SPF: What's in a number?

Browse the article SPF: What's in a number?

What's in a number?

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Agency: Dreamstime
Will you be able to lie in the
sun for 12 hours with an
SPF 70 sunscreen?

This summer, sunscreens with SPF 70 hit the market. The fair-skinned of the world might be tempted to take this as an open invitation to slather some on, jump onto a lounge chair and bake for 12 hours without getting burned. The higher the number, the better, right? Before you throw caution to the wind, though, you should know a few things about SPF. What is it, exactly? What do the numbers mean, and how high can they go?

We use sunscreen to block ultraviolet light from damaging the skin. There are two categories of UV light -- UVA and UVB -- that we consider in terms of sunscreen. UVB causes sunburn, and UVA has more long-term damaging effects on the skin, like premature aging. SPF, or sun protection factor, numbers were introduced in 1962 to measure a sunscreen's effect against UVB rays.

To determine a sunscreen's SPF, testers round up 20 sun-sensitive people and measure the amount of UV rays it takes them to burn without sunscreen. Then they redo the test with sunscreen. The "with sunscreen" number is divided by the "without sunscreen" number, and the result is rounded down to the nearest five. This is the SPF.

SPF numbers start at 2 and have just recently reached 70. To figure out how long you can stay in the sun with a given SPF, use this equation:

Minutes to burn without sunscreen x SPF number=
maximum sun exposure time

For example, if you burn after 10 minutes of sun exposure, an SPF of 15 will allow you to be in the sun for up to 150 minutes without burning. But before you grab your calculator and head for the beach, you should know that this equation is not always accurate. People usually use far less sunscreen than the amount used in testing. In the real world, the average sun worshipper uses half the amount of sunscreen used in the laboratory, which could result in a sunburn in half the time.

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UVB Absorption

So, now we know that a higher SPF number means more sun-exposure time. It also indicates the level of UVB absorption, but this number doesn't increase exponentially, which can be confusing. For example, an SPF of 15 absorbs 93.3 percent of UVB rays, but an SPF of 30 absorbs 96.7 percent. The SPF number has doubled, but the absorption rate has increased by only 3.4 percent.

Because of the confusion about UVB absorption, the FDA proposed a cap on SPF numbers. Any sunscreen higher than 30 SPF would be a "30-plus." Thirty was the decided cap because above that, the percentage of UVB absorbed and overall protection of the skin increases only slightly, but people may misinterpret these higher SPF numbers as a much higher level of protection or even a guarantee of all-day protection.

SPF	% UV absorbed
2	50
4	70
8	87.5
15	93.3
30	96.7
50	98

As helpful as the FDA was trying to be, the cap is clearly not in practice: Neutrogena and Hawaiian Tropic recently released sunscreens boasting an SPF of 70. With that number, a person who normally burns within 10 minutes can now enjoy 700 minutes -- almost 12 hours -- in the sun without any damage. Right?

Well, not exactly. We now know that most of us apply only about half the required amount of sunscreen. Also, despite waterproof or sweatproof labels, all sunscreens decrease in effectiveness when exposed to water or sweat. If you don't apply the correct amount and then reapply after exposure to water, a 12-hour bake in the sun could give you a very serious sunburn.

Protect Yourself

The bottom line is that a sunscreen with a higher SPF does offer higher protection against UVB rays, but once you get past SPF 30, protection doesn't increase dramatically, and the higher number may give you a false sense of protection. Instead of letting SPF be your only guide to sun protection, avoid a burn by following a few simple sunscreen rules.

zulema011, SXC
If you don't reapply, you could end up with a sunburn like this. Ouch.

1. Know thyself: If you are whiter than a sheet of paper, if your Aunt Linda has skin cancer, or if you are sensitive to the sun because of a medication or a medical condition, take extra measures. Stay out of the sun as much as possible, wear a hat when you are out, use a broad-spectrum sunscreen with a high SPF and reapply that sunscreen often.

2. Broaden your spectrum: The SPF number indicates protection only against UVB rays -- many sunscreens, even those with a high SPF, allow UVA rays to be absorbed by the skin. UVA protection is usually indicated by a "broad-spectrum" label. Look for this to ensure the most well-rounded sun coverage.

3. Here comes the sun: To be effective, sunscreen needs to be fully absorbed into the skin, so apply it 15 to 30 minutes before you even step into the sun.

4. Reapply yourself: Whether you're lying by the pool or mowing the lawn, you'll probably be exposed to sweat or water, the natural enemies of sunscreen. To be safe, reapply after you swim or sweat.

5. Full exposure: No matter how high the SPF, sunscreen can protect only the skin it covers. The most commonly missed spots are the temples, ears, back of the neck and top of the feet. If you are sometimes guilty of losing your focus while applying your lotion, try one of the sunscreens that contains disappearing colorants, so you can identify unlotioned areas before they burn to a crisp.

For more information on SPF numbers and sunscreen, check out the links on the next page.

New Protection against UVA Rays
The high SPF numbers on some new sunscreens might not be that useful, but their protection against UVA rays will be. The skin absorbs UVA rays from the sun, tanning beds and sunlamps. These rays won't give you a sunburn, but they can cause long-term skin damage. You should always use a broad-spectrum sunscreen to ensure protection from both UVA and UVB rays. The latest sunscreens boast new chemicals, like Mexoryl, which has proven to be one of the most effective UVA-blockers out there.

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SPF: What's in a number?