Dear EarthTalk: I am considering buying Honda's natural gas Civic. What exactly comes out of a natural gas vehicle's tailpipe, and how harmful to the environment is natural gas extraction and refinement? Which is greener, a hybrid or natural gas car?

"" Alex Neal, San Diego CA

Honda's natural gas Civic GX, which debuted in 2006 in California but is now becoming available in other parts of the country, just may be the cleanest mainstream car on the road. At least the American Council for an Energy-Efficient Economy (ACEEE) thinks so. The nonprofit group publishes an annual Green Book listing the greenest (and meanest) cars of the year, and put the Civic GX at the top of its 2007 environmentally friendly car list, edging out Toyota's hybrid Prius.

Although neither car is a slouch when it comes to fuel economy and reduced emissions, the natural gas-fueled Civic scored slightly better than the Prius on both counts in ACEEE's battery of tests. It also scored better in terms of the pollution generated in the manufacturing processes.

Natural gas is the cleanest burning of all fossil fuels. According to the U.S. Department of Energy, the burning of natural gas emits 117,000 pounds per billion (ppb) BTUs of carbon dioxide as compared to gasoline's 164,000. Its 92 ppb of nitrogen oxide emissions are considerably lower than gasoline's 448, and its mere one ppb of sulfur dioxide emissions is dwarfed by gasoline's 1,122. Natural gas also emits just seven ppb of particulates compared to 84 for gasoline, and it emits no mercury whatsoever against the trace amounts emitted by gasoline-burning engines. Natural gas combustion does generate slightly more carbon monoxide than gasoline, at 40 ppb versus 33, but the difference is negligible.

The big trade-off for Civic GX owners is the car's limited 220-mile range between fuelings. The gasoline-powered Civic can go 350 miles on a tank; the Prius, even with just an 11-gallon tank, can go considerably further operating at as much as 55 miles per gallon in highway driving. While a few dozen natural gas refueling stations have popped up around the U.S., they are few and far between. For those who need to make longer trips but still value a greener ride, a hybrid may be the best bet, as it will produce only marginally worse emissions while taking advantage of the ubiquity of gas stations out on the road.

Those who already use natural gas for home heating can pay $5,000 for a car fueling system installed in their garage or driveway. While that cost may seem high, owners can save about $1 per gallon over gasoline and can also get a federal $1,000 tax rebate. (Also, like the Prius, the purchase of the Civic GX itself qualifies for a federal tax break of $2,000 as well as up to another $2,000 in state and local incentives where applicable.) Some Honda dealers lease home systems for between $34 and $79 monthly. Honda pegs the fuel cost at 3.75 cents/mile, compared to 8.8 cents/mile for the gasoline-powered Civic.

Regarding the extraction and distribution of natural gas, the fuel is often sourced along with or near oil reserves, and involves similarly invasive drilling methods. Accidents do happen from time to time and, though natural gas does not spill like oil and cause ground and sea-level ecosystem disturbances, it rises into the atmosphere where it contributes directly to global warming.

CONTACTS: Database for State Incentives for Renewables and Efficiency, ; ACEEE's Green Book, ; Honda Civic GX, /civic-gx.

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