State air quality officials will give businesses more time to comply with California's tough diesel emissions standards for trucks, school buses and construction equipment.

The Air Resources Board unanimously passed the more relaxed rules at its meeting Friday.

The board says the changes were needed because fewer diesel trucks and construction equipment are in use because of the down economy.

Clean air advocates said the new rules only ensure more harmful soot would be emitted into California's air.

Companies now will have four more years, until 2014, to begin upgrades to construction fleets.

The board says the amended rules still achieve the same amount of diesel emissions reduction over time, and would save vehicle owners $1.5 billion.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

SACRAMENTO, Calif. (AP) — Businesses will have more time to comply with California's tough diesel emissions standards for trucks, school buses and construction equipment under more relaxed rules air quality officials are expected to adopt Friday.

The Air Resources Board said the amendments would give businesses struggling during a recession more time to replace or upgrade aging equipment.

"These changes streamline and simplify the rules, and in some cases offer companies and truck owners more time to comply," said Mary Nichols, the board's chairman. "At the same time, we continue our steady progress in cleaning up the air and protecting public health, achieving the same amount of reductions of soot from diesel vehicles we had originally planned for."

The board has acknowledged that its staff overestimated pollution created by construction equipment, buses and trucks. It says there are about 200,000 fewer construction vehicles in California than the board initially thought, mostly due to fewer vehicles in use because of the recession.

The amended rules would still achieve the same amount of diesel emissions reduction over time, at savings of $1.5 billion for those being regulated, Nichols said.

Soot from diesel exhaust is associated with a number of human ailments, including cancer, heart disease and other diseases. The state's regulations seek to reduce emissions 50 percent by 2014, with a 70 percent reduction by 2020.

California's 2007 off-road diesel emissions rules require tractors, bulldozers, pavers and other types of equipment be replaced or retrofitted over the next 15 years.

The improvements were originally scheduled to begin in March 2010 for larger fleets, with smaller fleets starting later. The new rules push the start date back to 2014, giving companies more time to start using cleaner equipment.

"The decision to significantly revise the state's off-road diesel emissions rules gives hope to thousands of construction workers fearful for their job security while safeguarding California's air quality," said Brian Turmail, a spokesman for the Associated General Contractors of America.

"The new measure will give contractors time to modernize their fleets with the more efficient equipment coming on line in the next few years while simultaneously delivering lower diesel emissions levels than the original rule ever contemplated," Turmail said.

For big rigs, trucks and buses, the new rules give owners an extra year, to 2012, to install particulate filters.

Also, the new rules give owners of large school buses another year to install filters, and smaller buses included in the original 2008 regulations are completely exempt. The board said smaller buses being used by California schools are new vehicles, so they comply with the current regulations anyway.

The new rules also exempt 150,000 lighter trucks previously covered under the regulations. Owners would still be required to replace lighter trucks that are 20 years old.

Clean air advocates lambasted the board's decision, saying it will result in poorer air quality, especially in low income neighborhoods.

"The brunt of the pollution burden will be felt by the low-income communities living near major highways, agricultural areas and industrial centers," said Elizabeth Jonasson, the Coalition for Clean Air's campaign and outreach associate in the San Joaquin Valley."

The air board is meeting about the diesel rules one day after approving the nation's most extensive system of financial incentives for power plants, refineries and other major polluters to emit fewer greenhouse gases.

Under the so-called cap-and-trade rules, regulators would enforce limits on heat-trapping gas emissions beginning in 2012, eventually including 85 percent of the state's worst polluters.

Some businesses that would fall under the new rules say the system could dampen California's already flagging economy and lead to an increase in the price of electricity.