The pH of skincare is a hot topic, and an issue that can have a dramatic impact on your skin. Fortunately, it’s also a fairly easy concept (at least as far as chemistry concepts go). Let’s dive right in.
What is pH?
Water consists of two hydrogen atoms and an oxygen atom: H20. In a water-based solution, some of those molecules will split up into two component parts: hydrogen ions, or H+, and hydroxide ions, or OH–.
The term “pH” refers to the concentration of hydrogen ions in an aqueous, or water-based, solution. As the concentration of H+ ions increases, the solution gets more acidic. The opposite is also true: as the concentration of OH– ions increases, the solution gets more alkaline.
The pH scale runs from 0 to 14: 7 is neutral, less than 7 is acidic, and more than 7 is alkaline. Water has a neutral pH of 7; that’s because when there are only H20 molecules, you can only ever have an exactly equal proportion of H+ and OH– ions. If you create a new H+ ion by breaking up a water molecule, you’re also creating a new OH– ion. This satisfying balance holds pure water at a pH of 7.
When you add something acidic to water – for example, glycolic acid – it will add more H+ ions to the solution without adding OH– ions to balance it out. That means the overall concentration of H+ ions goes up, and the pH goes down.
You may have noticed we’re only talking about water-based solutions. What about products without water, like facial oils? Water-free, or anhydrous, products actually don’t have a pH at all. That’s a measurement that’s reserved for water-based products. Asking the pH of an anhydrous product is like asking the weight in kilograms of the word “December” – it’s just not a measurement that applies.
One final note about how pH is measured: the pH scale is logarithmic, not linear. While you might expect something with a pH of 2 to be twice as acidic as something with a pH of 4, it’s actually 100x more acidic. Every time you go from one integer to the next in the pH scale, you’re changing by a factor of ten.
Okay, that’s the last of the math for this post. I promise.
pH and Skin
Since your body is mostly water, you might expect it to have a neutral pH, but your skin is actually quite acidic – usually between pH 4.2 and 5.6. There’s an interesting biological reason for this. While your skin is acidic, your blood is slightly basic (tightly controlled between 7.35-7.45). Most microbes like bacteria and fungus can only survive in a specific pH range. Those that survive well on the acidic skin will die if they make it into the alkaline bloodstream; those that want an alkaline environment and would flourish in the bloodstream will die on your acidic skin. Just one more way your skin keeps your body safe from the world. What a champ your skin is.
This acidic outer layer, known as the skin’s “acid mantle”, is critical for maintaining healthy barrier function and robust microflora. The process your body undergoes to build and maintain your skin barrier involves several pH-dependent enzymes. At too high a pH, these enzymes can’t function properly and as a result, your skin barrier suffers. And once you’ve raised the skin’s pH, it takes hours to return to normal; in one study, it took an average of 2 hours after cleansing to get back to the starting pH.
That’s why pH is so important in skincare. Raising your skin’s pH out of that acidic range can impair your skin’s ability to function on a basic level. With a compromised skin barrier, you’re leaving yourself open to attack from external sources – like acne-causing bacteria and environmental irritation – as well as internal sources, like inflammation and dehydration.
Consistent use of high pH products can cause redness, flaking, inflammation, and breakouts. In one 1995 study, a group of acne-prone young people washed their face daily with either high-pH soap or a low-pH cleanser for 3 months. The high-pH soap group ended up with an average of 5% more pimples, while the low-pH group saw a decrease of about 32%. [Source]
In another study, 200 folks with acne were compared with 200 folks without acne. 77.5% of those with acne had a high skin pH, compared with just 6% of those with healthy skin. [Source]
And it’s more than just acne: according to this 2006 paper, “Changes in the pH are reported to play a role in the pathogenesis of skin diseases like irritant contact dermatitis, atopic dermatitis, ichthyosis, acne vulgaris and Candida albicans infections.” [Source]
So if you have acne-prone, sensitive, or otherwise compromised skin, using low-pH skincare is ESPECIALLY important for you.
Why Soap is Bad for Skin
Before I get into it, I want to clarify that this section is just talking about the ingredient “soap”: fats or oils mixed with a high-pH base. Usually, soap comes in solid form (e.g. bar soap). Not all cleansers are soaps.
So! The soap-making process is a fairly simple one, and it’s been around for millenia. The first recorded soap formula was from Babylon in the 2000s BCE. To make soap, you simply take a fat or oil (like olive oil or lard) and add an alkaline ingredient (often sodium hydroxide, NaOH).
Plant and animal fats are made of triglycerides, which you can think of as a tree trunk with three branches coming off of it. The trunk is a glycerol base, and the three branches are each a fatty acid. When you add a strong base like NaOH to a triglyceride, it snaps all the fatty acid branches off of the trunk. These loose fatty acids then bond with the sodium (Na+) ions from the NaOH. These fatty acid salts are what make up soap; the glycerol trunks can be siphoned off, or they can be left in the product to make glycerin soap.
Adding a strong base is intrinsic to the process of creating soap; it can’t be done without it. That means that soap inherently has a high pH – often in the 9-10 range – and can wreak all the havoc mentioned in the previous section.
Why is There Lye in my Skincare?
You may have noticed a troubling ingredient in your favorite skincare products: sodium hydroxide, otherwise known as lye or caustic soda. This is the same ingredient that’s commonly used to make soap. It’s also highly caustic, extremely dangerous, and should be handled with utmost caution.
So why is it in your toner?
Remember, something becomes acidic when it has a surplus of H+ ions, and alkaline with a surplus of OH– ions. Certain skincare actives like AHAs (glycolic acid, mandelic acid, etc.), BHAs (salicylic acid), and vitamin C are extremely acidic, and will naturally have a pH as low as 1.5-2. This means there are a TON of H+ ions floating around.
To raise the pH up to a level that’s friendlier for your skin, you need to neutralize some of those H+ ions. When you add a strong base like sodium hydroxide (NaOH), you add a bunch of loose OH– ions in the mix. (“Strong bases” like NaOH dissociate completely in water into Na+ and OH–; there’s no intact NaOH left.) The loose OH– ions bond with some of the H+ to form H20, or plain water. Sodium hydroxide simply lowers the concentration of H+ ions by turning some of them into water, thus raising the pH.
That means there’s no actual lye in the final product – it’s all been turned into water. Due to the nature of labeling requirements, companies have to list every raw ingredient they add to the formula, not every ingredient that’s currently present in the final product, so even though there’s no sodium hydroxide in its final form, it still shows up on ingredient lists.