The trend comes amid a nationwide push to displace fossil fuels with home-grown renewable energy like biodiesel and ethanol, which the White House hopes will eventually reduce US dependence on foreign oil shipments.

"The greening of the waste management is here, but it's not going to happen overnight," said Bruce Parker, director of the National Solid Wastes Management Association.

New York, the largest city in the country, is the latest to pump biofuels into garbage trucks.

Starting July 1, the city will fuel its fleet of 4,500 garbage trucks with a blend of 5 percent biodiesel and 95 percent regular diesel known as "B5," according to Rocco DiRico, assistant commissioner for support operations in New York City's Department of Sanitation.

Using B5, New York's garbage trucks will emit 3 percent less particulate pollution, the soot-like emissions thought to trigger health problems such as asthma, according to the National Biodiesel Board.

B5 will also decrease the garbage fleet's greenhouse gas emissions by 5 percent, said DiRico.

New York City's switch to biodiesel follows close on the heels of San Francisco's conversion of 335 garbage trucks to biodiesel in March.

By switching to "B20" biodiesel, a blend of 20 percent biodiesel with regular diesel, San Francisco cut its greenhouse gas emissions from garbage trucks by 21 percent, according to the city.

Biodiesel, unlike regular petroleum-based diesel, is made out of biological sources such as animal fats or vegetable oils.

Several other cities across the country, including Seattle, Washington; Portland, Oregon; Berkeley, California; and Grand Rapids, Michigan, have also begun using B20 in their garbage fleets in the last few years.

Together, the six cities' use of biodiesel cuts regular diesel use by more than 2.3 million gallons per year. The United States uses about 50 times that amount of diesel in a day.

But the trend toward using biodiesel is growing.

Waste Management the company that uses B20 for Portland's garbage trucks, is piloting biodiesel for use in eight other states, said a company spokesman.

NorCal, which operates San Francisco's fleet, plans to begin using B20 for its trash operations in another six cities in Northern California, according to Mike Sangiacomo, the company's president.

But biodiesel is not a panacea, said Frank O'Donnell, spokesman for Clean Air Watch. In addition to using greener fuels, O'Donnell said, cities should install particulate pollution traps on diesel vehicles and explore other alternative fuels, such as natural gas, that cut emissions further.