But there are no requirements for sun protection. That can vary by vehicle, and even by window within a vehicle.
Windshields offer the most sun protection, according to Pete Dishart, who leads product development at Pittsburgh Glass Works in Pennsylvania. His company supplies glass to nearly every major automaker, including Toyota, Mercedes-Benz, General Motors, BMW and Hyundai.
By law, windshields must be made of laminated glass, which means they're formed from three parts: two 2.1-millimeter layers of glass separated by an 0.8-millimeter piece of stretchy plastic. The glass is made to break easily if something — the driver's head, for example, or a deer — comes in contact with it. The plastic then stretches to absorb that impact.
The layer of plastic helps windshields absorb nearly all of the sun's ultraviolet rays. Plastic is naturally good at absorbing UV rays, Dishart says, and can be made with extra UV absorbers to protect even more. He says windshields absorb 100 percent of UVB rays, which cause sunburn, and around 98 percent of UVA rays, which don't cause sunburn but can do long-term damage to the skin.
That gives windshields a sun protection factor — or SPF — of 50 or more, the equivalent of some of the strongest sunscreens, Dishart said.
Sunroofs, too, often contain UV-absorbing technology that can block around 90 percent of UV rays. That's partly to keep the car cool as well as protect occupants from sun exposure.
But side and rear windows are a different story, and their SPF can vary a lot depending on the vehicle.
Side windows are usually made from cheaper tempered glass, which is around 4 millimeters thick and doesn't include a layer of plastic. They have an SPF of around 16, Dishart says.
Concerned about the level of sun exposure in your car? Here's what you can do:
•Seek out vehicles with more protection. Glass makers have a trade group — the Enhanced Protective Glass Automotive Association — that keeps tabs on which cars have laminated windows and publishes the results.
•Explore window tinting and aftermarket films. Companies like Solar Gard offer window films that can block UV rays. Glass-tinting companies and auto dealers can install them on a four-door car for around $200. There are also do-it-yourself kits at auto parts shops. But beware: States govern how much windows can be tinted, and those laws vary.
•Protect yourself. The Skin Cancer Foundation recommends applying a sunscreen with an SPF of 15 or higher around 30 minutes before you drive. Make sure to cover your face, neck, arms and hands. If you have a sunroof and thinning hair, you should also wear a hat, since skin cancer can develop on the top of your head.