For many, the new-car smell is a bonus that comes with buying a new vehicle — an olfactory reward to enjoy each time the owner slides behind the wheel.

That smell, however, also could make them — and their passengers — sick.

That's because the plastics and textiles used in vehicle interiors contain a number of harmful chemicals, including antimony, bromine, chlorine, and lead. Repeated and concentrated exposure to any of these chemicals may contribute to a variety of acute and long-term health issues such as birth defects, impaired learning, liver toxicity, and cancer.

Not only are vehicle occupants exposed to these chemicals by breathing, but frequent exposure to the sun's heat and ultraviolet light can increase interior levels of these chemicals and possibly exacerbate their toxicity.

When these conditions combine with the fact that the average American spends almost two hours in a car every day, the potential exists for vehicle interiors to become a major source of indoor air pollution. Children, with their systems still developing, are most vulnerable to such exposure.

The visible evidence of chemicals being released into interiors is the film that sometimes appears on the inside of windshields. Known as "fogging," this phenomenon occurs when chemicals in the materials of the interior evaporate and then condense on the windshield or windows. This process happens over time, but accelerates in high temperatures.

To educate consumers, the Ecology Center, a nonprofit environmental advocacy group located in Ann Arbor, Mich., has been publishing a guide to toxic chemicals in cars since 2007.

"Our first guide reported results from more than 200 models from the 2006 and 2007 model years, representing 12 different manufacturers," said Jeff Gearhart, research director for the Ecology Center. "We found concentrations of some toxic chemicals were, on average, five to ten times higher than those found in homes and offices."

Experts at the Ecology Center test vehicles with a portable X-Ray Fluorescence (XRF) device which can identify the elemental composition of any material. "We test a number of components, including the seats, the dashboard, and headliner," said Mr. Gearhart. "These large components represent major sources of the chemicals we are concerned about."

Since its first report, the Ecology Center has worked with auto manufacturers to use less-toxic materials in vehicle interiors. Based on the findings in its most recent guide, available online at HealthyStuff.org, the efforts seem to be working. "The scores have improved each year, and for 2009, the Chevrolet Cobalt was our best overall vehicle," said Gearhart. "This marks the first time a car built in the United States by a U.S.-owned company has earned this distinction."

HealthyStuff.org Best Picks

VehicleTypeScore
Pontiac G5Small Car0.6
Chevy CobaltSmall Car0.7
Toyota CorollaSmall Car0.7
Honda AccordFamily Sedan0.7
Nissan Cube KromStation Wagon0.7
Honda Insight Hybrid Small Car0.8
Jeep WranglerSUV0.8
Toyota PriusFamily Sedan0.8
Honda Civic HybridSmall Car0.9

Source: HealthyStuff.org

The Cobalt tested with the least amount of toxic chemicals in its interior plastics and textiles, earning it a rating of 0.7 on the center's scale. The scale awards a rating of 0.0 to a vehicle with no detectable levels of objectionable chemicals; the worst offender gets a rating of 5.0. (The Cobalt's corporate cousin, the 2009 Pontiac G5, earned a rating of 0.6, but it is no longer available.) The vehicle with the worst rating on the 2009 list was the South Korean–built Chevrolet Aveo, which earned a rating of 4.9.

"We're pleased with the Cobalt's performance in this guide," said Doug Pickett, engineering group manager for materials engineering at General Motors. "The fact that six of the best vehicles by type are from G.M. shows that this issue has been important to us for a while."

While the Cobalt received top honors, Mr. Pickett explained that the Aveo's poor showing was the result of that car's reliance on older materials technology. "As a vehicle moves through its product cycle, we are able to incorporate less toxic materials into its updated design," he said. "This shows we are listening to our customers and that what is important to them, like less toxic interiors, is important to us."

Working with automotive suppliers and the plastics industry has proved essential to G.M. and other manufacturers in developing and manufacturing plastics that contain less-harmful chemicals. "Rather than dictate how to formulate plastics, we provide a list of more than a hundred chemicals banned in our vehicles, including formaldehyde and Class 1 carcinogens such as benzene," said Mr. Pickett. "This approach helps ensure the plastic is safe and remains safe throughout its life cycle."

HealthyStuff.org Worst Picks

VehicleTypeScore
Chevy AveoSmall Car4.9
Mitsubishi EclipseSport/Sporty Car4.7
Hyundai TucsonSUV4.1
Mitsubishi Eclipse SpyderConvertible3.9
Volkswagen JettaSmall Car3.9
Chevrolet ImpalaLarge Sedan3.7
Volkswagen BeetleSmall Car3.7
Volkswagen EosConvertible3.5
Ford ExpeditionSUV3.4
Audi TT Roadster 2.0 FWDSport/Sporty Car3.3

Source: HealthyStuff.org

Using less-toxic materials isn't just good for vehicle occupants. It also benefits the workers who build the vehicles and helps reduce the environmental considerations at the end of the vehicle's useful life. "When a vehicle is scrapped, the majority of its plastics often end up in a landfill," said the Ecology Center's Mr. Gearhart. "This releases both known and unknown chemical hazards into the environment."

What, then, can concerned owners do to lessen the potential toxicity of their vehicle interiors? "Limit solar exposure with a sun shade or by parking in a garage or shaded area whenever possible to reduce interior temperatures," Mr. Gearhart said. "Leave the windows down for five minutes before you get in. And weekly vacuuming can remove any harmful dust that accumulates."

For new-car buyers, Mr. Gearhart recommended using the guide to toxic interiors much as they would "Consumer Reports" or fuel economy information. "Consumers can use the guide to narrow the list of vehicles they want to shop, and then arrange an extended test-drive to note any possible sensitivities. It's easier to do the research before purchasing a vehicle than trying to return it after an issue arises."

From a broader perspective, said Mr. Gearhart, reducing the levels of toxic chemicals in vehicle interiors comes down to public policy. "Legislation is being drafted to reform the outdated Toxic Substances Control Act, which is the federal law for regulating chemicals," he said. "The Environmental Protection Agency only requires testing on about 200 of the more than 80,000 chemicals now on the market since the law was first passed in 1976. Reforming this legislation would phase out the most dangerous chemicals from the manufacturing process, require industry to take responsibility for product safety, and use the best science to protect those who are most vulnerable to exposure."

Read More:

- Gallery: Toxic Car Interiors
- Car Safety on AOL Autos
- Research New Cars on AOL Autos