A few days ago I was sat in a restaurant, the sun was high in the sky and hitting me through the window. I could feel the heat from the sun on my skin and instead of enjoying my meal my mind went into overdrive; a million ran amok and it was all I could think of for the whole meal and journey home. For instance does glass used in windows enhance and strengthen UV rays? Will sitting beside a window damage my skin and can you tan/burn due to sunlight beaming through a window? I know some reading this will no doubt think I am a tad strange (you wouldn't be all that wrong) but I'm sure others have had a similar path of thought...I hope. After no less than a week of research (I put less effort into School, I kid you not) this is what I have found out...
So the good news is that normal glass, much like the type used to construct windows can absorb up to 97 percent of UVB rays (the ray that can cause sun burn) and up to 40 percent of UVA rays (the ray that causes the skin to age). According to online science journals this roughly equates to an SPF factor of 30, which I somewhat find reassuring. In my mind I was a little worried that window glass could work like a magnifying glass intensifying the risk and rate of skin damage, I'm really glad to learn this is not the case.
Car windows are constructed slightly differently and not every car has the same type of windows, for instance tinted windows will block 100% of UVB rays and up to 80% of UVA rays, I don't know all that much about cars so I will end this here. If the windows are not tinted then generally it is the same level of protection as a standard window (like the ones at home, if the wording of this post is in any way confusing).
Now on to the somewhat bad news, typically most will not burn due to the sun shining through the window but it is not completely unheard of, as with any period of prolonged sun exposure burning/tanning is always a possibility. So in a nutshell windows filter out a good proportion of the sun's harmful rays (both UVA and UVB) so there is no real need to worry, as long as you apply SPF your risk of burning and damaging the skin is pretty minimum.
The official advice from Cancer Research in regards to windows is as follows: "If you are driving long distances or sitting in your conservatory every day for long periods of time, you need to make sure you are using sun protection on sunny days."
I know that puts my mind to ease and hope it does the same for you.