Key Takeaways

• pH runs from 0 to 14, with 7 as neutral, and each whole step is a tenfold change in acidity.

• A 2022 review in Molecules puts the skin’s own pH in a slightly acidic range of 4.0 to 6.0.

• The same review describes traditional soap as alkaline, while syndet cleansers can be formulated with a pH closer to the skin’s own.

• FDA says how much exfoliation an Alpha Hydroxy Acid (AHA) product gives depends on the type and concentration of the acid, its pH, and the other ingredients.

What pH Measures, and Where Skin Sits

pH is a measure of how acidic or alkaline something is. The scale runs from 0 to 14: Anything below 7 is acidic, 7 is neutral, and anything above 7 is alkaline, or basic. It’s also logarithmic, so a pH of 5 is 10 times more acidic than a pH of 6.

Skin sits on the acidic side. A 2022 review in Molecules on soaps and syndets describes the skin as having “a slightly acidic pH, between 4.0 and 6.0.” That mildly acidic state is often called the “acid mantle,” and a guide to supporting the moisture barrier covers the daily habits that can help keep skin comfortable.

Why Cleanser pH Gets So Much Attention

Cleansers are where pH comes up most. The Molecules review explains that traditional soap is made through saponification, which “requires the use of a strong base,” so the finished product is alkaline. A syndet, short for synthetic detergent, is made differently, and the review notes that the processes used to create syndets “allow for their pH to be adjusted to be similar to that of the skin.”

The review links high-pH soap to changes in skin lipids and pH, and it says that for people with dry skin conditions, traditional soap can leave skin with “a red, rough and scaly appearance.” Its suggestion for those conditions is “mildly acidified syndets.” If your face tends to feel tight after washing, non-stripping cleansers are a reasonable place to start.

How pH Shapes Acid Exfoliants

pH also matters for exfoliating acids. FDA’s page on Alpha Hydroxy Acids says AHA products cause “exfoliation, or shedding of the surface skin,” and that “the extent of exfoliation depends on the type and concentration of the AHA, its pH, and other ingredients in the product.” So two products with the same acid at the same percentage can exfoliate to different degrees.

FDA also notes that AHAs may increase the skin’s sensitivity to the sun. If you use one, a broad-spectrum sunscreen every day is a sensible habit, and a guide to AHA exfoliants can help you compare formulas.

Frequently Asked Questions

How Does a Neutral pH of 7 Compare With Skin?

Neutral sits a little above the skin’s own range of 4.0 to 6.0, according to the Molecules review. Traditional soap sits further from skin still, since the review describes it as alkaline.

What Does “Syndet” Mean on a Cleanser?

It’s short for “synthetic detergent.” The Molecules review sorts skin cleansers into two types, soap-based cleansers and syndets, and notes that both cleanse “through surfactant action, which leads to the lowering of the surface tension of the skin to facilitate the removal of dirt from its surface.”