Never turn on the A/C immediately as soon as you enter the car. Open the windows first for a minute or two.
Why this precaution? The car dashboard, sofa and air fresheners emits Benzene, a cancer causing toxin (carcinogen- take note of the heated plastic smell in your car). 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 Benzene level indoors is 50 mg per sq. ft. A car parked indoors with the windows closed will contain 400-800 mg of Benzene. If parked outdoors under the sun at a temperature above 60 degrees F, the Benzene 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 toxins.
Solution is simple.
As soon as you enter your car, open the windows (all of them) and/or the doors for couple of minutes, giving the contained air to exhaust out. In case of hurry, you can also drive on for a minute with windows open before shutting them up, forcing the toxic Benzene out of your breathing atmosphere inside the car.