Crash Course on AHAs, BHAs, Salicylic Acid, Vitamin C, Etc

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AHAs, BHAs, Salicylic Acid, Vitamin C… Help!

There’s a lot to learn when it comes to AHAs, BHAs, acids in general, vitamins, etc so I’ll try to write about the topic in a way that’s easy to understand and also as a guide for me to come back to when I need a quick reference.

In this blog post I’ll cover: what AHAs and BHAs are, benefits, a look at sun sensitivity, usage guidelines, pH, some formulation guidance and more.

Let’s get started.

Alpha Hydroxy Acids or α-Hydroxy Acids (AHAs) & Beta Hydroxy Acid or β-Hydroxy Acid (BHA)

AHAs and BHAs are both chemical exfoliators.

The FDA (U.S. Food & Drug Administration) requires that all retail packaging of products containing alpha hydroxy acids contain a sunburn alert:

Sunburn Alert: This product contains an alpha hydroxy acid (AHA) that may increase your skin’s sensitivity to the sun and particularly the possibility of sunburn. Use a sunscreen, wear protective clothing, and limit sun exposure while using this product and for a week afterwards.

With this alert out of the way, let’s get to know each acid now.

1. Alpha Hydroxy Acids or α-Hydroxy Acids (AHAs)

Alpha hydroxy acids are a group of natural acids found in foods. Alpha hydroxy acids include citric acid (found in citrus fruits), glycolic acid (found in sugar cane), lactic acid (found in sour milk and tomato juice), malic acid (found in apples), tartaric acid (found in grapes), and others.Alpha hydroxy acids are most commonly used for skin conditions such as dry skin, wrinkled skin, or acne. Source.

Alpha hydroxy acids (see here, here and here), or α-Hydroxy acids (AHAs) are chemical compounds consisting of a carboxylic acid substituted with a hydroxyl group on the adjacent carbon. What does this mean? It means that both the hydroxy group and the carboxyl group are separated by ONE carbon atom.

2. Beta Hydroxy Acids or β-hydroxy acid (BHA)

Beta-hydroxy acid — also called salicylic acid — exfoliates skin, unclogs pores and can improve texture and color. It also helps with acne. Salicylic acid exfoliates skin, unclogs pores and can improve its texture and color. It also helps with acne. Source.

A beta hydroxy acid or β-hydroxy acid (BHA) is an organic compound that contains a hydroxy functional group and a carboxylic acid functional group (check here and here) separated by TWO carbon atoms.

Benefits

What AHAs and BHAs do is help remove dead skin cells. That’s why they’re known as chemical exfoliants.

AHAs – AHAs soften the keratin in the horny layer (epidermis) which is the outer layer of the skin. They work as keratolytic agents and also bring moisture to the skin, making AHAs humectants (list of common humectants here). They’re useful as anti-acne treatments as well as anti-aging treatments as they help with:

• Sun damage
• Hyperpigmentation
• Increases skin’s elasticity
• Increases skin’s firmness
• Stimulates collagen synthesis
• Reduces appearance of wrinkles & fine lines
• Helps brighten skin

BHAs, on the other hand, work as comedolytic and desmolytic agents. What this means is that they work to disrupt cellular junctions as well as softening them and unclogging the pores on the skin. BHAs can have a drying effect on the skin because they lack humectant properties but they can penetrate into the pores so they’re used in acne-prone skin products as well as clogged pores.

Sun Sensitivity

As chemical exfoliators and when used above 2%, AHAs & BHAs increase the skin’s sensitivity to the sun (more pronounced with AHAs than BHAs). This means that after using products containing AHAs & BHAs the skin will be more prone to getting burnt as well as damaged when exposed to the sun.

If you use products with these chemical exfoliants you MUST use good quality sunscreen.

Remember the sunburn alert mentioned above!

AHAs Chart

Let’s have a look at glycolic acid, lactic acid, citric acid and malic acid.

