The main reason is medical. When the car is parked in the shade, it produces between 400 and 800 milligrams of benzene, also known as benzol. It’s a colorless liquid with sweet smell, which easily dissolves in water and mixes with air. It is also flammable.
It is used in the production of plastics, explosives, pints, and as a solvent. At high temperatures, the amount of benzene produced in the car reaches 4000 milligrams, even 40 times more than what is allowed by the rules! SO, according to this, if upon entering the car our windows are closed, we directly inhale large amount of benzene and we’re hurting our kidneys, liver and bones.