In a way, you’re all right.
Most glass, in cars or houses, blocks the kind of sunlight, called ultraviolet B (or UVB), that causes sunburns, said Dr. Robert Stern, chief of dermatology at Beth Israel Deaconess Medical Center. So you’re unlikely to get truly burned through the window. But glass does let in ultraviolet A (UVA) light, which can cause suntans, though rarely burns. It could take hours, though, to get a tan trying to use your car as your tanning salon.
But both UVA and UVB can cause skin cancer and skin aging, so you can be damaging your skin even through sunlight filtered through a window if you have long exposures, cautioned Dr. Thomas Ruenger, a dermatologist at Boston University School of Medicine.
For young kids, especially those in rear-facing car seats, some dermatologists say it makes sense to lather up with sunscreen before long car trips because kids get sun from both side and rear windows, noted Dr. Sandy Tsao, a dermatologist and clinical director of laser surgery at Massachusetts General Hospital. Light-shielding curtains that stick on to windows with suction cups can also help.
Adults or kids taking certain antibiotics such as doxycycline (Vibramycin) or sulfamethoxazole (Bactrim) that increase sensitivity to sunlight should also be careful on long, sunny car trips.