Is your sunscreen really protecting you?

What do SPF numbers really mean? My patients ask me that all of the time. For starters, SPF stands for sun protection factor. It measures how much ultraviolet B light (UVB), a sunscreen blocks from reaching the skin.

And because UVB is responsible for sunburns and skin cancer, you want to limit the amount you are exposed to.

Dermatologists routinely recommend using a sunscreen with at least an SPF 30, which allows only about 3 percent of UVB to reach your skin.  That’s not much different from the 2 percent of UVB you get with SPF 50, but the equation only works if you put on enough sunscreen.

Average sunbathers apply half the amount of sunscreen used in laboratory testing, meaning they could burn in half the time. Use at least one ounce of lotion (about the amount that fits into a standard shot glass) for full coverage.  And beware of sunscreen that claims to be waterproof and perspiration-proof; sweat, water and excessive toweling can decrease its effectiveness. For full protection, it’s best to reapply throughout the day.

Pay attention to the age of your sunscreen. Lotions lose their effectiveness over time, so get rid of any sunscreen more than three years old, or if the bottle has passed its expiration date.

And here’s something else that people forget: While sunscreen will block harmful UVB rays, many don’t protect you from the sun’s other ultraviolet rays: ultraviolet A light, or UVA. UVA rays have more long-term damaging effects on the skin, such as premature aging. Although UVA rays do not cause burns, scientists have linked excess UVA exposure to malignant melanoma, damage to the immune system and other skin cancers like squamous cell carcinoma. Make sure your sunscreen states that it is a complete blocker of both UVB and UVA.

Always wear sunscreen with at least SPF 30, even if you will be outdoors only for a short time.  Remember, long-term sun exposure can lead to skin cancer, cataracts and suppression of the immune system.

For more information about the sun and your skin, talk to your dermatologist.