Understanding Sunscreen Use:
Get The Skinny On SPFs

by Patricia E. Reykdal and Donald L. Smith

Conventional wisdom has it that all you need to do is multiply the amount of time it will take to sunburn by the sunburn protection factor (SPF) in order to calculate the time a sunscreen will protect you from sunburning. Unfortunately, relying on this simple formula will cause you to become painfully aware of the fact that what you don't know about sunscreen use can hurt you. A simple Skin Typing/Subtyping questionnaire can be found on page 34. You will need to know this value in order to use the information in this article.

Calculating Your "Natural" Photoprotection

Let's take a look at the photoprotection provided by our natural skin color in relation to different sunlight intensity levels.

As you can see from the chart below, a Skin Subtype 2A (the most sun sensitive Skin Type 2) takes 21 minutes to sunburn in sunlight with an intensity of 6 Ultraviolet Index Units (UVI). This time is referred to as the time to sunburn (TTS). Therefore, we say that a Skin Subtype 2A has a TTS of 21 minutes that is provided by their natural skin color. On the other hand, a Skin Subtype 3A has a TTS of 42 minutes, and a Skin Type 5 has a TTS of 72 minutes.

If we only use the information above and multiply it by an SPF 8 sunscreen product, we would assume that a Skin Subtype 2A has 168 minutes (8 x 21 minutes); a Skin Subtype 3A has 336 minutes (8 x 42) and a Skin Type 5 has 576 minutes in the sunlight intensity found on a typical summer day.

Application & Dose

The Food and Drug Administration requires that sunscreen products be tested at a concentration of 2 mg (of active ingredient) per square centimeter of skin. For an adult full-body application of sunscreen, this translates into 1.25 ounces of product or approximately one full handful.

The problem is that numerous studies have found that people routinely only use 25 percent of the recommended amount, and a new study shows that the amount of sunscreen used is only about only 20 percent of the recommended application dose.

Since the strength of the sunscreen product (the SPF value) must be reduced by this same amount, it means that an SPF 8 has the real-world photoprotective power of an SPF 1.6!

Now, we can see that the real-world photoprotection for a Skin Subtype 2A is only 34 minutes (1.6 x 21 minutes) instead of 168 minutes. This means that the individual has 134 minutes (more than two hours!) of time where they think they are protected--but are not.

This "false expectation of photoprotection" explains why many studies show that the probability of sunburning actually increases for those who use sunscreens as compared to those who do not use them. A Skin Subtype 3A actually would be protected for 67 minutes (269 minutes less than originally assumed for an SPF 8), and a Skin Type 5 would be protected for 115 minutes (461 minutes less than originally assumed for an SPF 8).

Since the FDA currently does not require sunscreen manufacturers to clearly state the recommended application dose, is it any wonder that people using products containing SPF have a false feeling of security? Is it also any wonder that not understanding this basic fact of sunscreen use can hurt you?

The Ultraviolet Index Reading

There are several ways to determine the UVI (ultraviolet index) reading for your particular locale. Most newspapers list the UVI forecast for their market areas, and many television stations also provide this information.

If you are in an area where this information is not available, you can access the National Weather Service Web site, accessible at www.nws.noaa.gov, and get both a text and a map presentation of this information. Additionally, you can purchase your own hand-held UVI radiometer for about $180, which allows you to measure the UVI reading on a real-time basis.

Skin Type 1 Individuals & The Sun

Individuals who are true Skin Type 1s lack the genetic ability to develop facultative pigmentation, which is better known as a photoprotective tan. These people have no alternative but to use a sunscreen, wear protective clothing and avoid the sun as much as possible.

All other Skin Types/Subtypes can significantly augment their level of natural photoprotection by acquiring and maintaining a tan--ideally on a year-round basis. Then, if they find themselves in a position whereby they will be unavoidably exposed to sunlight for an extended period of time, they will wish that they had acquired and maintained a tan. Even so, there is a level at which their natural skin color (constitutive pigmentation), and acquired tan (facultative pigmentation) are inadequate to cope with an extended period of intense exposure to UVR.

The next level of defense is the prudent, proper use of a sunscreen product (with the protection time calculated according to the real-world application dose).

The Bottom Line

Failing to understand the importance of the real-world application dose of sunscreen products containing SPF can cause you to sunburn because of a reliance upon a faulty means of calculating the protection time provided.

Since most people only apply 20 percent to 25 percent of the recommended amount, we recommend that you build this information into your calculations. An SPF 8 would, therefore, be an SPF 1.6 to SPF 2; an SPF 15 would be an SPF 2.4 to SPF 3; and, an SPF 30 would be an SPF 4.8 to SPF 6. Using these adjusted SPF values will compensate for the real-world application dose.

By knowing the UVI reading for the day in question, and the application dose adjusted SPF levels, you will be able to better predict your true TTS. The bottom line is that it is always better to be safely conservative than painfully sunburned!