SPF is more than just numbers

Using sunscreen regularly is one of the best ways to protect your skin from sunburns, some types of skin cancer, sun-spots, wrinkles, and premature aging. But with myriad new products touting higher SPF ("sun protection factor") numbers, how do you know which one to pick? You might be thinking, "Higher SPF, must be better!", but that's not necessarily true, so here are some tips:

UVA VS. UVB? GO FOR BOTH. The sun gives off two types of radiation: ultraviolet A (UVA) and ultraviolet B (UVB). Both can damage your skin. The 'SPF' part only refers to protection from UVB, which causes sunburns. The UVA rays penetrate deeper into your skin, causing wrinkles and sun-spots, and also making it harder for your body to fight off disease. So make sure your sunscreen shields you from both types of rays; look for the words "broad spectrum" on the label.

MAGIC NUMBER 30: On average, about six percent of the sun's UV radiation gets past an SPF 15 sunscreen and into your skin, compared to only three percent with SPF 30. That's why you should pick a minimum SPF of 30 when you're outside (even if it's not sunny). But as SPF numbers get higher, they only provide slightly higher protection (e.g., going from SPF 60 to 90 only gives you roughly one percent more coverage). So pick a higher SPF if you tend to burn easily, but there's probably no need to go overboard with SPF 100 and the likes.

REAPPLY, REAPPLY, REAPPLY: Time to bust a common myth: a higher SPF number doesn't mean it will last longer, regardless of whether you use a lotion, spray, or cream. Whatever you're using, you still have to reapply every two hours, especially if you have been swimming or sweating. Your best bet for protecting your skin is to stay vigilant and reapply, reapply, reapply.

Regardless of your skin color, sunscreen is a must, so don't leave home without it.

Be sure to tune in to Dr. Sanjay Gupta every weekend on HOUSE CALL. You'll find the answers to your medical questions Saturday and Sunday at 7:30 a.m. ET on CNN.