American drivers use 400 million gallons of gasoline every day, and every one of them comes at a cost.

"Burning one gallon of gas dumps 25 pounds of carbon dioxide into the atmosphere, and uses up 2.8 to 6.6 gallons of water (depending on where the well is located)," says Michael Wang, a senior transportation and energy scientist at Argonne National Laboratory near Chicago.

One of the few who has attempted a "well-to-wheel" lifecycle analysis of gasoline, Wang calculates how much carbon that gallon generates from the moment it's pumped to when you finally burn it on the way to the grocery store.

And that's merely the beginning, he says. There are hundreds, if not thousands of other, hidden variables involved in gasoline production.

No one, for example, has any idea of a species-by-species breakdown of the oil industry's impact on biodiversity, even though everyone admits it is substantial.

Yet while there's never going to be anything close to a "green gasoline," there are definitely greener oil companies.

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That started in 1986, when Congress reacted to what's now known as the Bhopal gas tragedy, a 1984 industrial disaster at a Union Carbine plant in Bhopal, India.

It passed the Emergency Planning and Community Right to Know Act, mandating that companies report the amount of toxins — defined by the EPA's Toxic Release Inventory (TRI), and based on a list of the 600 most dangerous chemicals — they release into the air, water, ground and/or transfer to storage facilities.

Using this data, the Political Economy Research Institute (PERI) at the University of Massachusetts put together both a 2002 and 2005 Toxic 100 list — rankings of the 100 worst air-polluting American companies.

"What we computed," says Michael Ash, Associated Professor of Economics and Public Policy at UMass and one of the main researchers involved with the Toxic 100, "is long-term chronic human health impact.

"For example, what we call the 'toxic score' is a combination of how toxic are the chemicals released, how far they spread, and how many people are affected by that spread."

Using this metric, Conoco Philips was found to be the third worst offender in 2002, having dumped 8 million pounds of pollutants into the air.

But by 2005, the company had cleaned up its act a bit, lopping 1.5 million pounds from its previous total and dropping down to 13th place.

Meanwhile, Exxon Mobil scored higher on the bad list, being the 9th worst polluter in America in 2005. By comparison, BP held down 29 and Royal Dutch Shell was at 42. If you want to vote at the pump, then Marathon Oil — all the way down at 96 — seems the best bet.

PERI didn't stop there. It did a separate report, called "Justice in the Air," which looked at the distribution of pollution impact based on demographics such as race and income.

"What's really surprising," continues Ash, "isn't just that Exxon Mobil is the 9th worst polluter, it's that 70 percent of their pollution burden falls on low-income minorities."

Meanwhile, in 2006, Sierra magazine published "Pick Your Poison: An Environmentalist's Guide To Gasoline," an attempt to take this sort of investigation even further.

The magazine's staffers admitted it was not an easy task.

"These companies are among the largest and most powerful enterprises on the planet," wrote Sierra senior editor Paul Rauber. "The complexity of their organization and activities, the vastness of their reach and the huge number of variables involved make objective ranking difficult."

Because of this, Sierra settled on an appropriate three-tiered approach: bottom, middle and top of the barrel, only ranking from best to worst.

Just like PERI, it ranked Exxon-Mobile as among the worst, for reasons stretching from the amount of oil it's spilled into the oceans to its troubling (as assessed by groups like Amnesty International) human-rights history.

In the middle of the pack was Shell, whose "black marks" for its politics and pollution levels were balanced against the $1 billion it's pledged to developing alternative energy sources and its continued reduction of greenhouse gases. (Shell's 2002 total was 10 percent below 1990 levels; by 2005 it had dropped below 15 percent.)

Topping the list, with best marks, was Sunoco, the only oil company to sign the Coalition for Environmentally Responsible Economies (CERES) principle, which requires full transparency in environmental reporting and has resulted in Sunoco releasing all of its ecological impact data for public scrutiny.

And just this past February, Greenopia.com, a "greenwashing" watchdog organization, released a possibly more accurate oil-company-ranking guide.

It worked with data for the year 2007 and did a 12-part analysis.

Measuring everything from oil spills and air pollution, through transparency and stances on climate change, all the way to emissions-per-barrel of oil and energy efficiency, Greenopia then did something no one else had tried — it adjusted for scale.

"If you look at everyone else's rankings," says Greenopia senior researcher Doug Mazeffa, "Exxon Mobil always ends up as the worst offender.

"People get stuck on the 141 million tons of greenhouse gases they emit — which is so much more than any other oil company — but they're also the largest oil company in the world.

"If you scale up Marathon to Exxon's size, then their numbers become two to three orders of magnitude higher. Adjusted for scale, you quickly find out that Exxon isn't the worst offender."

On Greenopia's list, for reasons given as "lackluster reporting" (Greenopia couldn't even find a figure for greenhouse-gas emissions, something every other company reports front and center) and "questionable commitment to the environment," Citgo, which is controlled by the government of Venezuela, earns that dubious honor.

And while Sierra gave top marks to Sunoco, it sits at number two on the Greenopia list, with the best in show being BP.

"It's a tough call between those two companies," continues Mazzefa, "but BP is just solid in all the major criteria, plus they've pledged 8 billion dollars towards renewables over the next few years, and that makes them one of [the top] renewable donors in the world."

Perhaps the best news about all of these efforts is that the oil companies seem to be paying attention.

"Since the EPA began their TRI in 1987," says Ash, "toxic releases have fallen by 50 percent."

That not only means you can use these lists to choose at the pump but, even better, that your choice might actually count for something.