Turns out the increasingly eco-conscious buyers of the sporty German cars have been unwittingly pumping smog into the air — because of software VW installed to cheat on U.S. emissions tests.

The world’s largest automaker has admitted selling 482,000 such diesels since 2009, California and U.S. regulators announced Friday. The scandal could cost the company billions of dollars in fines and lawsuit judgments and threatens to stunt the development of all diesel vehicles.

How Volkswagen cheated on emissions rules

Autos editor Brian Thevenot explains how Volkswagen used a software trick to hide illegal pollution levels in half a million diesel cars.

Autos editor Brian Thevenot explains how Volkswagen used a software trick to hide illegal pollution levels in half a million diesel cars.

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VW’s software trick allows the cars to emit up to 40 times the legally allowed amount of nitrogen oxide, environmental officials said. The automaker will have to recall all the vehicles and modify the emissions systems at its own expense, regulators said. Additionally it could face a fine of about $18 billion, or $37,500 per car, federal environmental officials said.

“It's pretty ugly,” Kelley Blue Book analyst Karl Brauer said. “Volkswagen has far outstripped everyone else in selling diesel cars. This challenges everything they've been saying about those vehicles.”

Nitrogen oxide is among the auto pollutants that put more smog into California’s skies, Air Resources Board Executive Officer Richard Corey said.