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-- By Eric Engstrom, MPH
© 2000, RPK&A, Inc.

All right, you do the math. The SPF on that tube of sun-stuff means "sun protection factor," and it tells you the degree to which the product in the tube is supposed to (and "supposed to" is the critical thing there) protect your skin from sunburn. An SPF of two gives you twice as much protection as your normal skin has; an SPF of four quadruples your normal protection, and so on.

But what is "your normal protection?" Well, unfortunately, it varies. It depends on your skin type and hair color, all of which depend on your gene pool. Mostly it's not hard to figure out in a basic way: the lighter your natural skin and hair, the less inherent sun resistance your skin has. Fair-skinned redheads will tolerate less exposure than olive-skinned, dark-haired Mediterraneans.

Let's say you can usually take about 20 minutes of intense, midday sun before you fry and burn. The suntan lotion with an SPF of four will let you stay out for about 80 minutes, theoretically. You can increase that time with a higher SPF (and you better do that for certain highly sun-sensitive skin areas, like your nose and lips). An SPF of eight might give you two and a half hours outside.

But somebody else with a different skin type might have a completely different experience. And "how long is safe" depends on other things, too, like:

  • Time of day. The sun is more intense when it feels more intense (go figure), between about 10 a.m. and 2 p.m. (but account for daylight savings time; that means 11 a.m. to 3 p.m. for most of us).

  • Time of year. Midday sun in January in North Dakota will burn you, but not as fast as in July.

  • Water. Some sun products are waterproof, and some aren't; some that claim to be waterproof are actually just water-resistant. Where might you encounter water? Well, there's sweat (exercise, heat); there's water you swim in (pool, ocean). If the product doesn't say it's waterproof (and sweat-proof), it's not. Which means you've got to re-apply it after swimming. Even the waterproof products don't last forever on wet skin; better safe than sorry, so put on some more (again, especially on sun-sensitive skin) when you've been soaked.

  • Body parts. Skin varies in its toughness and in its sun-resistance. Thin skin (think: nose, lips, ears, top of your toes) burns faster. Skin that has seldom or never seen the sun (you know those areas, right?) will burn sooner than skin that's been around the track before.

  • Medications. Some commonly-used drugs can make your skin more sensitive to sun. Tetracycline (used to treat acne and some minor infections) is famous for this, but it's not the only one, nor the only antibiotic, to do it. Whenever you pick up a prescription, the pharmacist should tell you if the medication makes you more sun-sensitive; remember to ask. And some over-the-counter and alternative products can reduce your sun resistance, too-notably, St. John's wort, which some people use to relieve depression.

  • Sun-protection products. These lotions, creams, roll-ons and sprays vary in quality, effectiveness and value. More expensive is not necessarily better-you might be paying for brand name, scents and packaging. My suggestion: get to know a product by testing it out-safely. Recommendations from friends (especially if they have similar skin to yours) can be useful; you can see the results on their skin, and decide whether that's what you want.

A final note: new research suggests that SPF doesn't correlate with protection against the long-term effects of ultraviolet radiation on deep skin tissues-that is, that preventing sunburn may not prevent premature wrinkling, thickening and aging of the skin.