The Skincare Series:
2. Understanding Acids

Acids on your skin? Sounds scary? It does, but they can do a lot for you. To see how, let's start with the top layer of your skin, the epidermis:

Picture of Epidermal Layers

The epidermis doesn't have any blood supply - any nourishment it gets is from the layer below it, the dermis. New cells are formed in the lowest level of the epidermis, the stratum basale, or basal layer. These new skin cells push the other cells upwards. As the cells move up, they grow larger, dehydrate and become harder until they eventually end up as hard, dead cells in the stratum corneum (horny layer). Most of your skin has only four layers; the stratum lucidum is found only on the bottom of your feet and your palms. The topmost cells fall off, and the entire upper layer (the stratum corneum) gets replaced every two weeks. The whole epidermis gets replaced about every four weeks.

This replacement process can be slowed by aging, hormones, diet changes, and even weather. Dead cells that hang around too long can make your skin look duller, your pores look bigger, and contribute to acne. That's why it's a good idea to exfoliate.

There are two ways to exfoliate: physical and chemical. Physical exfoliation is scrubbing with rough cloths, sponges, physical devices, brushes, or scrubs. Chemical exfoliators are primarily acids. We aren't entirely sure how these chemical exfoliators work, but we know they do. While physical exfoliators work well on most of your body, the skin on your face is much more delicate. Scrubbing can irritate or damage this fragile skin. Your skin also has an acid barrier (called the "acid mantle) that protects it. As you can see in the picture, the pH of the skin and mantle are around 5.5 (more on pH in a moment):

Picture of Acid Mantle

Physical exfoliation disrupts this acid mantle, but acids do not. Most acids also have additional benefits. Some acid molecules are small enough to reach into the deeper layers of your skin. That allows them to improve skin thickness, encourage faster cell turnover, and even encourage new collagen generation, which helps fine lines and wrinkles. Acids can also even out your skin tone and remove some discolorations. If used with care, the multiple benefits they provide would make them worth using, even if they did nothing for exfoliation!

But which products to choose, which acids to use? There are so many out there! The skincare industry is no help. There are so many ingredients and so many claims made about products that it is difficult to make the right decision. And the prices are unbearably high even though the core ingredients, the ones that actually do the work, are inexpensive! Brands price products that include them at a premium because they do work so well. And, of course, once one brand does it, greed pulls the rest of them in. Just look, you'll see the same ingredients in all products. But to justify the outrageous prices and suck consumers in, they add fancy-sounding ingredients with little or no evidence behind them. The actual concentrations of the truly effective elements are usually hard to find and sometimes surprisingly low. As a consumer, this is super annoying. And discouraging. After all, who wants to spend $150 only to find that it has done nothing for you?

The Solution: The Ordinary

A recent brand, "The Ordinary" (parent company Deciem), has solved most of these issues. Their product line is excellent. Their formulations are based on science, the concentrations are clearly stated on the bottles, and the prices are extraordinarily reasonable. At the moment, I am using two of the products, which last about 6 weeks. The total cost for both is under $14! These are extraordinary prices!

I also appreciate their marketing, which is informative and refreshingly free of hype. The problem with informative, hype-free marketing is that the consumer needs to know more about the products. Conventional wisdom argues that beauty consumers are too dumb and too lazy to inform themselves. But The Ordinary has shown this to be false. Consumers have responded to The Ordinary by buying from them in droves and have made The Ordinary a major player in a remarkably short time. Clearly, there is a market for intelligent beauty consumers!

What follows are the basics about acids that I've managed to learn as I tried to become an informed consumer of beauty products. Later articles will discuss specific acids.

Acid Basics

Acids are molecules that release hydrogen atoms when you dissolve them. The stronger the acid, the more easily it lets go of its hydrogen atoms when placed in a solution. These single hydrogen atoms are called hydrogen ions. The more hydrogen ions there are floating around in a solution, the more acidic it is. The pH values you see in product literature are there to tell you how acidic the solution is. To help you get a feeling for what pH values mean, here's a pH table that also lists some everyday household products:

pH table

Perhaps surprisingly, the lower the pH, the more acid the solution! The most acidic solutions have a pH of one, growing progressively less acid until you reach the neutral pH of seven. Every step on the pH scale changes the strength of the acid by a factor of 10, so apple juice (pH 3) is 10-times more acidic than tomato juice (pH 4). Distilled water is completely neutral with a pH of 7. Above that, we have the anti-acids, acid opposites, or "bases." These are the chemical opposites of acids. If you mix acids and bases together, they will react and neutralize each other. Try mixing vinegar and baking soda together sometime, and you will see what I mean. (If you have children, they love that activity.)

