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

The state Air Resources Board unanimously approved the new rule despite warnings that it could cause many of California’s small trucking companies to stop operating. 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 stay on the road for decades and are not as clean as newer models with tougher, federally mandated emissions standards.

More than 250 witnesses jammed the board’s meeting during two days of testimony on the rule.

Schoolchildren from Oakland, farmworkers and physicians from the San Joaquin Valley and representatives from environmental groups urged regulators to adopt the most sweeping diesel rule in more than a decade.

Truckers, loggers, independent dump truck and bus drivers and representatives of rural counties demanded the board delay what they called prohibitive regulations during a worsening economic recession.

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 California an estimated $2 billion in federal transportation funding.

“I don’t think anyone, no matter how committed they are to the cause of public health, truly believes that massive unemployment or large-scale bankruptcies of businesses are good for public health or for the economy,” Nichols said. “We are going to be looking for every possible way to make this rule feasible and livable.”

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

That’s a cost many small- and medium-sized trucking companies said they could not afford.

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.

“If this goes through, we’ll probably close our doors,” said Faulkner, who bought his first truck in 1988. “I’ve worked hard to build this to where it’s at and they’re going to tear it down.”

Critics also questioned whether technology being developed to cut nitrogen oxides would be ready in time.

The heavy-duty trucks that cart food, electronics, toys and other goods are the leading cause of diesel pollution in a state with some of the worst pollution in the country. The new rules will reduce ozone-eating nitrogen oxides and soot-forming particulate matter that can become embedded in lung tissue.

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 could be older than 13 years unless it had equipment to cut nitrogen oxide emissions.

Generally, the rule applies to any vehicle larger than a double-wheel Ford F-350 pickup truck, including those that come to California from other states, Canada and Mexico.

Tractor trailers, dump trucks, street sweepers, cranes, fuel delivery trucks, school buses, motor coaches and airport shuttles all must comply. Some military, emergency and vintage vehicles would be exempt, along with private motor homes, snow plows and those driven fewer than 1,000 miles a year. Vehicles in rural counties that meet federal air standards and some agricultural vehicles will get extra time to comply.

Regulators said the costs will be spread over 16 years and business could pass them on to customers. A staff report estimated consumers would see negligible effects such as 1 to 2 cents for a pair of shoes or a fraction of a cent extra for a pound of produce.

State officials said the cost is outweighed by an estimated $48 billion to $69 billion in health benefits for Californians who breathe diesel fumes.

The state also has several loan programs and bond money to help businesses replace their fleets.

The board directed its staff to report back in a year on the economic effects of the regulation and look for more ways businesses could get state funds.

It also gave small companies an extra year to meet the regulations — a step some environmental groups complained would delay the health benefits of the rule.

Board scientists estimate the amount of diesel particulate matter and nitrogen oxides emitted would be cut by about a third by 2023, preventing 9,400 premature deaths over 20 years, 150,000 asthma-related cases and 950,000 lost work days.

“We have lost people we love,” said Christine Cordero, a community health coordinator at the Center for Environmental Health based in Oakland. “We have seen people sick every day for years and decades from this problem. We cannot wait any longer.”

Regulators also adopted another rule requiring long-haul truckers to install aerodynamic devices and low-rolling tires on trucks and trailers to help cut greenhouse gas emissions.

That will change the look of an estimated 436,000 trucks and more than a million trailers at a cost of $10.4 billion. Regulators say the change will save about $14.7 billion in fuel, but several trucking companies disputed the board’s calculations.

Drivers at Save Mart Supermarkets, for example, travel an average of 45.7 miles per hour, far below the 62 mile-per-hour standard the board used to calculate the projected fuel savings, said Daniel Miller, a Save Mart vice president. Adding aerodynamic fixtures to large trailers also could make it hard for deliveries at narrow loading docks, he said.

“It will force us to replace our 53-foot trailers with smaller trucks, and then we’re running more loads and consuming more fuel and creating more greenhouse gases,” Miller said.

Nichols said companies that could not operate if they installed the devices would be exempt.

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On the Net:

California Air Resources Board: www.arb.ca.gov