Dozens of Greyhound buses roll in and out of Washington, D.C. each day. While they’re here, many of them sit idling at Union Station, in violation of the District’s environmental laws, according to D.C. Attorney General Karl Racine.

Racine and the District Department of Energy and Environment have filed a lawsuit against the bus company, alleging Greyhound buses regularly flout the city’s three-minute idling limit, polluting the air and endangering the health of commuters and riders. The suit seeks $216,000 in penalties from the company, as well as reimbursement for the cost of the investigation.

The anti-idling law applies to commercial vehicles — not regular passenger cars — and there are a few exceptions. If it’s below freezing, for example, the limit is five minutes.

Throughout 2018, investigators documented 50 Greyhound buses idling for longer than the three-minute limit. Many were idling for longer than 20 minutes. In early 2019, the Department of Environment sent 50 notices of violation to the company. Months later, the idling was still happening — inspectors documented four more violations in May and July.

Exhaust from vehicles is the largest source of air pollution in the city, causing health problems including asthma and heart disease, and accelerating global warming. Nationally, vehicle idling wastes an estimated six billion gallons of fuel, and generates 30 million tons of carbon dioxide. That’s equal to the emissions of five million cars over the course of one year.

In D.C., fines for idling violations start at $500 for the first offense, and climb to $1,000 for the second offense, $2,000 for the third, and $4,000 for offenses beyond that.

Greyhound is not the only company to regularly ignore the idling law, but other offenders may be harder to catch. A list of citizen complaints for the first quarter of 2016 shows 28 reports of illegal engine idling. Of those, 21 were investigated, but no violations were discovered. Three were determined to be legal idling situations, for example, a private passenger vehicle. In three cases violations were issued.

Only one case, out of the 28, was marked “action pending.”

Greyhound did not immediately respond to a request for comment.

This story originally appeared on WAMU.