Ten high-end cars that pollute the least

For instance, two visually identical cars that use the same engines can have different emissions outputs simply based on one model being all-wheel drive, and the other being rear-wheel drive. The rear-drive BMW 328i ties for eighth place on our list, with a Carbon Footprint of 8.3 annual tons of greenhouse gas emissions. The all-wheel-drive 328xi did not make out list because it gets worse fuel economy and emits 9.2 tons of greenhouse gases annually.

Milbourn suggests environmentally conscious consumers check the fuel economy and emissions ratings for all vehicles under consideration, as posted on the vehicle-related Web sites the agency maintains in conjunction with the Department of Energy.

With luxury status out of the equation, hybrids still figure prominently when it comes to tailpipe emissions.

Small hybrid cars like the Toyota Prius and Honda Civic Hybrid rated the highest on the non-luxury list, but a few hybrid SUVs, like the Ford Escape Hybrid, which tied for fourth place, and Toyota Highlander Hybrid, in fifth, also placed well.

Hybrids are the top choice for low emissions today, but a few years from now, a similar ranking based on tailpipe emissions might turn up a list of all-electric vehicles.

That hypothetical list might contain the $98,000 Tesla Roadster, an all-electric sports car slated to go on sale this spring. Another potential contender is the Fisker Karma, an $80,000 four-door hybrid luxury sedan that can be plugged in and recharged, traveling up to 50 miles on emissions-free electric power.

But until those vehicles and others like them come out, choosing a hybrid is the easiest way to pollute the least.

Our two least-polluting lists are based on examining each model's Air Pollution Score and Carbon Footprint ratings as determined by the EPA.

The Air Pollution Score represents the amount of health-damaging and smog-forming airborne pollutants a vehicle emits. Among these are hydrocarbons, nitrogen oxide, particulate matter, carbon monoxide, and formaldehyde. Scoring is based on a 10-point scale, with 10 representing a zero-emissions vehicle, meaning higher numbers are better.

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Carbon Footprint ratings estimate the tons of greenhouse gases each vehicle generates per year based on 15,000 miles of driving. These emissions include carbon dioxide, nitrous oxide, methane, and other compounds believed to be major contributors to global warming. One gallon of gasoline converts to 20 pounds of CO2 emitted into the atmosphere.

The EPA’s ratings cited in our lists assume emissions generated not just by burning fuel while driving, but from fuel production and refining as well. Scoring is expressed in the number of tons of greenhouse gases produced each year, meaning lower scores are better.

The EPA considers a vehicle's Air Pollution Score to be more important because it is directly related to air quality, and thus has an immediate impact on human health. Therefore, we use it as the basis for our ranking. Carbon Footprint rating is included for every vehicle but was primarily used to break ties in our rankings.

Many of the lowest-emission versions of some models aren’t available in all 50 states. Some specially certified “super low-emissions” (SULEV) or “partial-zero emissions” (PZEV) vehicles — which are rated at 9 and 9.5 on the Air Pollution scale, respectively — are offered only in California and the handful of other states that adhere to California's stricter air-pollution standards. They include Arizona, Connecticut, Florida, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington.

However, there are instances when a manufacturer sells the same version of the car that qualifies as SULEV or ULEV in other states and doesn’t bother going through the bureaucracy of certifying it as such in the states that don’t require it. In those instances, we use the highest EPA emissions ratings, being that, more than likely, consumers can purchase the SULEV- or ULEV-equivalent version of the vehicle in the states that don’t require this rating.

But in instances where a SULEV or ULEV vehicle is only sold in California and the other states mentioned above that have adopted its emissions regulations, we use the lower emissions score.

The best way to tell if a vehicle is SULEV or ULEV compliant even though it isn’t rated as such in the state where you live is to check what the EPA calls the “Underhood ID” to see if it matches that of the variant that officially qualifies as SULEV or ULEV.

Finally, to qualify for luxury status, a vehicle must be made by a company commonly regarded as a luxury or premium automotive manufacturer.

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