94% of the total PM ₁₀ and 90% of PM_2.5 emissions by 2030. Therefore, non-exhaust traffic-related pollution poses a great risk to the environment and human health.

Non-exhaust emissions may threaten our health but are less understood and more difficult to estimate. While there have been some breakthroughs into lower exhaust emissions with the development of electric cars, there is still concern over how to tackle non-exhaust emissions.

‘Non-exhaust emissions are produced by electric and combustion-engine vehicles.’

Is the pollution coming from the wheels?

Brake dust is the most important traffic-related non-exhaust emission. Experiments of cars inside wind tunnels and on the test tracks have shown a correlation between brake events and brake airborne particles. Studies have found that brake material consisting of metallic lining forms three to four times higher emissions compared to other materials used.

‘The greatest wear occurs during acceleration, braking, and cornering.’

Vehicle aerodynamic studies have also shown that particulate matter originates during the car's drag. This creates a force that resists car motion, different from friction, in which wheel tyres resist and disturb the airflow making the car less aerodynamic. And it is this disturbance that causes the brake and tyre particulates to become airborne 一 around 50–70% of brake debris become airborne rather than settling on the floor.

During braking, initially larger particles (less than 10 micrometres) are ejected from the wheel, followed by smaller airborne particles (less than 1 micrometre). This cloud of varying-size particulates is potentially detrimental for humans, and may even affect those not directly in the vicinity of a car, such as those waiting at a bus stop, but also reach those indoors with a distance of approximately 100 metres from the roads.

There is substantial research that supports the claim that brake dust is the most prevalent non-exhaust emission source, in comparison with particles from tyre and road particulates. Nonetheless, more research is needed into non-exhaust emissions to correctly assess the environmental and human exposure to these traffic-related components.

Some of the limitations of real-world experiments are the difficulties in sampling dust as particles are lost during the collection process. Differences in pavement materials may also limit the total mass of dust picked up. Results vary from location to location, and climatic conditions and traffic behaviours may also account for conflicting results.

What’s inside this dust?

The dust or wear and tear from brake pads, tyres and roads contain a variety of components that are ejected from the wheels into the atmosphere. A study sampling the streets of three European cities (Girona, Zurich and Barcelona) found mainly road wear minerals such as aluminium, calcium, iron and vanadium from the asphalt.

Another component found was brake dust metals (iron, copper, zinc, chromium, tin and antimony. Brake pads include antimony (Sb), which is a friction material, and a considerable amount of which is released during braking. Additionally, antimony has been identified as carcinogenic.

Tyre dust consisted of organic carbon, sulphur and zinc. The latter metal was the most abundant heavy metal from tyres. This heavy metal distribution to the atmosphere is exacerbated by the car’s driving speed and road abrasion.

Antimony (Sb) is used in braking pads, which is suspected to pose a human cancer risk and should be deterred.

Car particulate matter is associated with cancer, heart disease and psychiatric disorders

Brakes and road dust contain heavy metals, Polycyclic aromatic hydrocarbons (PAHs) and sulphides, which are known to be carcinogenic. PAHs from road pollution and other sources can be inhaled, and at higher exposure levels can irritate the respiratory airways. They are also known to cause cancer.

The International Agency for Research on Cancer (IARC) has classified outdoor air pollution as carcinogenic to humans and 15% of lung cancers can be accounted for by exhaust and non-exhaust pollution.

A study on brake pad particles assessed the toxicity of exposure to lung cells in in vitro laboratory settings. Results showed that heavy metals, on a cellular level, decreased cell junction proteins which promotes cell death and increased pro-inflammatory cytokines.

Another study found a link between air pollution and psychiatric disorders in the United States and Denmark. Disorders like schizophrenia and bipolar are associated with complex genetic markers, but environmental factors also play a role.

While assessing a large population in Denmark, the study found that air pollution increases the risk of bipolar disorder, schizophrenia, personality disorder, and major depression. While in the United States a similar increase of bipolar disorder and major depression was observed, but the absence of signal from other disorders may be from problems with datasets and differences in exposure, as well as country-specific genetic variations. Although these studies did not discern the source of air pollution, they included PM₁₀ and PM_2.5 as well as PAHs as sources.

Additionally, PM₁₀ has been associated with cardiovascular disease and even linked to some deaths. A small study of nine young males assessed the potential exposure to brake wear emission of individuals inside a car, specifically for those whose occupation involved driving. The results observed were that those car pollutants may affect the heart’s autonomic physiology, increasing heartbeat frequency and pro-inflammatory and pro-thrombotic factors in drivers.

It is also important to mention that many of the studies into public health rarely discern between exhaust and non-exhaust emissions-since the two contribute to the overall PM total mass.

Future research

There is more research coming from the collaborations between engineering and health fields. At the University of Bristol, The Core Aerosol Science (CDT) in Aerosol Science is offering PhD projects on the behaviour of inhaled aerosols, providing a better insight into air pollution. One of the researchers from the University of Cambridge’s Department of Engineering, collaborating with the CDT programme, is lecturer Dr. Megan Davies Wykes, an expert in environmental fluid mechanics and air pollution. Dr Davies and others will be researching the transport and dispersion of non-exhaust pollutants in the flow around a vehicle, to predict the exposure of pedestrians and others.

Another project from the University of Cambridge on the Health Effects of Non-Exhaust Particle Emissions will be led by Dr. Adam Boies and Professor Anne Willis. Their studies predict that with the increase of electric cars with heavier batteries, non-exhaust emissions will continue to increase and they intend to investigate exactly how this will happen.

Non-exhaust emissions from car brake and tyre dust play a considerable role in air pollution. Exhaust (fuel combustion) and non-exhaust emissions contribute to the overall air PM mass. In particular, non-exhaust emissions contribute to PM₁₀ emissions due to their larger particle size. Experimental studies were able to replicate real-life vehicle emissions originating from the wheels, showing that braking events correlate to the ejection of braking material into the air.

The car industry has been improving their technologies for car performance, alternatively these technologies could be applied to better understand car debris, including its source of origin, distribution and fate. Further studies into particle transportation may help better quantify their significance and implication on human health and environmental risks. In the meantime, the automobile industry should be deterred from using antimony and other possible carcinogenic materials in the fabrication of brake pads due to its carcinogenic potential and other health effects.

Featured imaged: Reinhart Julian | Unsplash

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