Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Harry Reid (D-Nev.) are directing the Architect of the Capitol to switch the Capitol Power Plant, a major air polluter, to 100 percent natural gas.

The CPP, which provides the legislative complex with steam and chilled water, is “a shadow that hangs” over efforts to green the Capitol, the Congressional leaders said in a letter Thursday to Stephen Ayers, the acting Architect of the Capitol.

The plant “continues to be the number one source of air pollution and carbon emissions in the District of Columbia,” the leaders argued. While the Architect of the Capitol has taken steps to make the plant cleaner and more efficient, “more must be done,” they said.

Pelosi and Reid said their staffs have received “conflicting information” on what it would cost to operate the CPP with natural gas. They called on Ayers to develop “realistic budget numbers” for retrofitting the plant and a plan to retrofit at least one coal boiler this summer and the remaining boiler by the end of the year.

While it will cost more to use natural gas, it is worth the investment since it will allow the CPP to eliminate about 95 percent of sulfur oxides and at least 50 percent of carbon monoxide, the leaders wrote. The switch will also demonstrate that “Congress can be a good and conscientious neighbor” to Capitol Hill residents, Pelosi and Reid said.