Unless you're taking a drive in a convertible with the top down on a bright, sunny day, you won't get a burn or increase your risk of developing skin cancer just from sitting in a car-even if it is sunny outside. "UVB rays are primarily responsible for tanning, burning and most forms of skin cancer, and they can't travel through glass," explains Dr. Weinstock. "UVA rays [which do penetrate glass] are 100 times weaker, so it's highly unlikely they can induce tanning or burning, or up your cancer risk much on their own," he says. One caveat: It's presumed that UVA rays are the main culprit behind premature aging, wrinkling and discolorations. When it comes to sun protection, your rule of thumb should be to use a full-spectrum sunscreen inside-and outside-your car every day.