Sun Protection Factor (SPF) measures the length of time a product protects against skin reddening from UVB, compared to how long the skin takes to redden without protection.
Do the sun's ultraviolet rays penetrate the window glass in your cars and at home?
Yes, they do penetrate. Windows and car windshields somewhat reduce exposure to ultraviolet B (UVB) radiation. Only car windshields are partially treated against ultraviolet A (UVA) rays. So, consider the use of a UV-protective window film on windows in your home and vehicles.