The European Union says nitrogen dioxide levels should not exceed 40 micrograms per cubic meter of air. NO2 levels on Oxford Street, however, peak at more than 450 micrograms per cubic metre, while average day and night levels since the beginning of the year have been 135 micrograms per cubic metre, more than three times the EU’s safety limit.

The tall, Queen Anne-style buildings along Oxford Street form an urban ‘canyon’ that stops pollution from the road level from diffusing into the sky.

According to Dr Robert Carslaw, principal air quality scientist at King’s College London and author of the study, the level of pollution is largely due to the use of diesel fuel by black taxi cabs and red double decker buses, and the location of the street in the heart of the city. 

The use of diesel is ironically an unintended consequence of early efforts to fight climate change. The European Union originally favoured diesel fuel over gasoline because it emits less carbon dioxide. Now scientists say diesel’s contaminants cause harm that exeeds its relative advantages.

London's Queen Mary University recently linked nitrogen dioxide exposure to pneumonia, especially in young children. It can also cause asthma and heart attacks. The World Health Organisation also says nitrogen dioxide can inflame the bronchitis  

Since 2009, Londoners have received more than 30 official warnings about the levels of pollution outside. Adults and children with lung and heart problems have been told to reduce physical exertion and avoid outdoor activity. But because air pollution and particulates in London are not visible, unlike the smog from unburnt fuel that shrouds urban centres including Beijing and Delhi, politicians don’t feel the same pressure to tackle the problem.

The London Mayor Boris Johnson has set up an Ultra-Low Emission Zone which will only allow the cleanest vehicles into central London. But the programme will start in six years and will deal with just seven percent of the total of London over the limit in 2020. Other measures the mayor's office has adopted include a toll drivers pay to enter central London, a thriving bike-hire program and growing public transport network.

Heather Acton, Westminster City Council cabinet member for sustainability, said the borough is exploring and implementing policies such as retrofitting to reduce the carbon output of buildings in Westminster. It has installed air quality monitoring stations, including
one in Oxford Street. And it has called for a 20 percent reduction in the number of buses on Oxford Street.

But that’s not enough to satisfy some Londoners, who are calling for a rapid switch to trams, electric buses and taxis. Some want London to follow New York and introduce traffic-free days to promote its main shopping area.

Meanwhile, the environment ministry said it had no plans to warn shoppers about hotspots such as Oxford Street but would continue to issue warnings on Twitter and elsewhere online.