Clean-burning natural gas in cars and trucks could reduce the air pollution plaguing many of America's cities, according to Mountain Fuel Supply Co. and other natural gas suppliers.

Many cities face drastic sanctions from the Environmental Protection Agency because carbon monoxide and ozone pollution are beyond federal health standards.Gas suppliers hope the country's cities will begin converting their fleet vehicles to burn natural gas, thus reducing carbon monoxide and ozone pollution that results from burning gasoline.

Two vehicles that burn compressed natural gas were shown off to the national mayors conference Tuesday: a pickup truck owned by Mountain Fuel and a new Buick sedan owned by Southwest Gas Corp., an Arizona utility.

"There are a number of major advantages" with vehicles burning natural gas, said Susan Glasmann, Questar Corp.'s assistant manager for corporate communication. Questar is Mountain Fuel's parent company.

"Clean air is an important factor," she said.

Compressed natural gas - called by the initials CNG - releases far fewer air pollutants when it's burned. Also, there's not the problem with volatile hydrocarbons escaping whenever a gasoline tank is filled.

Evaporating hydrocarbons are a serious source of air pollution, leading California to require special aprons around gasoline nozzles so that fumes are captured.

"The cost (if using natural gas) is about half the cost of gasoline," Glasmann said.

In 1980, sparked by the international oil crisis, Mountain Fuel converted 25 utility vehicles to natural gas. Since then, four died a natural death when the trucks wore out. But the rest are still on the road and have had no problems.

The cost of converting a vehicle is about $1,800 or $2,000.

Originally, the conversions were to save money and gasoline, but now, with lower fuel prices and better mileage, the emphasis is on cleaning up the air.

Glasmann showed the two CNG tanks that power one of the trucks. Each holds the equivalent of about seven gallons of gasoline, and the gas is at 3,500 to 3,600 pounds pressure.

"The natural gas that powers an appliance at home comes in at about 4 ounces," she said.

The drawback is that a vehicle will get 120 to 300 miles (depending on whether it has one or two tanks, and their size) per filling - and there are no natural gas service stations.

Mountain Fuel has two fleet service stations for its vehicles in Salt Lake City.

So to make a real contribution to the war on air pollution, whole city fleets must convert to natural gas, with the necessary filling stations.

John Banchero, manager of materials and equipment services for Mountain Fuel, said when vehicles burn CNG, "there are some emissions from it, but they're well within any guidelines." In fact, they can be minuscule.

Any danger of CNG leaking or exploding? "Almost none," he said.

Glasmann said that rather than pooling and posing an explosion threat, as gasoline and propane do, CNG just diffuses into the atmosphere. "Insurance-wise, it's not perceived as being any different" than using gasoline, she said.

The tanks are reinforced fiberglass and aluminum.

"They've actually fired high-powered rifle bullets in them, dropped them into burning waste, subjected them to crashes - they've never had a failure," Banchero said.