Alternative fuels aren't solving Phoenix's air-pollution problem, and it's doubtful that they will anytime soon

Nor can anybody say whether Phoenix buses run as clean as they would if powered by the latest diesel engines. More advanced diesels are in the works, meaning natural-gas buses soon could be as obsolete as natural-gas pickups.

Another consideration regarding liquid natural gas: No one makes any in Arizona. So the natural gas used in Phoenix buses is trucked here from Colorado, presumably in diesel-powered tractor-trailers. This offsets some of the presumed pollution savings of the natural-gas buses.

Much hope is staked on bio-diesel these days, which can be made from fryer grease, among other things. Whether straight up or blended with normal diesel, bio-diesel fuel runs cleaner as far as some pollutants go, but adds more ozone-forming chemicals to the air. Unmodified diesel engines can run on B20, a mixture of 20 percent bio-diesel and 80 percent diesel, though extra maintenance problems crop up.

When Rudolf Diesel first fired up the engine that bears his name, it ran on a bio-fuel — peanut oil. That was more than 100 years ago. So why isn't bio-diesel everywhere? Because it costs more than petroleum products, it reduces fuel mileage, it's relatively scarce, and environmental benefits are uncertain.

In other words, it has the same problems as other alternative fuels.

"There are tradeoffs with using alternative fuels over gasoline," says Carol Weisner, an environmental protection specialist with the EPA in San Francisco. "You have to look at the whole life cycle to measure emissions impacts. It's complicated."

Clearly, reducing air pollution locally and worldwide will be a challenge.

One 2006 study shows that even bicycles, the ultimate in alternative-energy transportation, may be worse for the atmosphere than automobiles.

The problem, according to the study's author, Karl Ulrich of the University of Pennsylvania, is that switching from an SUV to a bicycle might make you healthier, thus increasing your lifespan and your total contribution to greenhouse gas in the atmosphere.

Apparently, then, the most effective cure for air pollution is to die young.