Even more than premature aging, the most alarming consequence of sun exposure is skin cancer—the most common form of cancer. Dr. Oz says there are actually three types of skin cancer.

Basal cell cancer, which starts at the base of the skin, is the most common form. Typically caused by sun damage, it starts with a scaly area that, if it advances, will eventually gain a pearly appearance. Squamous cell tumors get a scaly appearance early. As they advance, the skin puckers in.

The third type of skin cancer is melanoma, which Dr. Oz calls "the big one." Melanoma comes from melanin, the cells that make pigment, so there are a few indicators.

To identify melanoma, doctors use the "A, B, C, D" method:

• Asymmetry: "It's hard to draw a line down the middle," Dr. Oz says.
• Border: "The border's irregular and it sort of melts into the skin around it. That's a classic sign of a melanoma."
• Color: If a mole has a "rainbow coalition" of colors, that means it's probably melanoma, Dr. Oz says.
• Diameter: Dr. Oz says mole size does matter. "If you can cover over the mole with an eraser head, you'll probably be okay," he says. "But if you've got hundreds of moles or a family history or you've had a melanoma in the past, you need to be screened pretty aggressively."