Metro is now the only major transit agency in the U.S. with a fleet run entirely on alternative fuels. Officials say the shift has sharply cut emissions of cancer-causing pollution. After almost two decades of effort to reduce vehicle emissions, the Los Angeles County Metropolitan Transportation Authority retired its last diesel bus Wednesday and became the only major transit agency in the nation with a fleet that is totally equipped with alternative-fuel technologies. In an urban area where diesel buses began operating in 1940, the MTA now has 2,221 buses powered by compressed natural gas, as well as one electric bus and six gasoline-electric hybrids. Transit officials estimate that the elimination of diesel engines has reduced the release of cancer-causing particulates from the bus fleet by 80% and greenhouse gases by about 300,000 pounds a day in one of the smoggiest areas of the country. MTA officials say that compressed natural gas buses cost more to buy and maintain than those powered by diesel but that the increased expenses are offset somewhat over the long run by lower fuel costs.