“German authorities have identified traffic as the main source of the pollution,” wrote the European Commission in a decision paper on February 20th.
Of particular concern are levels of nitrogen oxide from diesel exhaust, which have not fallen as expected after environmental green zones were imposed in many cities - in fact, levels of the gas have risen in many areas, the paper said.
Nitrogen oxide levels were found to exceed legal limits in 33 of 57 German regions tested, including Berlin, Berlin, Stuttgart, Munich, Cologne, Dortmund, Düsseldorf and Hamburg.
The German government had asked Brussels to give it until 2015 to reduce the pollution in these areas, but the Commission said action had to be quicker than that.
“The affected regions must act as fast as possible,” said a spokesman from the office of EU Environment Commissioner's Janez Potocnik.
“We ask the regions to act immediately to reduce traffic from private individuals,” Jens Hilgenberg of Friends of the Earth Germany (BUND) told the paper, and said this was the only way to reduce pollution.
In a bid to drastically cut emissions, some regions may consider banning cars altogether, wrote the Bild newspaper on Thursday.
The warnings, which were also issued for other European Union countries including Austria, the UK, France and Italy, must now be met with concrete plans to improve air quality and reduce pollution.
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If levels do not drop below acceptable limits, offenders could face legal action and possible fines from the European Court.