You could drink the exhaust of the Honda FCX Clarity. The four-door sedan—the first hydrogen fuel-cell car available to the general public—emits only water. Powered by the electricity generated when hydrogen and oxygen combine to form H 2 O and with upholstery fabric made of fermented corn, the Clarity sure sounds green. But is it the “zero-emission sedan of the future,” as Honda claims?

Not yet. Most hydrogen fuel is derived from natural gas in a process that releases plenty of carbon dioxide, so the car and its 134-horsepower electric motor fall short of being footprint-free. Still, fueling a vehicle like the Clarity emits less than half the CO 2 released by its gas-guzzling counterparts for a given distance, says John Turner of the National Renewable Energy Laboratory in Golden, Colorado. The Clarity’s fuel efficiency equivalent of 68 miles per gallon clobbers even the feel-good 48 mpg of the Toyota Prius, and the car can go 270 miles on a $20 tank.

Just don’t plan on taking it cross-country anytime soon. Honda begins leasing the Clarity this summer ($600 a month, with limited availability) in Santa Monica, Irvine, and Torrance, three Southern California communities with rare access to hydrogen fueling stations. The company is working toward a greener, more abundant hydrogen supply line; a research station at its R&D headquarters turns water into fuel using solar power. The downside? It refuels only one Clarity a day.