A new proposal to clean Southern California's air would bring new technology to the war on smog, using roadside sensors to measure pollutants spewing from tailpipes as vehicles accelerate.

For now, authorities rely on smog checks, the California Highway Patrol, and other motorists to catch drivers who violate anti-smog regulations. Under a regional plan approved by state regulators, a camera would photograph the license plates of vehicles whose emissions are too high. A notice would then be sent to the owner, requiring that the vehicle be brought in for a smog check.

While few doubt the need to clear the region's skies, the program could become an expensive headache for motorists with persistent car trouble.

The sensor, an infrared beam that changes color if pollutant levels are too high, could snag vehicles that passed their last smog checks but fell out of compliance before their next check is up. Smog checks, which cost about $50, are required every two years.

"If a vehicle develops a problem and starts to become a high emitter, you don't necessarily have to wait two years until the problem is fixed," said Sam Atwood, a spokesman for the South Coast Air Quality Management District.

Officials in California and Colorado have tested the sensors before, but Atwood said he believed the planned rollout in parts of Los Angeles, Orange, Riverside, and San Bernardino counties would be the most widespread use in the nation. No firm start date has been set, but the board's vote called for the sensors to be in place by a 2010 federal deadline.

The proposal was added at the AQMD's request to a broader plan approved by the California Air Resources Board, which is designed to cut more than 1,200 tons a day of smog-creating pollutants throughout Southern California. The sensors would eliminate an estimated 16 tons a day, Atwood said.

Failure to meet the 2010 deadline could result in the loss of billions of dollars in federal highway funds.

Smog has been linked to asthma and other health problems, especially in children and the elderly.