Are there any environmental benefits of diesel-powered cars? (Ask E).

--Bill Darcy, Concord, NH

The high fuel economy of diesel-powered vehicles, such as the Volkswagen Golf GL TDI, may seem like a no-brainer for environmentalists looking for a new car. But don't be fooled by the numbers. Though a four-cylinder, five-speed TDI gets 42 miles per gallon (mpg) in the city and 49 mpg on the highway, compared to 24 city/31 highway' for a similar gasoline-powered Golf, that diesel engine emits a lot more harmful pollutants. Swedish and German studies have reported that diesel-powered cars have twice the cancer potency level of gasoline-powered vehicles. And the Union of Concerned Scientists reports that diesel exhaust accounts for 70 percent of the cancer risk from California's polluted air.

The particulates, or soot, in diesel exhaust cause a host of health problems, from asthma to lung disease to premature death, according to the Environmental Protection Agency (EPA). Under current EPA regulations, diesel vehicles can emit 16 times more particulates into the air than gas vehicles can.

The EPA's Green Vehicle Guide rates cars and trucks on a scale of zero to 10 (10 being the best) based on their emissions of smog-forming pollutants. While a gasoline-powered Golf received a rating of seven, the Golf TDI got a one. The gargantuan Ford Excursion and Hummer H2, SUVs with gas mileage in the single digits, both received twos. CONTACT: EPA, www.epa.gov/greenvehicles; Union of Concerned Scientists, [phone], www.ucsusa.org.