Beijing is infamously known for its dangerous levels of smog, but a European city actually has greater concentrations of nitrogen dioxide: London.

Encouraging people to drive vehicles that run on diesel fuel instead of gasoline to lower carbon dioxide emissions has had the unintended effect of increasing contaminants in London air, Bloomberg reported.

"Successive governments knew more than 10 years ago that diesel was producing all these harmful pollutants, but they myopically plowed on with their CO2 agenda," said Simon Birkett, founder of the nonprofit Clean Air in London, as quoted by Bloomberg. "It's been a catastrophe for air pollution, and that's not too strong a word. It's a public-health catastrophe."

Contaminants from diesel vehicles have counteracted efforts including a driving toll to enter central London, a popular bike rental program and an expanding public transportation infrastructure.

"Nitrogen dioxide is a problem that you get in all big cities with a lot of traffic," said Alberto Gonzalez Ortiz, project manager for air quality at the Copenhagen-based European Environment Agency, as quoted by Bloomberg. "In many cases it's gotten worse because of the new fleets of diesel cars."

As the city has switched to a different fuel source, diesel cars have been emitting three times as much NO2 as traditional gas-powered vehicles. Regulations to curb NO2 emissions were passed in September.

"The challenge is much greater that we had thought just a few years ago," said Matthew Pencharz, environment and energy adviser to London Mayor Boris Johnson, as quoted by Bloomberg. "A lot of that is due to a well-meant EU policy that failed. We're stuck now with these diesel cars--about half our cars are diesel, whereas 10, 15 years ago, it was lower than 10 percent."

In 2012, London and around 300 other European sites passed the European Union's limit on NO2 emissions, including the cities of Paris, Rome, Athens, Madrid, Brussels and Berlin.

London passed a clean-air act in 1956, following the "Great Smog" that killed 4,000 people in 1952. As cold air held in industrial emissions and coal fumes, a thick blanket of smoke covered the city.

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