Glycolic Acid

Glycolic acid is of easy penetration due to the small size and is soluble in water. It can be used in concentrations up to 70% although only by professionals. It is used at pH 3 to 5.

pKa value: 3.83

Lactic Acid

Lactic acid is of easy penetration due to its small size as well and also soluble in water. It can be used in concentrations up to 20% by professionals only. It is used at pH 3 to 5. I use lactic acid as a PH adjuster when necessary.

pKa value: 3.86

Citric Acid

I also use citric acid as a pH adjuster, but it can be used as a humectant as well. The size of citric acid is large so it’s not as easy for it to penetrate the skin as glycolic or lactic acid. It is used at PH 3.5 and above.

Malic Acid

Used as a humectant as well as pH adjuster. Of large size (so not easy for it to penetrate the skin). It is used at pH 3.5 and above.

pKa value: 3.40

BHAs Chart

Salicylic Acid

Salicylic acid is well known for being used in product aimed at acne. Suggested usage rate is 2% or below. It is used at pH 2.5-4.

pKa value: 2.97

Salicylic acid is soluble in oils, propylene glycol and ethanol.

About pKA Values

In the above tables we showed the different pKa values. Basically, the lower the pKa value the stronger the acid is. pKa is important because it determines whether the acid prefers to be in free acid form or in ionised form.

When the pH of your formulation is the same as the acid’s pKa value, half the acid will be in ‘free acid form’ (active!) and the other half in ‘ionised form’ (dissociated, inactive).

For example, a product with 3% AHA with a pH value the same as the pKa value (for example, let’s say, lactic acid at both pH and pKa at 3.86), 1.5% of the acid will be active and the other 1.5% won’t.

This is important because only the ‘free acid form’ can go through the oily layers of the so called stratum corneum. Therefore, this form will be the one that is considered to be an ‘active’ when it comes to your product. So when the acid is in ‘ionised form’, as it won’t pass the stratum corneum it doesn’t count as an effective exfoliant.

Calculating How Much of an Acid in a Product with a Particular pH is Active

To calculate how much of an acid in a product with a particular pH is active you’ll need to multiply:

1) the percentage of the acid in your formula together with

2) the percentage of the acid ‘free acid form’ at that very particular pH level

**Find here a free acid calculator table.

Let’s go back to our 3% lactic acid amount in our product. If the pH of our product is 4, then around 42% of the lactic acid (as shown on the table when you click on the link above) will be in ‘free acid form’.

>> Amount of lactic acid: 3%

>> pH of our product: 4

>> Taking into account the two amounts above, when checking out the table we will see that the percentage of the acid in its free acid form at the pH 4 is of: 42.01%

Here’s then how we calculate how much of the lactic acid in the active form:

3% x 0.42 = 1.26%

1.26% is the amount of lactic acid in its active form within the product.

I hope it makes sense!

What’s the Ideal pH when Using Acids in a Product?

This is why general recommendations say to keep your acid product in the pH range above 3.5.

Difference Between Acids & Peels

It is better to used these acids in low quantities (up to 2% when used daily and up to 5% for occasional treatments). When used at higher amounts (10% and more) it would become a chemical peel.

Just as a note, glycolic and lactic acids can be used in products at concentrations up to 10% in the final formulation with a pH ≥3.5 (directions for use must include the daily use of sun protectives).

More on this here, here and here.

About pH

pH is a measure. It measures how acid or how alkaline is a water solution.

This means it measures the actual concentration of hydrogen ions (H) in the actual solution. When the hydrogen ions concentration is high the pH is low. When the hydrogen ions concentration is low, the pH is high.

Here’s the pH equation:

pH = -log[H+] concentration

Which is read: the pH is equal to minus the logarithm of the H+ concentration.

Basically, when you add an acid to water it will release H+ ions. This will cause the pH to go down. Once we understand this it’s easy to also understand that when you add AHAs and BHAs to water they will increase the H+ concentration because they’re acids hence lowering the pH.