As we mentioned earlier, the skin and acid mantle have a pH which is (ideally) around 5.5. You might have noticed in the pH table that soap has a pH up in the anti-acid zone of the table! It's fairly high, around 10. So when you use soap to wash your face it tends to neutralize the acidity of the skin, leaving it closer to a neutral pH of 7. The problem with that is that bacteria just love a neutral pH. Those little guys just flourish. As you probably know, bacteria can contribute to many skin problems, so this is another reason I like acid for exfoliating and cleansing the skin.

Another number you will see mentioned in product literature is an acid's pKa value. That's a measure of how strong the acid is. Remember that the strong acids give up their hydrogen atoms easily when they are dissolved? So pKa is also a measure of how soluble an acid is when you put it into a liquid. So if you place hydrochloric acid, a very strong acid with a pKa of -6.3, into a solution it will dissolve entirely and release all of its hydrogen atoms. But the kinds of acids that are used on our skin have a pKa between 3 and 4. In this case, only some of the molecules will give up their hydrogen atoms. Others won't - they remain complete and undissolved.

Undissolved atoms are electrically neutral (they are called "free acid"). If they dissolve and give up a hydrogen ion (which is positive), then they have a negative charge. Why do we care about this? Because your skin will only let neutral molecules inside. This makes the neutral molecules (the free acids) the biologically active ones. We say they are "bioavailable". Meanwhile, your skin blocks all charged particles from entering. That's where pKa becomes important: we want to put enough acid into a solution that we have plenty of free acid molecules, and pKa helps us do that.

It turns out that when the pH of a solution is equal to the pKa value of the acid, then 50% of the acid is free, and 50% is dissolved. Let's say you have an acid with a pKa of 3. Depending on the pH, you will have the following situations:

pH Free Acid Ionized Acid
4 10% 90%
3 50% 50%
2 90% 10%
1 99% 1%

Let say you buy a bottle of pKa 3 acid from a company at a pH of 3. The bottle says the concentration of the acid is 8%. Since the pKa and pH values are equal, you instantly know that half (4%) of that acid is biologically active (free acid) and 4% is not. This is a pretty good situation, since you know that at least 1/2 of the active ingredients are going to go where you want them.

To drive home the relationship between pH and bioavailability, here are the results from a real-world study that I found a real word study that compared the absorption of two of the most commonly used skin acids at different pHs:

Acid pKa pH Absorption after 24 hours
Glycolic Acid 3.83 3 27%
7 3.5%
Lactic Acid 3.86 3 30%
7 10%

Look at the huge difference in absorption having a low pH makes! All of this really does matter.

(Here's the study referenced: (Kraeling MK & Bronaugh RL, In vitro percutaneous absorption of alpha-hydroxy acids in human skin, J Soc Cosmet Chem 1997, 48, 187.)

Since we get more bioavailable acid at a lower pH, why not make all products at a pH of zero? Well, because that would burn your face off! Low pH's do burn our faces off to varying degrees anyway. You'll know from the stinging, redness, itchiness, and peeling whether or not your skin can handle it. (I wrote about this a bit in Part 11, where I talked about your skin threshold.)

What happens to the other 4% of our ionized acid? Does it just sit there on top of the skin? No, it doesn't. As the free acid gets absorbed into the skin, the percentage of free acid on the skin's surface drops. As that happens, some of the ionized acids reconnect with their hydrogen atoms and become free acids. So, in theory, given enough time, all the acid on your skin could become bioavailable. Some companies use this to market acid products at higher pHs as "time-release" or "buffered". Since there is less free acid, there is also less skin irritation. But if the pH is too high, the acid will be absorbed too slowly and will be too dilute on your skin. It's like the difference between drinking an entire bottle of vodka vs. dumping the bottle into a swimming pool and drinking all the water in the pool. Only one of those will get you drunk.

The swimming pool analogy makes the point that concentration matters. An acid with a pKa of 3 at 10% concentration with a pH of zero WILL burn your face off. But a 50% concentration with a pH of 4 will burn it just as badly.

The Take Home Message

Acids are good.

pH changes the amount of acid in your product that will be absorbed by your skin. Lower pHs increase the amount of acid absorbed and cause increased irritation (with no time-release properties).

pH's higher than the recommended range might work, but not as well. They might work so poorly that you won't see any effect. Increasing the concentration of the acid can help it work at a higher pH but will also increase the potential for irritation.

That's it for now. Part 3 introduces some great and useful skincare acids! If you have comments or questions, please send the nice ones to me! To do that, you can