A bus company that left its buses’ engines running while parked in Boston neighborhoods has settled a lawsuit by agreeing to pay US$5.6 million in damages for the resulting air pollution.

The settlement came in a citizen-enforcement lawsuit under the Clean Air Act, which allows any person to sue alleged polluters for the harm done to human health and the environment. The case was based on a Massachusetts law that limits vehicle idling to no more than five minutes.

Like nearly all cities around the world, Boston has trouble keeping its air clean and safe, in part due to vehicle emissions. Those emissions contributed to nearly 42,000 deaths in the U.S. in 2024, or more than four people every hour.

But the law, and the legal case against the bus company, didn’t target all vehicles. Rather, they were aimed at the easier target of idling vehicles – those whose engines are running for no real reason except that someone didn’t bother to turn them off.

Reducing air pollution

Many states, including Massachusetts, have laws regulating vehicle emissions, in part because vehicles are responsible for significant amounts of different types of air pollution.

Managing environmental pollutants is an increasingly complex practice, with new sources and pollutant types emerging all the time. It is a critical profession I have long worked to train........

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