American Taxi in Santa Ana, Calif., has become the first taxi company in the nation to power its fleet exclusively with clean-burning natural gas, a move made in anticipation of proposed smog-fighting rules in the region. In April, the South Coast Air Quality Management District will vote on a proposed regulation for Southern California that would make public vehicle fleets and private fleets that serve airports operate on clean-burning fuels. Meanwhile, a new study by a group that analyzes vehicle pollution for the federal government argues that hybrid diesel-electric buses are just as eco-friendly as those powered by natural gas. New York City officials felt vindicated by the study, as they have resisted the move toward natural gas buses and made a significant investment in hybrid-diesel technology. But some enviros criticized the study, saying that while natural gas and hybrid-diesel buses might produce comparably low levels of particulate matter, diesel particles are believed to cause more health problems.