Metal particles from the abrasion of brake pads (brake abrasion dust, BAD) may cause inflammation and reduce the ability of immune cells to kill bacteria, similarly to particles derived from diesel exhaust particles (DEP), according to a new study led by researchers at King’s College London.

The researchers say this suggests that particulate pollution from brake wear—richly metallic, non-tailpipe-derived particles—also impairs the ability of immune cells to ingest respiratory pathogens and could be contributing to increased susceptibility to airway infections and other negative effects on respiratory health.

Representative images and particle size distributions of brake abrasion dust (BAD) taken via SEM at 1000× and 4000× magnification (Ai–ii). Selley et al.

In the open-access study published in the RSC journal Metallomics, researchers obtained dust from a brake pad testing factory that tests a broad range of drum brakes from a mixture of buses and trucks currently used in Europe under conditions representative of urban driving and high-speed braking. To test its effect on immune cells, they grew macrophages—a type of immune cell that is on the front line of our defences in the lungs and kills bacteria by engulfing and digesting them—in the lab.

When they exposed the macrophages to particulates from diesel exhaust and brake dust respectively, both reduced the ability of the macrophages to take up and destroy bacteria. They were tested with Staphylococcus aureus, a common infection in the lungs.

By adding metal chelators, chemicals that can bind to metal ions, stopping them from reacting and causing damage, they prevented the negative effects on immune cells. This suggested that the metal content of the particulate dust was causing ill effects. The particulates from both sources also caused the macrophages to produce immune-signalling molecules which drive inflammation.

The scientists were surprised to find that the metals in both brake dust and diesel exhaust had similar effects on the immune cells, as the diesel exhaust lacked the iron and copper common in brake dust, which is known to generate reactive oxygen species (ROS) in the body. ROS are chemicals which can cause damage to cells on entering the lungs in the body.

There were many other metal particles they had in common (such as arsenic, tin and antimony), but the researchers concluded the biggest culprit was most likely to be vanadium, as it was the only metal that was taken up by the cells increasingly as the dose of brake dust and diesel exhaust particulate matter increased.

At this time the focus on diesel exhaust emissions is completely justified by the scientific literature, but we should not forget, or discount, the importance of other components, such as metals from mechanical abrasion, especially from brakes. There is no such thing as a zero-emission vehicle, and as regulations to reduced exhaust emissions kick in, the contribution from these sources are likely to become more significant.

—Dr Ian Mudway, Senior Lecturer School of Population Health & Environmental Sciences

It is estimated that only 7% of PM_2.5 pollution from traffic comes from tailpipe exhaust fumes at roadside sites—the rest comes from sources such as tire, clutch and brake wear, as well as the resuspension of road dust.

Brake dust is the source of approximately 20% of total PM_2.5 traffic pollution.

Diesel fumes and brake dust appear to be as bad as each other in terms of toxicity in macrophages. Macrophages protect the lung from microbes and infections and regulate inflammation, but we found that when they’re exposed to brake dust they can no longer take up bacteria.

Worryingly, this means that brake dust could be contributing to what I call ‘London throat’—the constant froggy feeling and string of coughs and colds that city dwellers endure—and more serious infections like pneumonia or bronchitis which we already know to be influenced by diesel exhaust exposure.

—Dr Liza Selley, MRC Centre for Environment and Health at King’s College London and Imperial College London

This research was conducted in cells in the lab; further research is needed to fully understand how brake dust particles could contribute to ill health in humans.

The researchers were supported by the Medical Research Council, the In Vitro Toxicology Society and the National Institute for Health Research Health Protection Research Unit in Health Impact of Environmental Hazards held at King’s College London in partnership with Public Health England and Imperial College London.

Resources

• Liza Selley, Linda Schuster, Helene Marbach, Theresa Forsthuber, Ben Forbes, Timothy W. Gant, Thomas Sandstrom, Nuria Camina, Toby J. Athersuch, Ian Mudway and Abhinav Kumar (2020) “Brake dust exposure exacerbates inflammation and transiently compromises phagocytosis in macrophages” Metallomics doi: 10.1039/C9MT00253G