In a statement issued today (17 May), the EC said that it is referring France, Germany, Hungary, Italy, Romania and the United Kingdom to the Court of Justice of the European Union for failing to respect agreed air quality limit values and for "failing to take appropriate measures to keep exceedance periods as short as possible".
Fumes from diesel vehicles are thought to be the main factor behind the illegal levels nitrogen oxide in the atmosphere. The commission began pushing for the clean-up in 2015.
Around 400,000 people die prematurely every year across the bloc because of pollutants they breathe, according to the EU's executive.
However, EU Environment Commissioner Karmenu Vella told a press conference in Brussels the six, which include Europe's top four economies, had not acted quickly enough.
"The commission had to conclude that ... that the additional measures proposed are not sufficient to comply with air quality standards as soon as possible, and therefore are being referred to court", Vella said.
The Commission said it was standing up for the right of citizens to breath clean air.
"It is my conviction that today's decision will lead to improvements for citizens on a much quicker timescale".
The EU introduced limits for particulate matter (PM10) and nitrogen dioxide (NO2) in 2010.
Government research shows clean air zones CAZs, in which cars are deterred from city centres by pollution charges, are by far the most effective solution to air pollution but ministers have refused to make them compulsory.
"The European Commission's action, which could end with the United Kingdom having to pay fines, should be another strong catalyst for ministers who have failed for nearly a decade to take the necessary action".
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