If you’d like to know more about water and organic molecules check out this link.

What’s crucial for me to remember here is that pH is a very important factor when it comes to skin irritability as well as AHAs and BHAs efficacy. High pH products will be less irritating than low pH products but they need to stay on for a while in order for the chemical exfoliators to have an effect on the skin. When the pH is too high the body doesn’t have time to rate of acid absorption is actually slower than the rate the body removes and breaks down these acids, the fast rate at which they get broken down means the exfoliation won’t have the needed time to take place.

pH below 4 is the ideal one when it comes products using AHAs and BHAs.

**If you have sensitive skin and you use these chemical exfoliants regularly you don’t have to mix with physical exfoliants. If you don’t use them that often you can combine physical and chemical exfoliants but you should leave two or three days in between at least.

Other Important Facts

• • AHAs & BHAs shouldn’t be used on oil based products (balms, salves, etc) because they need water in order to get activated

• • AHAs are water soluble

• • BHAs are not water soluble so you’ll need a solvent if you want to incorporate them into your water products

• • It is recommended you use acids on your night time routine

• • 2% and below can be used during the daytime (use a great quality broad spectrum sunscreen if so)

• • Rates: up to 2% for daytime use, up to 5% for occasional treatments (up to 10% for non-sensitive skin)

• • Over 10% acids becomes a chemical peel rather than an exfoliator

• • Avoid acids if your skin is sensitive to the sun or just sensitive in general

• • AHA fruit extracts can be added to the formula in the cool down phase

• • BHA Salicylic acid is oil soluble. You can use a solubilizer to dissolve it in water. You can also dissolve it in ethanol.

• • Salicylic acid (a BHA) is a bit more challenging to work with as it is oil soluble. Some solubilizers can help with dissolving it in water and it’s also soluble in ethanol and propanediol. It needs to be dissolved in water to get activated

• • If dissolving in ethanol the ratio is 20% ethanol to 0.5% salicylic acid

• • AHAs and salicylic acid can be added to the water phase (hot) as they’re not sensitive to the high temperatures. Willow bark extract and Fruit Acid Complex are added in the cool down phase

• • Being water soluble AHAs can be added to creams, toners, etc. BHAs will need the help of a solvent. They’re NOT SUITABLE in only oil-based products as they need water to get activated!!

• • It is recommended that you use AHAs and BHAs products as part of your nigh time routine. If using as part of your daytime routine, make sure that the product has acid concentration of 2% or below. Whether you apply the product day or night, do apply a quality broad spectrum sunscreen before leaving home in the morning.

• • For sensitive skin acids are best avoided.
  • Use ingredients that can be soothing to the skin such as actives: allantoin, panthenol, aloe vera, bisabolol and ingredients that add conditioning to the skin such as olive oil, jojoba oil, shea butter and coconut oil.

Emulsifiers

Products with AHAs and BHA will have a pH of 3.5 to 4. You should only use emulsifiers that work within this range. Non-ionic emulsifiers can work well. More on emulsifiers here.

Water Phase Thickeners

Natural gums (infographic here by skinchakra) swill work well but carbomer WILL NOT WORK as pH needs to be neutral in order for the gelling effect to take place.

Incompatibilities

Carbomer as mentioned above (it will not work due to the lower pH).

Niacimide needs a pH of 6 (AHAs are effective at lower pH – 3.5 to 4 as mentioned above). If you mix Niacimide in formulations that are acidic can cause hydrolysis and the appearance if nicotinic acid which is irritating and can also burn.

Willow Bark as An Alternative to Salicylic Acid

An alternative to salicylic acid is willow bark, also soluble in water (a source of natural salicylic acid-like ingredients without the side effects of irritability). As salicylic acid is not easy to work with when it comes to solubility willow bark can be a great alternative.

Combining Acids

Acids can be combined together (making sure we keep track of the amounts used). Glycolic acid is usually combined with lactic acid.

Vitamin C