A clean-air proposal with support from the governor is taking aim at a largely unchecked pollutant in northern Utah: diesel fumes.

The measure seeks to require emissions tests for cars and trucks that take diesel fuel, and it would apply only to counties with existing testing programs, says sponsor Rep. Patrice Arent, D-Salt Lake City. It would will be a big change for Utah County, which to date hasn’t required the testing for diesel vehicles.

About a dozen people, mostly air quality advocates and local representatives from northern Utah health agencies, told a legislative panel Friday the bill is a smart, low-cost move. The House Transportation Committee voted unanimously to pass the proposal on to the full House.

Ashley Soltysiak, policy director for HEAL Utah, told the panel that of all the factors contributing to poor air, “we think this is one of the major ones we can improve upon.”

An analysis of Davis County, which adopted the diesel emissions testing in 1996, shows that when diesel cars and trucks are found to be out of compliance, they pollute four times the limit and seven to eight times more than gas vehicles, Soltysiak said. Federal regulators require emissions testing for cars that take gasoline, but not those powered by diesel.

Diesel fumes emit compounds which can mix with other chemicals to form ozone and fine-matter pollution. That becomes a problem, Arent said, during northern Utah’s winter inversions, when low temperatures cause a layer of hot air to act like a Tupperware lid, sealing in sooty air at ground level.

The particulates—tiny fibers about 30 times smaller than the width of a human hair and 3 times smaller than dust particles—can lodge deep in the lungs and enter the bloodstream. They can contribute to asthma and make people more prone to respiratory infections, according to the federal Environmental Protection Agency.

Denni Cawley, the executive director of Utah Physicians for a healthy environment, said the potential for inversion-related health side effects is too big to ignore. The bill, she said, is “a move in the right direction.”

Arent estimates the change would lead to emissions testing for over 20,000 diesel vehicles that are currently are unchecked. And she has wide support. Sen. Curt Bramble, of Provo, has signed on to sponsor the bill in the Senate. And Gov. Gary Herbert highlighted the measure in a speech Thursday night.

Several automakers manufacture a diesel and gas version of each model, said Arent, so “just from a fairness perspective, why should one be inspected and the other one should not?”

Michael Kennedy, R-Alpine, questioned how much impact the bill could have on Utah's air quality and noted earlier it costs around $20 to test, and more for repairs and upgrades.

The change would reduce emissions significantly—by 170 tons a year—replied Bryce Bird, director of the Utah Division of Air Quality.

Meantime, budget committees are considering growing a program to help low-income residents afford the repairs needed to pass the emissions test, which tend to range from $500-$1,000, Arent said. The bill she added, would require free software upgrades by health departments doing the testing.