Dirty diesel buses will be phased off California streets by 2007 under a far-reaching regulation unanimously adopted yesterday by the California Air Resources Board. The rule, the first of its kind in the nation, will require buses to use alternative fuels or cleaner diesel technology. It represents the first step in a major new effort by state officials to reduce the public’s exposure to diesel exhaust, which has been linked to cancer, asthma, and other health problems. Many of the state’s 75 transit agencies backed the new standard as reasonable, though they are concerned about the cost of implementation, an estimated $13 million a year statewide. Enviros said the standard is a step forward, but they would like the dirty buses phased out more quickly.