Research indicating wood-fired boiler heaters spew carcinogens and lung-clogging matter has raised concern across the United States.

The U.S. Environmental Protection Agency is considering guidelines for states to regulate wood-fired boilers, The New York Times reported Monday. Dozens of local governments have already taken such steps.

Proponents say the boilers save owners thousands of dollars on heating bills per year, while reducing U.S. dependence on foreign oil. But the New York attorney general’s office maintains the boilers generate as much pollution per hour as 45 cars or two heavy-duty diesel trucks, the Times said.

The fast-growing alternative outdoor energy source was designed to heat farmhouses. Roughly 150,000 units are in use, a number about double that of two years ago say scientists who are studying the boilers’ environmental impact.