A spot of Christmas shopping in a busy town centre may damage your health as well as your bank balance.
Diesel vehicles are becoming increasingly popular
Air pollution from diesel traffic can worsen lung function in people with asthma, a team of international researchers has said.
The first "real-life" study showed lung function was worse in patients who spent two hours on London's Oxford Street compared with nearby Hyde Park.
The results are published in the New England Journal of Medicine.
Study leader Dr Paul Cullinan said previous studies had looked at the effects of pollution on a population level, for example comparing air quality with admissions to hospital, or in a laboratory, but not in a real-life scenario.
A total of 60 adults, half of with mild asthma and half with moderate asthma, walked for two hours along Oxford Street, where only buses and taxis are allowed, and then on a separate occasion walked for two hours in traffic-free Hyde Park.
Lung function tests done before and after the walks showed a greater reduction in lung capacity after participants had been exposed to diesel traffic than in the park and more inflammation in the lungs.
The negative effects were greater in those with worse asthma to start with.
Diesel engines can generate more than 100 times more particles than petrol engines, said the researchers.
The smaller the particle, the deeper it can be inhaled into the lungs and very small particles may even be absorbed into the bloodstream.
Researchers found three times as many ultra-fine particles (less than 0.1 microns in diameter) on Oxford Street compared with Hyde Park.
Oxford Street also had more than three times more nitrogen dioxide in the air and six times as much elemental carbon.
Dr Cullinan, honorary consultant in respiratory medicine at Royal Brompton Hospital in London, said the results were applicable to other urban environments.
"However, we don't know if you would find the same effects with petrol traffic or in people without asthma," he said.
"The real message is not for individuals with asthma but for people who plan traffic and build engines.
"With over five million people in the UK suffering from asthma, it is important that we that we urgently consider practical ways to reduce harmful emissions from diesel vehicles."
Dr Keith Prowse, chairman of the British Lung Foundation: "It very clearly and vividly demonstrates the negative impact diesel fumes have on lung health, especially in asthmatics."
Leanne Male, assistant director of research at Asthma UK said it was already known that living near a busy road was associated with worsening of asthma symptoms but it was unclear which chemicals were to blame.
"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."