The U.S. government’s Environmental Protection Agency (EPA) ordered automaker Volkswagen to recall close to half a million cars September 18 for apparently using software to skirt smog-reducing emissions standards.

The EPA has accused the company of installing an illegal “defeat device” in four-cylinder diesel Audi and Volkswagen vehicles from model years 2009 through 2015 that would essentially let the cars cheat to pass official emissions tests.

The device uses software to determine when the vehicle is going through a government emissions test and enables its full emission systems controls; after the test, during regular use, it turns these controls off so that the car pollutes a lot more heavily.

Specifically, the device the EPA accuses Volkswagen of installing in its cars would help it circumvent ozone- and smog-creating nitrogen oxides emissions standards. The pollutant has been linked to a range of health problems, including asthma attacks.

“Using a defeat device in cars to evade clean air standards is illegal and a threat to public health,” Cynthia Giles, EPA assistant administrator for the Office of Enforcement and Compliance, was quoted in The New York Times.

“Working closely with the California Air Resources Board, EPA is committed to making sure that all automakers play by the same rules. EPA will continue to investigate these very serious violations.”

The allegations leveled by the EPA would apply to roughly 482,000 diesel cars sold in the U.S., including VW Jetta, Beetle, Golf and Passat models from model years 2009 through 2015. Model year 2009 through 2015 Audi A3 cars would also be affected.

At a maximum civil penalty of $37,500 per vehicle found in violation, this could mean the fines VW would face could be as much as $18 billion.

An Environment Canada spokesperson said September 18 that the organization has lined up its emissions standards with the EPA’s, and that it would also be looking into whether Volkswagen has made any infractions upon Canadian regulations.

“The Department is in discussions with its U.S. EPA counterparts to further examine this issue and assess potential implications for Canada,” Mark Johnson wrote in an email to Autofocus.

“Environment Canada takes matters related to environmental pollution very seriously. When violations are found, enforcement action is taken in accordance with compliance and enforcement policies.”

In November 2014, U.S. administrators fined Hyundai and Kia a combined $300 million for violating the Clean Air Act by overstating how fuel efficient their cars were. It was the largest ever penalty for a Clean Air Act violation.