Every single time you enter your vehicle you are making this mistake that is dangerous for your health and for those with you.
Now, you probably asking yourself: Why!? What am I doing!?
When you park your car outside almost always you park it with the windows closed. When you park it on a direct sunlight on a temperature higher than 16 degrees Celsius the level of benzene that accumulates in your car can reach 2000-4000 mg, which is almost 40 times more that allowed. So when you enter your car and sit inside with the windows closed you are inhaling all that benzene, not knowing how damaging this is to your kidneys, liver and bone tissue.
If you search in your cars manual you will find that they recommend to open your windows before turning your A/C and yet they do not clarify why.