World's highest level of toxic gas found on London's main shopping street...

As if the danger they represent to cyclists weren’t bad enough, new research reveals that London’s buses are a major source of the emissions that have given Oxford Street the world’s highest concentration of toxic nitrogen dioxide, which can trigger asthma and heart attacks.

According to The Sunday Times (£), researchers set up a monitoring station on Oxford Street and found a peak level of nitrogen dioxide of 463 micrograms per cubic metre of air (mcg/m3), over 11 times higher than the European Union’s limit of 40mcg/m3.

On average, including nights when the traffic is lighter and emissions level drop, the level of nitrogen dioxide is 135mcg/m3 — still four times the limit.

David Carslaw, an emissions researcher at King’s College London, said: “To my knowledge this is the highest in the world in terms of both hourly and annual mean.

“Nitrogen dioxide concentrations [on Oxford Street] are as high as they have ever been in the long history of air pollution.”

Diesel-powered taxis and buses are the main source of nitrogen dioxide, researchers say, making Oxford Street’s notorious wall of buses a double health hazard.

But it’s not just London that’s choking on nitrogen dioxide. In central Glasgow NO2 levels have averaged 67mcg this year and peaked at 213mcg/m3. In Bath they averaged 48mcg and reached 269mcg/m3.

Dr Benjamin Barratt, who works alongside Carslaw, said: “Oxford Street is full of buses and black cabs, all diesel powered.

“It’s a quirk of modern diesels that they produce more NO2 then in the past. This is because they have been designed to use NO2 to burn off particulates but, in the stop-start conditions found in congested streets, the systems are inefficient and produce far too much NO2.

“This is compounded by the high buildings on either side, which concentrate it to dangerously high levels.”

Richard Dickinson, chief executive of New West End Company which represents traders in Oxford Street, Regent Street and Bond Street, said a reduction in traffic was essential.

He told the Evening Standard: “We are working closely with the relevant London authorities to look at longer term traffic reduction initiatives and we are keen to see ideas rapidly put in place. Businesses in the West End want action.”

London’s City Hall hit said it has already cut the number of buses on Oxford Street by a fifth and called the figures “misleading”.

The danger to cyclists from pollution has long been a source of controversy even among experts. A 2010 study found that cyclists inhale five times more toxic nanoparticles than pedestrians and drivers.

But when air pollution in the South-East of England hit the top of DEFRA’s scale earlier this year, Dr Paul Cosford, director for health protection at Public Health England, said that normally healthy people should have nothing to worry about.

If you are concerned, it seems the best option is to avoid roads with lots of bus and taxi traffic. A 2012 study by Sustrans found significantly lower levels of pollution on London Greenways than on nearby main roads.

City Hall maintains that it’s not really that bad, and Mayor Boris Johnson is busy fixing it anyway.

A spokesman for the Mayor said: “Contrary to any misleading claims, London’s levels of air pollution are lower than many world cities. We know this because we are serious about monitoring pollution levels.

“To prevent severe episodes that cities like Beijing experience, the Mayor has overseen an ambitious package of measures including a comprehensive monitoring network which specifically target our most polluted areas.

“These measurements have led to the mayor reducing the number of buses on some of London’s busiest streets with high traffic congestion, like Oxford Street, and taking the most polluting buses and taxis off the road.

“This approach has already halved the number of Londoners living in areas above nitrogen dioxide limits.

“The mayor is clear more needs to be done, which is why he has announced an Ultra Low Emission Zone in central London from 2020 which will be a world first game changer for the capitals air quality.”

Our official grumpy Northerner, John has been riding bikes for over 30 years since discovering as an uncoordinated teen that a sport could be fun if it didn't require you to catch a ball or get in the way of a hulking prop forward.

Road touring was followed by mountain biking and a career racing in the mud that was as brief as it was unsuccessful.

Somewhere along the line came the discovery that he could string a few words together, followed by the even more remarkable discovery that people were mug enough to pay for this rather than expecting him to do an honest day's work. He's pretty certain he's worked for even more bike publications than Mat Brett.

The inevitable 30-something MAMIL transition saw him shift to skinny tyres and these days he lives in Cambridge where the lack of hills is more than made up for by the headwinds.