Brake dust could be more harmful to health than diesel exhaust

SOUTHAMPTON, England — As cities worldwide crack down on diesel vehicle emissions, a more insidious form of air pollution has been quietly growing alongside increased traffic – brake dust. Research concludes that the particles released when vehicles brake may actually be more harmful to human lung cells than diesel exhaust, with copper-rich brake pads emerging as a particular concern.

This finding comes at a critical time, as the shift toward heavier electric vehicles means more brake wear and potentially higher exposure to these harmful particles. While governments have made substantial progress in reducing exhaust emissions, brake dust remains largely unregulated despite contributing up to 55% of all traffic-related fine particles in urban areas.

Researchers at the University of Southampton and their collaborators examined how tiny particles from different types of brake pads affected human lung cells, focusing on the delicate air sacs where oxygen enters our bloodstream. They compared brake dust from four common types of brake pads against diesel exhaust particles. Much like comparing different recipes to see which ingredients might cause problems, they tested low-metallic, semi-metallic, non-asbestos organic (NAO), and ceramic brake pads.

Their findings, published in Particle and Fibre Toxicology, painted a concerning picture: brake dust from copper-enriched NAO and ceramic brake pads caused significantly more cellular stress and inflammation than both other brake pad types and diesel exhaust. These copper-rich particles triggered inflammatory responses and altered cell metabolism in ways that could potentially lead to disease.

Modern brake pads contain a complex mixture of materials that help vehicles stop safely. NAO brake pads, the most common type in the U.S. due to their low cost and good performance, were developed to replace asbestos-containing pads. However, manufacturers added copper fibers to maintain heat conductivity – a role previously filled by asbestos. This copper content turned out to be problematic.

When researchers exposed lung cells to NAO brake dust, copper accumulated inside the cells steadily as exposure increased. Using specialized molecules that bind to specific metals – like a magnet that only attracts one type of metal – they confirmed that copper was driving the harmful effects.

Perhaps most concerning was the discovery that copper-rich brake dust triggered a cellular response called “pseudohypoxic HIF signaling.” In simple terms, this means the cells behaved as if they were starving for oxygen even though plenty was available – similar to a false alarm that keeps cells in an unnecessary state of emergency. This same mechanism has been linked to various diseases, including certain cancers and scarring of lung tissue.

Some U.S. states, including California and Washington, have already begun restricting copper in brake pads – but these rules were originally created to protect fish and aquatic life from copper washing off roads into waterways, not to address human health concerns. This study suggests these restrictions may have the unexpected benefit of protecting human health as well.

As vehicle fleets transition toward electrification, the issue of brake dust pollution may become even more pressing. Electric vehicles tend to be heavier than conventional vehicles due to their batteries, potentially generating more brake wear particles. While electric vehicles can use regenerative braking to help slow down, they still rely on traditional brakes, especially for sudden stops.

“There is a huge body of evidence showing that airborne dusts are damaging to our health. Unfortunately, while the switch to electric vehicles (EVs) will eliminate exhaust emissions, which include toxic gases as well as dust, it will not eliminate road, tire and brake dust,” write study co-authors James Parkin and Matt Loxham, in an article published by The Conversation. “Studies indicate that, because they tend to be heavier, electric vehicles can generate more non-exhaust dust than petrol or diesel vehicles – the zero-emission label is clearly not accurate.”

The researchers say governments should urgently reconsider how they regulate vehicle-related air pollution. While reducing exhaust emissions remains important, addressing non-exhaust sources like brake dust – particularly their composition – may be crucial for protecting public health in our increasingly urban world.”