SAN FRANCISCO — The California Air Resources Board (CARB) conditionally approved the use of diesel hybrid-electric buses (HEBs) for use by some transit agencies in California on June 24, according to The San Francisco Chronicle. According to CARB sources, the Board approved the purchase of diesel HEBs only for transit properties on the diesel path. A temporary waiver through 2006 allows the purchase of these buses as long as the drive train, after application of a .75 factor (25-percent offset) for hybrid technology to the emissions of the selected diesel engine, meets the current applicable standard. After 2006, the drive train will have to be certified, and the board did not decide what that standard would be. That will most likely be pushed off until next spring. The decision won praise from Municipal Railway (MUNI) chief Michael Burns, who deemed it terrific news. "This is the next step for us in reaching zero emissions by 2020," said Burns, who added that the San Francisco transit system would start moving ahead to request bids from engine manufacturers. Under Proposition I — passed in March by more than two-thirds of San Francisco voters — Muni must replace by 2007 its pre-1991 diesel buses, which number around 150 of the 540 vehicles in the diesel fleet. The air board's decision "means that Muni can implement the will of San Francisco voters and replace its dirty diesel buses without delay," said Linda Weiner, a campaign director at the American Lung Association. Diesel exhaust is a major source of particulate pollution that can trigger respiratory and heart problems. The bus uses an electric engine and an electric drive system, paired with a diesel engine smaller and cleaner than the one in a conventional bus, using less fuel and emitting less nitrogen oxides, particulate matter and hydrocarbons. Burns has been holding out for the diesel-electric bus, while rejecting the natural gas buses preferred by the coalition of environmental groups that put Prop. I, or the Healthy Air Enforcement Act of 2004 on the ballot.