3. AHAs, BHA
and Other Acids

Now that you have the acid basics down, let's discuss specific acids. Information about how to actually apply these to your skin will be in Part 5.

Remember that most of these acids are also skin irritants and may increase your skin's sensitivity to sunlight. If you missed the Skin Threshold and Sunscreen discussions in Part 1 of this series, I encourage you to read those now.

I favor The Ordinary's products because of their high quality and low price. Still, you can find the ingredients below in products from many different manufacturers. I provide links to The Ordinary's website for specific products when available. I don't make any money from this.

The AHAs

AHAs (alpha-hydroxy acids) are a group of acids with useful skin properties. Here's how a chemist would think of them (these are called 'skeletal' diagrams):

Skeletal diagrams of AHAs

We won't be talking about all of these! All of the AHA's are exfoliants. Also, depending on the particular AHA you are using, you can get the following benefits:

They also help to keep the skin moisturized. Let's talk about a few in detail.

Glycolic Acid

This is the most popular AHA and can be found in multiple products. As you can see from the chemical drawings, it is also the smallest AHA molecule. Its size allows it to penetrate more deeply into your skin, but increased penetration also increases the risk of irritation. It is also used (often with other AHA acids) in high concentrations as a peel or mask. It is by far the most studied of all the AHAs.

Best for
Exfoliation, fading pigmented skin
Harshest of all the AHAs
The Ordinary
Glycolic acid 7% Solution, 240 ml, $8.70

Lactic Acid

Lactic acid is the second most researched AHA after Glycolic Acid. It is known for being gentler, more hydrating, and more effective at treating sun-damaged skin. It also increases the ceramides in your skin. Ceramides are one of three oil lipids that keep our skin waterproof, so lactic acid helps keep your skin hydrated.

People sometimes recommend yogurt masks because yogurt contains lactic acid. Unfortunately, the concentration of lactic acid in yogurt (1% or less) is too low to make this practice truly useful.

Best for
exfoliation, fading pigment, moisturizing
The Ordinary 5%
5% Lactic Acid, 3o ml, $6.50
The Ordinary 10%
10% Latic Acid, 30 ml, $6.80

Mandelic Acid

Mandelic acid has only been studied for its use in peels. The skeletal drawing shows it is a much larger molecule so that it will be less penetrating and gentler on the skin. People with bronze-ebony skin who suffer from rebound pigmentation (increased skin pigmentation due to irritation) often use it.

Best for
exfoliation in darker skin
The Ordinary
10% Mandelic acid solution, 30 ml for $6.80

Other Acids


There is only one BHA, which is Salicylic acid. It's a widespread ingredient structurally related to aspirin, with whom it shares anti-inflammatory properties. Like the AHAs, it is an exfoliant. It is also found in wart and corn removers and very common in skin cleansers.

It is a big molecule and quite oily. Perhaps because of this, salicylic acid's effects seem less influenced by pH. Typical concentrations are 0.5-2%.

You can buy Salicylic acid masque's from The Ordinary, but I prefer to use it in a cleanser (see Part 1 ) and in combination with multiple AHAs as a peel (see, for example, my personal skincare routine described in Part 5.)

Azelaic Acid

This is a crucial ingredient in many prescription-only acne creams. It's often prescribed for pregnant and nursing women since it's safe. And pregnancy-associated acne is real. Typical concentrations are 15-20%. Side effects are unusual unless you have sensitive skin. If you do, you may experience redness, peeling, and stinging.

You can use azelaic acid to treat post-inflammatory hyperpigmentation (PIH). These are dark spots left by skin injuries such as acne. PIH is very common in darker skin. There have been recent concerns about the safety of hydroquinone, a kind of bleach that is a common treatment for PIH. If you are worried about hydroquinone, Azelaic acid, combined with retinoic acid, is almost as effective and might be safer. Azelaic acid is an effective treatment for rosacea. Its anti-inflammatory properties don't work for inflamed vessels, but it will reduce the lumps and bumps that often appear.

Best for:
treating acne, including pregnancy-associated acne, fading pigmented areas. It helps rosacea.
Mild. Safe for pregnant and lactating women.
The Ordinary
10% Azelaic acid suspension. 30 ml for $7.90

Ascorbic Acid

The most common and studied water-soluble vitamin C form. It has multiple benefits: treating pigmentation and increasing collagen. It's an antioxidant that protects against damaging free radicals from UV radiation and pollution. Free radicals are the leading causes of skin aging.

The biggest problem with ascorbic acid is that it breaks down in water very quickly. Combining it with Vitamin E and ferulic acid can slow this but add a lot to the cost of the product.

My sweetie Dr. Davey is not a big fan of ascorbic acid. He recommends adding this last (if at all) to your skincare routine. It is very acidic (pH of 2-2.5), so it tends to be very irritating. It is also less effective than Retinol and AHAs. My Dr. Davey also believes ascorbic acid is less effective for PIH than Hydroquinone.

Best For
treating pigmentation, increasing collagen, fighting free radicals that cause aging.
Recommended Use
Try other acids first.

Hyaluronic Acid

Hyaluronic acid is a very hyped ingredient. I am writing about it here primarily for that reason. It occurs naturally in your body. It is a primary ingredient in many dermal fillers.

Its acidic properties have nothing to do with its use in skincare formulations. It is too large a molecule to penetrate very far into your skin and is not an exfoliant. However, it is a remarkable humectant (hydrator) for the skin. One gram of hyaluronic acid can hold onto six liters of water! There are plenty of other suitable humectants (such as vaseline), but they may not feel as nice. They also lack the hype factor.

There is no reason to avoid products containing hyaluronic acid. But it is also no reason to purchase one. If a product containing hyaluronic acid is interesting to you for some other reason, by all means, buy it.

That's it for now. In Part 4, we will talk about the best acid for skin! Don't miss it! Meanwhile, if you have comments or questions, please send the nice ones to me!