Kim and Kate have a combined question… Kim uses acne products that contain salicylic acid (a beta-hydroxy acid) and she’s worried that applying moisturizer on top of the sal acid may be making it less effective. Kate’s been told that she should wait 15 minutes after showering before applying an AHA because the water in skin will dilute the acids. So she’s concerned that she’s not waiting long enough after she showers before applying her AmLactin
The Right Brain attempts an answer:
The answers to these questions were surprisingly tricky to track down. Concentration is a crucial factor for AHA efficacy in products like Alpha Hydrox . According to the FDA , the Cosmetic Ingredient Review board has determined that AHA`s are safe at concentrations up to 10% for over the counter consumer products. For products used by trained cosmetologists, the concentration can be as high as 20 to 30%, and for doctors it’s an astonishing 50 to 70%. These higher concentrations are safe only if followed by thorough rinsing and application of a sun protection product. (That’s because AHAs can increase your sensitivity to UV radiation. But that’s a topic for another post.)
But while everyone agrees that AHA and BHA functionality is affected by concentration (and pH) we couldn’t find any direct evidence that to show that either skin moisture or moisturizing lotions can reduce the effective concentration of these active ingredients.
We did find out that BHAs penetrate skin quickly, so it’s unlikely that use of a moisturizer on top of sal acid would cause significant problem. But if your just-showered skin is really wet when you apply an AHA, it is at least theatrically possible that you’re reducing its efficacy. Better safe than sorry, we say. So until we can find some data to the contrary, we’d recommend making sure your skin is dry before using AmLacTin . If you’re still worried about sealing in moisture in towel dried skin, you may want to use a conventional moisturizer.