California, a state plagued by smoggy skies and rising asthma rates, adopted the nation's toughest diesel emission standards Friday for the trucks and buses that crowd its highways.

The state Air Resources Board approved the rule despite warnings it could shut down many small trucking companies in the state. Many of them rely on the older, dirtier vehicles targeted by the change.

The regulation comes one day after the board adopted a sweeping plan to reduce the state's greenhouse gases, which is expected to change everything from the way factories operate to the fuel Californians put in their vehicles.

Starting in 2011, the diesel rules will speed up the replacement of thousands of polluting trucks and buses that typically stay on the road for decades and are not as clean as newer models that have tougher, federally mandated emissions standards.

Board chairwoman Mary Nichols said California has a legal obligation to clean up pollution and meet federal air standards. Failing to meet those targets could cost the state an estimated $2 billion in federal transportation funding.

Air regulators estimate the emissions standards would cost businesses, school districts and transit agencies $5.5 billion over 16 years.

Many trucking companies say they cannot afford to comply.

Ron Faulkner, president of Tulare-based Faulkner Trucking, estimated it would cost him $7 million to replace 26 of his 35 aging trucks by 2014. He said he doesn't know if he can afford it, since his company only turns a profit of $50,000 a year.

"I've worked hard to build this to where it's at and they're going to tear it down," he said.

Nearly a million vehicles will have to be replaced or retrofitted with smog traps, filters or cleaner-burning technology beginning in 2011. By 2014, all trucks must have soot filters, and by the time the rule is fully implemented in 2023, no truck or bus in California will be allowed to be older than 13 years unless it has equipment to cut nitrogen oxide emissions.