The amount of UV radiation that penetrates car windows depends on a number of factors, including the type of glass, the thickness of the glass, and the angle of the sun. The UV protection provided by car windows is also not uniform. The windshield typically provides the most protection, while the side and rear windows allow more UV rays to pass through.
For the best protection from the sun, it is recommended to keep the windows rolled up and to wear sunscreen and sunglasses when driving. It is also a good idea to avoid parking in direct sunlight whenever possible.