Diesel fumes from public transport are putting asthma sufferers at risk, new research suggests.
Scientists from Imperial College London asked 60 people with mild to moderate asthma to walk along London's Oxford Street for two hours at a spot where only buses and taxis are allowed.
They found the volunteers experienced worse symptoms, reduced lung capacity and inflamed airways both during the walks and afterwards.
But when they took a similar stroll in a section of London's Hyde Park where traffic is banned completely, they experienced fewer problems.
The study is the first to analyse a real-life setting and find that traffic fumes have a harmful effect on asthma sufferers.
Leanne Male, assistant director of research at Asthma UK, said: "We know that living near a busy road is linked to worsening of asthma symptoms, and there is a wealth of anecdotal evidence showing that people with asthma believe that traffic fumes have an adverse effect on their condition, with nearly half telling us that it discourages them from walking in congested areas.
"We do not yet know enough, however, about the specific chemicals in exhaust fumes responsible for triggering these symptoms, and hope that further research like this will provide the answers to help us improve the lives of the millions of people with asthma in the UK who are affected by traffic fumes every day."