Los Angeles Metro retired its last diesel bus on Wednesday, making the agency the first transport service in the United States to have all its buses running on alternative fuels.

The Metropolitan Transportation Authority (MTA) called the development a milestone, praising L.A. Metro as a pioneer in promoting alternative fuels.

"What Metro has achieved transcends Los Angeles County," MTA board chairman and Supervisor Don Knabe said. "We proved from both a technical and economic standpoint that a large transit agency can operate with alternative, clean-burning fuels, and this has led many other transit agencies to follow our lead."

The development also drew compliment from Mayor Antonio Villaraigosa.

"When I was growing up in Los Angeles, I remember days when the air was too smoggy to go outside and play, and today we understand how crucial clean air is for the health of our kids and communities," Villaraigosa said. "I'm proud that our entirely clean-fueled fleet is a key element of greening Los Angeles, and we're simultaneously improving mobility, customer service and air quality with these new clean-fueled buses."

Though diesel-engine technology has come a long way, old-style buses were known for spewing black clouds of cancer-causing soot. In 1993, Metro's board of directors started plotting their elimination.

After experimenting with methanol and ethanol buses, Metro ultimately went with buses that run on compressed natural gas (CNG) .

Today, it has 2,221 CNG buses, plus six gasoline-electric hybrid buses and one electric bus.

The city's Department of Transportation also uses alternative fuels in all of its 205-bus DASH fleet. The agency is in the process of converting its 105-bus Commuter Express fleet to alternative fuels as well.

Jane Warner, president and CEO of the American Lung Association in California, said that reducing the use of diesel would greatly improve air quality.

"By acquiring cleaner fueled buses, Metro is helping to address the region's serious pollution problems and reduce smog-related illnesses and deaths," she said.