Open the windows after you enter your car.

In brief, the above article says:

According to research done by a U.C., the car dashboard, sofa, air freshener will emit Benzene, a cancer causing toxin (carcinogen). In addition to causing cancer, it poisons your bones, causes anemia, and reduces white blood cells. Prolonged exposure will cause Leukemia, increasing the risk of cancer. May also cause miscarriage. Acceptable Benezene level indoors is 50 mg per sq. ft.

A car parked indoors with the windows closed will contain 400-800 mg of Benezene. If parked outdoors under the sun at a temperature above 60 degrees F, the Benezene level goes up to 2000-4000 mg, 40 times the acceptable level. The people inside the car will inevitably inhale an excess amount of the toxin. It is recommended that you open the windows and door to give time for the interior to air out before you enter. Benzene is a toxin that affects your kidney and liver, and is difficult for your body to expel.