County commissioners repealed a county provision requiring the procedure for diesel-powered vehicles after learning the program was no longer required by the state to maintain air-quality standards.

A state diesel-inspection-and-maintenance plan requiring the tests was submitted to the U.S. Environmental Protection Agency - but never approved.

"The EPA determined that diesel-testing programs did not have enough scientific basis for determining pollution reductions," said County Commissioner Jerry Grover in a statement.

"Since the EPA would not acknowledge any pollution reductions from the program, the program will be dropped, effective 2006."

There's a catch: Owners of diesel vehicles with registrations due before January must still pass the diesel-emissions test, even if they wait until 2006 to do so.

And after the new year, diesel-vehicle drivers must still pass the regular safety inspection for registration.

The County Commission's decision may come as a surprise to motorists who assume the black exhaust of diesel-powered vehicles is more of a pollutant than gasoline exhaust.

Steve Alder, the Utah County Bureau of Air Quality manager, offers an explanation.

"Diesel engines may look like they pollute more because of the black cloud, but they don't discharge the same level of carbon monoxide as do gasoline engines," Alder said.

He says the bigger concern with diesel fuel is the amount of particulate matter expelled.

Utah County's particulate levels have been in compliance with national air-quality standards for roughly 10 years. Updated state and federal air-quality planning now shows that diesel emissions testing is no longer required to meet the national air quality standards, Alder adds.

Salt Lake County, however, will not be following Utah County's lead, according to Jim Brande, air pollution-control director for the Salt Lake Valley Health Department environmental health division.

"Air quality issues are different in Utah County and Salt Lake County," Brande said. "Our intent is to continue on with diesel-emission testing. We still see some good emission reductions here."

thollingshead@sltrib.com