Because glass filters unevenly: it blocks most UVB (the burn wavelength) but UVA transmission depends on the glass - and windshields (laminated) block UVA far better than side windows (tempered), which is why long drives pink up the window-side arm.
UVA penetrates deeper, drives photoaging, and contributes to skin cancer - the photoprotection literature flags window exposure as an under-appreciated source. Fixes: sunscreen on the driving side for long trips, UV window film, and do not trust shade-feel - glass can block glare and still pass UVA. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Why do I get sunburned through a window?
Because glass filters sunlight unevenly - it stops most of the burning rays but waves the aging ones through. The physics from the photoprotection literature: all commercial and car glass blocks the majority of UVB (the main burn-and-tan wavelength), but UVA transmission depends on the glass type - and UVA penetrates deeper, drives photoaging and pigment changes, and contributes to skin cancer. Car windows are the classic trap: windshields are laminated and block UVA well, but side and rear windows are usually tempered glass, which lets much more UVA through - the review of window photoprotection notes laminated glass offers better UVA protection than tempered. That is why professional drivers and long commuters accumulate one-sided sun damage, and why you can pink up on a long bright drive with the windows up. The fixes: sunscreen on the driving side for long bright trips, UV-filtering window film (it upgrades any glass to near-total block), and do not trust shade-feel - glass that blocks heat glare still passes UVA. Home and office windows count too if you sit in direct sun for hours daily.