Why Vitamin C needs to be acidic
Most Vitamin C skincare products on the market use ascorbic acid, because it's the most researched and proven form of Vitamin C. Ascorbic acid is water soluble, so it tends to come in a lightweight serum form — which many of us are familiar with.
The tricky part is: how do you make ascorbic acid get absorbed into the skin? Because as you know, our skin is waterproof.
Pinnell and colleagues, researchers at Duke University, published work in 2001 that showed ascorbic acid can penetrate through the skin barrier if formulated in a solution where the pH is 3.5 or lower. So Vitamin C serums on the market tend to be on the acidic side. For comparison: a pH of 3.5 is about the same acidity as white wine.
And of course, hydroxy acid products tend to be acidic as well.
Why acidic formulas aren't bad for your skin
Your skin is naturally slightly acidic and functions optimally at an acidic pH. There are two key reasons for this:
Hence a healthy, waterproof skin barrier relies on an acidic pH. Acidic formulas aren't necessarily bad for your skin — in fact, studies show they can be actively beneficial.
In 2020, Valle-Gonzalez and colleagues applied a glycolic acid formula at pH 3–4.5 and showed it was antibacterial against the acne-forming bacteria C. acnes — and the lower the pH, the better the antibacterial activity. Further, in 2010, Hachem and colleagues showed that hyper-acidifying the skin barrier with polyhydroxy acids led to a stronger skin barrier by increasing ceramide production and inhibiting its breakdown by serine proteases.
What to do if you have sensitive skin
Even though acidic topicals can benefit your skin, some people — especially those with sensitive skin — can find them irritating. This is because those with sensitive skin often have a compromised and leaky skin barrier. Topicals that should have trouble crossing the skin barrier or should stay on the surface don't behave as expected, and this can irritate the skin.
You can imagine that for some, using a Vitamin C product (which is acidic) and a hydroxy acid product (also acidic) can cause redness.
If you have easily irritated, sensitive skin, use niacinamide to fortify your skin barrier. In our opinion, niacinamide is the best skin conditioning ingredient on the market. Read our Deep Guide to Niacinamide →
How to use Vitamin C and hydroxy acids together
To curb irritation, here's what I recommend: start out by using Vitamin C and hydroxy acids separately.
Use Vitamin C serums in the morning as their antioxidant activity will fight the free radicals that are generated by UV rays during the day. Then use hydroxy acids like glycolic acid in the evenings, as glycolic acid can increase sensitivity to the sun.
If that doesn't work, another option is to switch to a hydroxy acid product that might be more suitable for sensitive skin — such as a polyhydroxy acid product, which tends to be more mild.
For those who don't experience any irritation, or whose skin has acclimated to both products, you can absolutely use them together in one routine. There is no reason you can't comfortably use both ingredients at the same time.
- Pinnell SR, Yang H, Omar M, Riviere NM, DeBuys HV, Walker LC, Wang Y, Levine M (2001). "Topical L-Ascorbic Acid: Percutaneous Absorption Studies." Dermatol Surg 27: 137–142.
- Valle-Gonzalez ER, Jackman JA, Yoon BK, Mokrzecka N, Cho MJ (2020). pH-Dependent Antibacterial Activity of Glycolic Acid: Implications for Anti-Acne Formulations. Sci Rep 10:7491. https://doi.org/10.1038/s41598-020-64545-9.
- Hachem JP, Roelandt T, Schurer N, Pu X, Fluhr J, Giddelo C, Man MQ, Crumrine D, Roseeuw D, Feingold KR, Mauro T, Elias PM (2010). "Acute Acidification of Stratum Corneum Membrane Domains Using Polyhydroxyl Acids Improve Lipid Processing and Inhibits Degradation of Corneodesmosomes." J Invest Dermatol 130: 500–510.
