Diesel fumes have been proven for the first time to cause coughing and shortness of breath.
British scientists now have physical evidence that car fumes cause nerves in the lungs to misfire.
It was known that people with asthma, taken to traffic-heavy Oxford Street in London, for example, are worse affected than those in less polluted areas.
But researchers can now explain the effect of diesel particles, which are so tiny that the body mistakes them for natural molecules and draws them deep into the lungs.
A team from Imperial College London has discovered diesel fumes trigger a receptor in the airway, starting a chain reaction which causes nerves in the lungs to fire wrongly.