Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

SPF Scale Indicates Length of Time You Can Spend in the Sun

Dear Doctor: I always reach for the highest SPF when I'm buying sunscreen, especially for the kids. But my mom says what matters most is the type of UV rays the sunscreen is blocking. Why is that? What does SPF actually measure?

Dear Reader: There's a lot of confusion when it comes to sunscreen. We've had patients admit that when they get overwhelmed by the labels and the choices while shopping for sunscreen, they sometimes give up and decide to go without. But sunscreen can help prevent skin cancer. In fact, it is estimated that exposure to the sun's UV rays is responsible for up to 90 percent of non-melanoma skin cancers and at least 65 percent of melanomas. And while it's not quite in the same life-or-death category, UV rays are also responsible for signs of premature aging like wrinkles, brown spots and sagging skin.

Let's start with the basics. Sunlight is the portion of the sun's radiation that reaches us at the Earth's surface. It's made up of three basic components -- the visible light that we see, infrared rays, which deliver heat, and ultraviolet rays. Although we neither see nor feel ultraviolet radiation, it still manages to make its presence known. On the plus side, it triggers our bodies to manufacture vitamin D, an essential micronutrient. But prolonged exposure to the sun's ultraviolet spectrum results in skin damage.

The longer wavelengths of ultraviolet light, known as UVA, are the so-called tanning rays and are known to age the skin. UVB rays, which cause sunburn, play a major role in skin cancer. Although you may hear the axiom regarding UV rays that A is for aging and B is for burn, the most recent research suggests that both types of UV rays play a role in each process. All of which brings us to your question about SPF factors and sunscreen.

SPF stands for sun protection factor. While the ascending numbers of the SPF scale seem to imply an increasing strength or concentration of the sunscreen product, what they actually represent is the length of time that you can spend in the sun before starting to burn. Let's say it takes five minutes in the sun without protection before your skin begins to redden. A sunscreen with an SPF of 10 means that when you apply it, you will have 10 times that amount of time -- 50 minutes -- before you begin to burn. A lotion with an SPF of 30 gives you 150 minutes. Bottom line -- the "factor" in SPF is the number by which you multiply your minutes-to-a-burn time.

By using sunscreen, you've applied a chemical barrier that helps keep UV radiation from reaching the skin. The trick here is to be sure you use a full-spectrum (sometimes known as a broad-spectrum) sunscreen. This means the product blocks both the UVA and UVB rays, which is essential for the best protection. And while this seems like a no-brainer, you really do need to follow the instructions on the product in order to get full protection. Apply at least 15 minutes before heading outside, use at least an ounce or two for adequate coverage, and reapply at least every two hours.

(Send your questions to, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)