How to Layer Skincare Products by pH Level
There tends to be a lot of confusion around what order you should be applying the products in your skincare routines, and it becomes even more complicated when phrases like “pH-dependent” get thrown around. Some will tell you that pH-dependent products must be applied directly after cleansing, in a specific order determined by pH levels, and with 20 minute wait times in between. Others will tell you, however, to just slap that vitamin C on whenever you want and don’t bother checking the pH level because who the hell cares!
Clearly, those are two extremes and my philosophy lies somewhere in between!
What does it mean when a skincare product is pH-dependent?
Some skincare products, namely acid treatments, need to be formulated at certain low pH levels to effectively penetrate our skin. If you apply an 8% glycolic acid product, that doesn’t mean 8% glycolic acid will be absorbed! Absorption depends on the pH level, amount of free acid particles, and the concentration of the product. You can think of the free acid particles as the only particles that are free to enter your skin - this penetration is what makes an acid effective. There will always be more free acid particles at lower pH levels which is why some acids require a low pH formulation.
Which skincare products are pH-dependent?
Not all acids are pH-dependent because other factors like molecule size triumph pH when it comes to absorption potential. (Large molecules can’t penetrate the skin regardless of a low pH level.) The acids that are dependent on a low pH level for skin absorption are alpha hydroxy acids (glycolic, lactic, malic & mandelic), ascorbic acid (vitamin C) and azelaic acid.
Optimal pH levels for skincare acids
Alpha hydroxy acids (glycolic, lactic, malic & mandelic): ph < 4 (source)
Ascorbic acid (vitamin C): ph 3.5 (source)
Azelaic acid: 4.9 (source)
Beta hydroxy acids (salicylic acid): pH doesn’t matter
Retinoic acid (retinoids): pH doesn’t matter
When should you apply pH-dependent Acids?
Acids are active ingredients that change the structure and function of the skin through penetration (like glycolic acid causing your skin cells to turnover quicker), rather than just temporarily changing its appearance (like a moisturizer plumping fine lines). That’s why I like to apply acids as close to my skin as possible, directly after cleansing. Applying any hydrating/moisturizing ingredients before acids can slow down their penetration. However, for those with sensitive skin, applying a hydrating/moisturizing step beforehand is a nice way to buffer the irritation that can be caused by acids.
You may have heard of some people using pH adjusting toners to force down the pH of their skin before applying acids so that your skin is already at a low pH for them to work effectively. I truly don’t believe this step is necessary though, especially if you're already using a low pH cleanser (which you should be!). Check out my post on the reasons the pH of your skin matters for more on that.
How to Layer Skincare Products by pH
Layering multiple acids in a single routine can cause some serious over-exfoliation and damage to your moisture barrier so I’ll never recommend it to anyone who doesn’t already have a lot of experience using acids on their own. I’m a big fan of alternating acid days instead!
But for those who do have experience with acids in your skincare routine already, I know that many follow the practice of applying them in order of lowest to highest pH level. However, since most skincare acids are formulated in close pH proximity, I don’t think it really matters until someone shows me some hard evidence to the contrary!
I think it’s more beneficial to learn about which actives function better in morning vs. nighttime routines and pair them up this way if you’re layering. For example, ascorbic acid (vitamin C) is a powerful antioxidant that enhances UV protection when used with sunscreen so it’d be best in your morning routine paired with a BHA that doesn’t cause any UV sensitivity. Then there is retinoic acid (retinoid) that does cause UV sensitivity and degrades in light so it’d be better in your nighttime routine paired with an AHA that also causes UV sensitivity. (AHAs actually cause increased UV sensitivity for multiple days after use so you’ll be at a higher risk for UV exposure regardless of morning vs nighttime application - but you get the point!):
Ascorbic acid (vitamin C): mornings
Retinoic acid (retinoids): nights
Beta hydroxy acids (salicylic acid): mornings or nights
Alpha hydroxy acids (glycolic, lactic, malic & mandelic): mornings or nights
Azelaic Acid: mornings or nights
Are wait times necessary between skincare product applications?
From some very particular skincare enthusiasts, I’ve heard of 20-30 minute wait-time suggestions between skincare acid applications to allow time for each product to force down your skin’s pH to its optimal level and then self-neutralize before applying the next acid. However, this is another thing I find to be unnecessary due to lack of evidence and think you can do without!