When we are in school, we are taught “1+1=2″.
Unfortunately, that’s not the way it works in Sunscreen Land. Keep the following rules in mind:
1.) With perfect application, an SPF 20 formula layered with an SPF 50 formula = SPF 50.
With perfect application, sunscreens do not have an additive effect. Instead, think of sunscreens more as having a limiting effect — they limit the number of rays that get through, but you can’t increase above the limit of the highest number.
For instance, if you apply a very thick application of an SPF 50 formula, it will allow 1/50 UVB rays through, or about 2% (source).
If you apply a very thick application of an SPF 20 formula, it will allow 1/20 UVB rays through, or about 5%.
If you apply both formulations perfectly at the same time, the SPF 20 formula allows 5% of rays through. The SPF 50 formula will block some of those, but 2% of the overall rays will still get through. This is regardless of which formulation you apply first.
2.) Almost no one achieves “perfect application.”
Formulations undergo rigorous testing to achieve SPF that involves very thick application of product — 2 mg/cm2 of skin, or about half a 8 oz. bottle to cover the average body. It covers a grid until it appears almost white, and light is reflected onto the grid to measure absorbance capacity to determine the overall effectiveness of the product. This is how the SPF rating is achieved for all sunscreens.
Unfortunately, the average person only applies 1/3 to 1/2 of the amount of sunscreen needed to achieve the SPF rating on the bottle. So your SPF 30 sunscreen is more like an SPF 10-15 with the average application.
It’s even worse with powder formulations. The average consumer only applies 1/14 of the amount of sunscreen needed to achieve the SPF rating on the bottle, so your SPF 15 powder formula is more like an SPF 1 formula:
3.) So it is true: Layering and reapplication of individual formulas has benefit with typical application.
Here’s where it gets tricky. As I’ve stated before, applying an SPF 30 sunscreen and an SPF 15 powder perfectly nets you a total SPF of 30, because you are stuck by the upper limit of maximal rays that can be blocked/higher SPF rating.
Applying an SPF 30 sunscreen and SPF 15 powder with typical use of application is still limited by the stronger formula/SPF rating. Assuming you are the typical consumer, the SPF 30 sunscreen nets you an SPF rating of 10-15, and the SPF 15 powder nets you an SPF rating of 1. If you were paying attention (and you better be, because there will be a quiz later — kidding!), then you are only getting a net SPF effect of 10-15. The higher number wins.
But layering still pays off with individual formulas. If you apply the SPF 30 sunscreen perfectly, you get 1/30 instead of the typical 1/15 rays blocked, or 3% versus 6% — a substantial enough difference.
And for the sake of argument, let’s say you apply the SPF 30 sunscreen very thinly and net SPF 10. And let’s say you pack on the SPF 15 foundation and net SPF 15. Well, guess what — you get SPF 15 protection. It’s not the higher SPF that wins, it’s the formula that provides the higher SPF when the amount of application is taken into account. Just looking at this from the stance of probability, the more formulations with sunscreen that you use, the better your probability of applying the right amount of active to get to the right dose to achieve maximal SPF.
4.) Do not layer formulas with octinoxate and avobenzone.
It has been shown in studies that octinoxate will degrade avobenzone (Photochemistry and Photobiology, 2005). For this reason, it is important that you avoid using formulas with these ingredients simultaneously after one another.
- 1.) With perfect use, you get the SPF rating of the higher formula. (Use SPF 30 sunscreen and SPF 15 powder, each perfectly, get SPF 30 protection.)
- 2.) With typical use, you get the SPF rating of the formula that provides the higher SPF when the amount of application is taken into account. (If you apply an SPF 30 sunscreen very thinly and net SPF 10, and pack on a SPF 15 foundation and net SPF 15, you get SPF 15 protection. It’s not the higher SPF that wins, it’s the formula that provides the higher SPF when the amount of application is taken into account.)
- 3.) Be sure to layer and reapply.
- 4.) Do not layer formulas with octinoxate and avobenzone.
For sunscreens we love at FutureDerm, check out our Sunscreens page!
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