SACRAMENTO, Calif. -- The California Air Resources Board has approved a landmark regulation aimed at reducing pollution emitted by older heavy-duty diesel trucks and buses.

The approval came despite criticism and protests from upwards of 100 business groups and testimony the regulation would drive small truck and bus operators out of business.

The In-Use Truck and Bus Rule requires that older polluting buses, trucks and other large vehicles be retrofitted to reduce exhaust emissions or replaced.

The regulation applies to diesel-fueled vehicles with a manufacturer's gross-vehicle weight rating greater than 14,000 pounds that operate in California, regardless of where the vehicle is registered or based.

For fleets with four or more vehicles, the regulation will require the installation of exhaust retrofits in 2010 and 2011 and accelerated engine or vehicle replacement from 2012 to 2022. Implementation is delayed for fleets with one to three vehicles.

A wide variety of commercial vehicle operators tried to convince the board that now was the worst time to force them to make expensive changes.

"The cost doesn't change just because the economy is bad. So the only option people have to comply in a bad economy is to basically sell their equipment. They don't have the money to fix it. They don't have the money to replace it," said contractor Mike Lewis.

While truckers and others showed up in force to speak against the rule, testimony from proponents, including moms with children and other family members suffering from asthma, carried the day.

Anna Sanchez held up a picture of her asthmatic daughter who she says is sick because of California's bad air.

To help with the costs, the state will make loans and grants available to companies to help them comply with the regulation.