Biofuels, good. Fossil fuels, bad. For a while now, that's been the basic message from the camp in favor of getting the world to kick its $5-billon-a-day oil habit. But there's a growing concern among scientists and clean-tech advocates that some green fuels are better than others.

The latest zinger? A Stanford University study last month said emissions from ethanol, the world's most widely used biofuel, could in some cases be worse for public health than gasoline pollution.

And tailpipe emissions aren't the only worry. Devoting more landto grow corn to make ethanol, or soybeans to make biodiesel, say, could hurt native crops and push up world food prices. “The availability of adequate food supplies could be threatened by biofuel production to the extent that land,water and other productive resources are diverted away from food production,” a United Nations report released earlier this month warned.

What's more, the process required to turn some of these materials into fuel can be dirty and energy intensive, limiting if not altogether canceling out the environmental benefits of using them.

All these red flags have prompted a group of researchers to call for a biofuel ratings system that would help consumers and others figure out which biofuels are truly green. “The fact that biofuel is made from biomass doesn't automatically mean it's good, and we need to have more information on that,” said Alex Farrell, assistant professor at the Energy and Resources Group, University of California at Berkeley.

Farrell co-authored a study on the need for a “green biofuels index.” He has also been named to the Roundtable on Sustainable Biofuels, an international group set up to draft the world?s first environmental and social standards for biofuels.

Creating a way to rate the impact of biofuels on global warming and the environment makes sense for several reasons. For starters, if the industry doesn 't identify the earth-friendliest fuels now, “we run the risk of creating very significant environmental problems as we rush to fix the oil problem,” Farrell said.

A biofuels index could also encourage more people to tank up with green fuels by raising awareness of a fuel?s benefits?once the benefits are clearly established by an index, that is. For now, drivers are confused about these new options. A survey by Pavillion Technologies in January found 44 percent of U.S. motorists don?t understand the difference between biofuels and regular gasoline. But 95 percent also said they could be encouraged to make the switch, according to Pavillion, which makes software used in ethanol manufacturing.

Much of the attention and scrutiny around biofuels has centered on ethanol. It's the main biofuel, making up much of the 5 percent that biofuels represent in the overall U.S. gasoline market. It is already blended with gasoline used in vehicles in something called an E10 blend of 90 percent gasoline and 10 percent ethanol. Even President Bush has jumped on the ethanol bandwagon, setting a goal of producing 35 billion gallons of biofuels by 2017, the bulk of them likely to be ethanol. To achieve that goal, the U.S. would have to ramp up production dramatically: it produced only 4 billion gallons of ethanol in 2005, the most recent year for which figures were available.

Before the U.S. races to meet these targets, however, a biofuel ratings system should be in place. That?s because ethanol from corn may not be the smartest bet. If the U.S. were to satisfy just 10 percent of its energy needs using corn-based ethanol, the country would be forced to divert into ethanol production all of the corn grown for animal feed and some of the corn now grown for export, disrupting prices and the market, Farrell says. Making matters worse, corn receives the most fertilizer and pesticide per unit area of any major crop, according to Farrell?s study. Depending on the source and technology, the conversion from corn to fuel can use up “large amounts” of water and lead to “significant” greenhouse gas emissions.

“We really have to look at corn-based ethanol as a bridge fuel,” said Ron Pernick, co-founder and principal of the clean-tech research firm Clean Edge in Portland, Oregon, suggesting its use will lead to other innovations down the road.

Clean-tech researchers are already scrambling to develop new green fuels, including biobutanol?another alcohol produced from biomass?and synthetic biodiesel?diesel made from natural materials but using a catalytic process instead of the usual fermentation process to create what backers say is more efficient diesel. Among the more exotic plans? Companies are working on making biodiesel made out of algae–it?s oily–and even poultry guts, Pernick said.

But there are pros and cons to other new fuels too, underscoring the need for standards. For instance, developing “cellulosic” ethanol may have a smaller impact on feedstocks because it is made from a variety of materials including willow and switchgrass, as well as wood shavings and other waste. But turning the stuff into the sugar, which is then fermented into alcohol to make fuel, currently requires intense heat, acid or additional enzymes, Farrell?s study says, suggesting potential environmental problems.

The other currently available biofuel?biodiesel–is simpler to produce than ethanol, resulting in fewer negative environmental effects. A small amount of biodiesel comes from recycled kitchen grease and other wastes, but most often it is made from oilseed crops like soybean. One main hitch: “There is no way you could grow enough plants on this earth to replace all the petroleum we use,” said Gretchen Zimmerman, a worker and owner of BioFuel Oasis, a Berkeley, California-based biodiesel service station. Some 95 percent of the oil Zimmerman?s outfit uses comes from restaurant and other waste. But the limited supply means the operation will likely remain a boutique business, with around 40 customers a day.

For her part, Zimmerman is pushing something that eventually may put her out of business: conservation. “We try to discourage people from using our product,” she said. Indeed, the Earth Policy Institute reported earlier this year that increasing auto-efficiency standards by 20 percent over the next decade would save as much oil as converting the entire U.S. grain harvest into ethanol.

It?s true that clean-techies and others must identify which biofuels make the most sense for the environment and public health to help stave off global warming and other problems. But at the same time, encouraging conservation could buy them more time to develop the best ones.