Have you ever sat in a car on a hot, Sunny day without window tint before and wondered how it was possible that you weren't getting a sunburn? Well, it turns out that there's a perfectly logical explanation behind this.
The Sun emits three forms of ultraviolet rays: UVA, UVB, and UVC. The UVC rays are blocked out by the atmosphere, but UVA and UVB continue to on to the Earth's surface. UVB is the one that causes sunburns, and windshields and car windows are manufactured with a protecting agent that prevents these rays from encountering your skin.
UVA rays, on the other hand, are only blocked out by the windshield; not the side windows. These rays are responsible for aging the skin and are linked to skin cancer and other complications. Window tint can help reduce exposure, but only by a fraction.
You can thank modern manufacturing for your protection from sunburn while you drive and enjoy a ride as a passenger. Things are always different, however, once you step out of the car, so remember to pack that sunscreen.