The Skin Cancer Foundation recommends wearing sunscreen with at least a 15 SPF — but what exactly does that mean?

SPF, or Sun Protection Factor, measures how long it takes for sun-exposed skin to redden with sunscreen, compared to without any protection.

For example, if it takes unprotected skin 20 minutes to begin to turn red, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer — about 5 hours. (Keep in mind it may take up to 24 hours after sun exposure for redness to become visible.)

Sunshine contains two types of ultraviolet rays: Ultraviolet A and B, or UVA and UVB. Sunburn can be blamed on UVB — short-wave rays, which are more potent and are considered the main cause of skin cancer.

SPFs of at least 15 block 93 percent of UVB. While SPFs higher than 30 block only 4 percent more UVB, they're a good idea for sun-sensitive individuals, skin cancer patients, and people at high risk of developing skin cancer. They also give you a bit of wiggle room in case you don't apply enough sunscreen.

UVA are long-wave rays. Although they generally don't cause sunburn, UVA rays go more deeply into the skin, and cause wrinkles and leathery skin. Latest studies show UVA not only makes UVB worse, it also may cause some skin cancers.

SPF measures UVB protection, but no comparable numerical standard exists for UVA. If you purchase a product with broad-spectrum protection, it shields against UVA as well as UVB — but it can't guarantee protection against all UVA wavelengths. Most broad-spectrum sunscreens and sunblocks with an SPF of 15 or higher do a good job against both UVA and UVB. If they also contain avobenzone, zinc oxide, or titanium dioxide, they should be effective against the entire UVA spectrum.

Even with the ideal sunscreen, some UV rays can get through to your skin and cause damage. The Skin Cancer Foundation considers sunscreen one part of a comprehensive sun protection program, along with sun-protective clothing, sunglasses, shade, and sun avoidance from 10 a.m. to 4 p.m. Remember, sunscreen only works if applied regularly. You'll need to apply more often if you're in the water or participating in sports.