In a study, lung cells became inflamed and less able to protect against bacteria when exposed to exhaust and brake dust particulates, while lung tissue was also damaged.

Metal-particle dust created by scraping brake pads account for a fifth of all tiny particle pollution on the roads, according to researchers.

By contrast, tail pipe fumes make up only 7% of the tiny particles found in roadside air – tyre wear, clutch scrapings and general road dust which may not be emitted from cars at all account for the rest.

“At this time the focus on diesel exhaust emissions is completely justified by the scientific literature,” said Dr Ian Mudway, who led the research at the MRC Centre for Environment and Health at King’s College, London.

“But we should not forget, or discount, the importance of other components, such as metals from mechanical abrasion, especially from brakes.”

While brake dust makes up 20% of total roadside particle pollution, non-exhaust emissions make up only 7.4% of particulates from cars.

The share is set to rise to 10% by 2030, this is partly due to the increasing weight of cars and particularly heavy electric vehicles increasing wear on brakes.

Dr Mudway said: “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.”

Air pollution causes an estimated 64,000 early deaths a year in the UK. Around three-quarters of these are due to particulate pollution.

Mike Hawes, Chief Executive of the Society of Motor Manufacturers and Traders, spoke in July 2019 on the subject: “The automotive industry is committed to improving air quality and has already all but eliminated particulate matter from tailpipe emissions. Brake, tyre and road wear is a recognised challenge as emissions from these sources are not easy to measure.

“A United Nations global group, including industry experts and government, is working to better understand, and agree how to measure, these emissions. Maintenance of the road surface, as well as further investment in new vehicle technologies, is essential to reducing these emissions, without compromising safety and we welcome further research in this area.”