Most Popular in:

Sun Care Treatments

Email This Item! Print This Item!

Understanding Sunscreens

By: Rebecca James Gadberry
Posted: June 23, 2008, from the June 2006 issue of Skin Inc. magazine.

page 4 of 7

A . Some UV rays still get through, even with the very highest SPFs. Because the actual reason requires more than a passing knowledge of physics to understand, it’s simpler to look at SPFs and how much protection they provide the skin.

The FDA has defined SPF as the amount of time it takes to produce minimal redness (erythema) in skin via UVB exposure. This is called the Minimal Erythemal Dose (MED), and it’s achieved when the skin attains the amount of UVB required to produce redness, more commonly known as a sunburn.

An SPF 2 enables the skin to be exposed to UVB twice as long as when a sunscreen is not used before the MED is attained. This means that an SPF 2 absorbs 50% of UV rays, allowing the other 50% to get through, so it takes twice as long for the skin to absorb the amount of UV needed to produce redness. An SPF 8 absorbs 87.5% of the rays, leaving 12.5% to reach the skin. If you multiply 12.5% by 8, you get 100%, so the skin requires eight times the amount of UV needed to produce redness when wearing an SPF 8. An SPF 15 absorbs 93.33%, allowing 6.67% UV to get through, and an SPF 30 absorbs 96.67%, allowing 3.33% to pass. If you follow the diminishing numbers, you’ll see that even the highest SPF will not absorb 100% of UV rays. This is why the FDA no longer allows the use of the term “sun block” to describe sunscreens.

Q. Does an SPF 30 offer twice the protection as an SPF 15?

A .Based on the numbers in the last answer, an SPF 30 offers less than 4% more protection than an SPF 15. But that isn’t the whole story. To achieve the extra 4%, the higher SPF must reach beyond UVB rays into the UVA part of the sun’s radiant spectrum. Because UVA is implicated in phototoxic and photoallergic reactions, as well as diseases such as lupus, this extra 4% can be enough to keep certain complexions safe in the sun.