Dirty driving: Top 10 worst polluters

updated 6:21 p.m. ET July 9, 2008

The Hummer H2 might be an obvious target for environmentalists, but unless it’s caked in mud, the hulking sport utility vehicle isn’t the filthiest ride on the road. That distinction goes to another SUV: the Volkswagen Touareg V10 TDI.

The Hummer H2 does make the fifth spot on our list of top 10 worst polluters, thanks to its overall heft and gas-guzzling V8 engine, but what pushes Volkswagen’s Touareg V10 TDI to the very top of the ranking is its large turbo-diesel engine. It dumps so much exhaust into the atmosphere that the U.S. Environmental Protection Agency gives this midsize SUV its lowest air pollution score of 1.

The EPA ranks vehicles on an air pollution scale of 1 to 10, giving a 1 to those that pollute the most and a 10 to those that pollute the least. Most vehicles get an air pollution score of 6. Highly efficient models like the Ford Escape Hybrid, Honda Civic Hybrid and Toyota Prius earn a 9.5, which is currently the highest rating any vehicle gets.

The EPA’s air pollution score is the main criterion for our ranking. It represents the amount of health-damaging, smog-forming airborne pollutants a vehicle emits. These primarily include nitrogen oxide, which combines with hydrocarbons to create smog; carbon monoxide, a colorless, odorless, poisonous gas; particulate matter that creates the black soot that lodges in lungs and deposits on buildings; and formaldehyde, a lung irritant and carcinogen.

Some heavy-duty pickup trucks and vans pollute more than the Touareg TDI, but they’re intended primarily for commercial use and weren’t included in our consumer-focused list. We also excluded exotic Bentleys, Ferraris and Lamborghinis because they're produced in such limited quantities and so few consumers would consider buying one.

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Five other diesel-powered SUVs are right behind the Touareg TDI in our ranking of worst polluters. These include the Jeep Grand Cherokee CRD, the second worst polluter; the Mercedes-Benz GL320 CDI, which is third; and the Mercedes ML320 CDI and R320 CDI, tied for fourth. All of them get an air pollution score of 1.

Not all of the ties on our list were avoidable, but to help break some of them we looked at each vehicle’s carbon footprint, which is the EPA’s way of quantifying other automotive emissions called greenhouse gases. These include carbon dioxide, nitrous oxide and methane. Scientists believe greenhouse gases contribute to global warming.

The primary greenhouse gas is carbon dioxide, but nitrous oxide and methane are also part of the mix.

The Volkswagen Touareg V10 TDI has a carbon footprint of 12.4, which means it emits 12.4 tons of greenhouse gases annually, according to the EPA’s standard calculations based on 15,000 miles of driving per year. Burning one gallon of gasoline emits 20 pounds of carbon dioxide, the EPA says.

The second-place Jeep Grand Cherokee CRD has a carbon footprint of 11.1 annual tons of greenhouse gases and the third-ranked Mercedes GL320 CDI puts out 10.6 tons annually.

Auto emissions are 75 percent to 90 percent cleaner than they were before more stringent federal emissions standards were enacted in 1970, says EPA spokesperson Catherine Milbourn. “Even the worst polluters on the road today are far cleaner than most vehicles were a few decades ago,” she says.

The worst polluter on our list, the Volkswagen Touareg V10 TDI, emits 83 percent less nitrogen oxide and 89 percent less carbon monoxide than the average vehicle built in 1970, according to EPA historical data.

Widespread use of catalytic converters, exhaust gas recirculation, particulate filters and reformulated fuel are the main reasons today’s vehicles pollute far less than their predecessors.

And things will only get better: Some of the biggest polluters on our top 10 list will be cleaning up their acts for the 2009 model year, thanks to the advent of new diesel engine technology.

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