Most UVB radiation is blocked by window glass but most of the UVA radiation goes on through. Times when the sun's UVA rays can reach you through window glass may be while sitting in an office building, at home near a window or while driving the car.
Here is a graphic example of what the UVA rays of the sun can do to you! The woman shown in the pictures above worked in the same room by a window for 15 years. The sun damage you see occurred on the side of her face (A) that was exposed to the UVA light coming through the window. The other side of her face (B) was not exposed to window light.
For many years UVA was not considered a serious problem. In fact, the SPF rating on sunscreen only measures the amount of UVB blocked. But, as you can see, UVA exposure can be very damaging.
Don't forget that UVA stays strong all year long and all day long.
1. Reprinted with the permission of Moulin G, Thomas L, Vigneau M, et al. A case of unilateral
elastosis with cysts and comedones: Favre-Racouchot syndrome.
Ann Dermatol Venereol. 1994;121:721-723.