Layering of different skincare products, one right on top of another, raises many questions and remains one of the murkier and more confusing subjects in skincare. In fact, a DermTV viewer recently asked something along the lines of, “Do two different products… say a 5% and a 10%... give you a 15% strength product if layered on top of one another?" She asked because her toner had 5% glycolic acid and her serum, 10. To begin to address this question, let’s first think about what happens if you apply the same product twice on your skin? For example, if I take a 10% glycolic lotion and apply it on the back of my hand, wait a moment, and then apply it again… Does it become 20%? Your gut probably says no, it's still 10% on your hand. Which is right. But now let’s see why. To make it easier to see, let’s pretend that this liquid represents a glycolic product. The yellow is the glycolic, and the red is the vehicle. The glycolic is 25% of the total volume of the product and therefore this represents a 25% glycolic product. And as you can see, I have two identical “doses” here. So now lets add my two doses together, which is the same as applying some product on my skin, and then applying the same amount of the same product over it. Will we double the concentration to 50% or will it stay the same at 25%. I need to pour slowly for you to see the different colors. So my first dose comes up to the second mark, and you can see that one and a half of the marked volumes is red vehicle, and one half of a mark is yellow glycolic, so its still 25% glycolic… no magic here. Now I’ll slowly add the second identical dose and lets see what happens. Voila! Three quarters of the final dose is red vehicle… It comes up to the third mark... And one quarter, or 25%, is yellow glycolic, which comes up to the fourth mark. This shows that even by putting a second dose on top, you still have the same strength glycolic. The point here is that by putting more of the same lotion on you don’t get a higher strength or concentration. Now coming back to the original question… Do two different products… a 5% and a 10%... give you a 15% strength product? From this experiment I hope you can see that the answer is no and that the final concentration is not additive. It’s actually a weighted average, and in fact, instead of 15%, you’d end up with 7.5% if the same volume of both products were used. This question really pleased me because it shows that you are taking this stuff seriously and really thinking about it.