The smell comes from the plastic, glue and carpeting in cars. When those materials are new, they're unstable and give off gases, including benzene and formaldehyde, two known carcinogens.

The United States doesn't have any standards to protect against volatile organic compounds, but other countries do. Research shows new car interiors can contain 128 times the legal limit in countries like Australia.

The harmful gases are even worse on hot days. The Environment Working Group says opening the car window or vents will help clear poisonous gases. But the group cautions to roll windows up in heavy traffic where smog could be an even bigger health threat. Pregnant women should use added caution, and drivers should not exacerbate the problem by using stain guards or cleaning solutions in a new car.

The health risks subside in about six months, when the smell fades.