Windshields offer the most sun protection, according to an executive 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 a 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, and can be made with extra UV absorbers to protect even more. The Pittsburgh Glass Works exec says windshields absorb 100 percent of UVB rays (which cause sunburn), and around 98 percent of UVA rays. Those UVA rays don't cause sunburn but can do long-term damage to the skin. That built-in protection gives windshields a sun protection factor - or SPF - of 50 or more, the equivalent of some of the strongest sunscreens.