The Modern Skin Alphabet: pH, AHA & BHA
One of the most important factors in skin care is pH.
The so-called acid mantle of the skin is a protective mixture of sebum and sweat. This thin layer spread out over skin’s surface with its dead cells is a chemical barrier against microorganisms, toxins and water loss. It has components that are oily and watery, and it is acidic. Normal healthy skin will have pH between 4.5 and 5.5. There is a very simple reason for this acidity: most skin aggressors in the environment don’t thrive in those conditions. By being acidic, skin fends them off, neutralizes them or just keeps them at bay. Acidic works for the skin.
pH exists only when things are wet. Skin doesn’t appear to be wet normally, however its mantle is, just enough to produce that important pH value. With water being neutral at pH 7, compounds that can dissolve in water even slightly will and do alter the pH according to their chemical nature. The pH scale does not involve big numbers – it runs between 0 and 14. Hydrochloric acid in the gastric juice has pH of around 1, Lye or Sodium hydroxide is at 13. These are extremes and extremely dangerous. Closer to the middle, with pH 7 in the middle – water – is where chemicals are milder. Unit differences between pH 1 and pH 2 for instance are ten-fold, which means that this small scale represents huge differences in actual activity and concentration of ions.
Many factors can and do alter the pH value of the skin – the surface of the skin is a highly dynamic destination. Skin will recover its natural pH but if continually exposed to factors affecting its natural state, this will result with increased stress and threat to its health i.e. it can lead to infections, irritations and many other skin problems. That is why it is said that healthy skin has pH 4.5 – 5.5. Skin with higher pH is unhealthy. For instance, increased pH of the skin is considered one of the reasons why acne persists. (Acids will lower skin’s pH, but fall into an uncommon occurrence in daily life apart from acid-based personal care products).
Except for oils, every skin care product has its own pH. This opens up two perspectives: the stability and efficacy of the product depend on its pH; how the product’s pH affects the skin’s normal pH levels.
Let’s now review the pH in AHA and BHA products. Both of these groups belong to weak acids, although they can quite seriously burn the skin. Their ‘weakness’ has to do with how much they dissociate in water. Strong acids will dissociate almost completely; weak acids will not. In other words, a skin care product that has 5% acids will have only a portion of that 5% as dissociated or free acid. pH as well as acid dissociation constant, pKa, are factors that will determine the content of free or dissociated acid in a skin care product. Within the same product (pKa doesn’t change for the same acid), the content of free acid will vary at different pH values. Concentration will affect the content of free acid in obvious ways: 5% and 10%-product at the same pH are not going to have the same amount of acid, dissociated or not.
AHAs are water-soluble and BHAs are lipid-soluble (they also dissolve in alcohols). For this reason, BHAs are considered particularly suitable for acne and skin with clogged pores however they both work well in proper formulations.
AHAs present in skin care products are: glycolic, lactic, citric, malic and tartaric. Among them, glycolic and lactic are the most popular and best researched, although citric acid has had a long use as well. Out of BHAs, salicylic acid is the only one used in cosmetics. There are also PHAs – poly-hydroxy acids – but they have only recently appeared in the cosmetic market.
The main reason acid-based products are used in skin care relies on their fantastic ability to exfoliate the skin effectively. Why is exfoliation so important? Because it increases the turn-around of cells in the skin, prevents the buildup of dead cells that creates a dull look of the skin and with aging, the practices a person has had throughout her life – sun exposure in particular – will start showing through thickening and discolouration. With regular use, the skin starts to look smoother, even-toned and fresh.
AHAs and BHAs are not active only on the skin surface, especially the AHAs, but also penetrate the dermis, because of their small molecular size. The pH inside the body, the physiological pH, is not acidic any longer but slightly alkaline – pH 7.4 on average. To stimulate collagen production in the dermis, to increase the production of hyaluronic acid and ceramides – all activities that AHAs have shown results with – AHAs must work in an alkaline environment. What that means is in those conditions the acids are no longer free but bound.
Interactions on the skin
An acid-based product is produced with exact, invariant pH value. This is based on manufacturer’s research and expectations for the product and will vary. How the product gets used is just as important as proper formulation.
A typical skin care regimen will include a cleanser, toner, serum, moisturizer and eye cream. Every beauty routine begins with the cleanser. Most cleansers contain a surfactant system i.e. foaming agents, and for the amount and the stability of foam they produce, their pH is usually higher than that of the skin. A product labeled ‘pH balanced’ can be misleading. This is a popular marketing term which implies that great effort has been put in creating a certain pH for the product (presumably close to skin’s own pH), when in fact pH is tuned all the time depending on the components and purpose of the product.
Washed with a cleanser with higher pH, the skin will be left with its own pH raised. The following product will thus be applied to a skin with higher pH, which means that that product’s pH will also be affected. When it comes to AHA and BHA products, skin’s increased pH can affect their activity and efficacy. This is a chain of cause and reactions that follows the dynamic of a two-way street.
Let’s go back to the skin washed with a cleanser that raised its pH: when applying an AHA-based product to a skin in that condition, the pH of the product will also be raised.
Once applied, the product must be left on the skin to work. Why? If another product is applied on top immediately, with its own pH higher, this will further alter the pH of the acid-based product.
Let’s assume that instructions have clearly stated to apply the acid-based product and leave it on by itself for a little while. While acids are working on exfoliating the skin, what else might be taking place during that period? The pH is also rising. The skin, whether it was left after cleansing with its normal healthy pH or raised pH, will nonetheless have a higher pH than the AHA/BHA product, and after about 20-30 minutes, the product will be brought to the level of the skin. The skin being acidic – 4.5 to 5.5 – some amount of dissociated acid will still be present, but this is considered negligible.
AHA/BHA products are pH dependent. There are those who support very low pH for most effective results. They consider below pH 2 the optimum pH if the acids were to work. Weak acids or not, that is a low pH and skins vary i.e. not every skin will react positively to such acidity. It is generally accepted that the AHA/BHA products need to have pH below 4 in order to be effective.
The FDA has issued guidelines on Alpha Hydroxy Acids in products for home use:
1. pH has to be 3.5 or higher, and
2. Concentration of the acids in a product has to be 10% or less.
Consumers also have to be warned that these acids may cause photo-sensitivity in some individuals and if they use them, especially in the summer, they must wear sun block.
Acid-based products need optimal conditions for giving optimal results – clean skin with healthy pH and enough time to work. For this reason, cleansers with AHA/BHA ingredients have limited effect; they are washed off after a minute at the most. If the cleanser used has a higher pH than desired, a toner with a suitable pH value can bring the skin back to its normal level and improve the efficacy of acid-based products.
There is also research that indicates that higher pH doesn’t mean no efficacy at all or inferior activity. What that implies is that not only dissociated acid is the biologically active form but the efficacy of the acids relies on both free acid and non-dissociated forms for the complex results they produce. In this, the issue is the matter of philosophy and interpretation of scientific results. In no small way the type of skin you have also plays an important role. There are thicker, resilient skins that can withstand high concentration and low pH without negative side-effects, although most skins don’t fall into that category; furthermore, over time skin changes, therefore caution is always advised.
In conclusion, relations between the skin and topical products applied on its surface – skin care products, make-up – are highly dynamic and they depend on many biochemical and physical factors. Out of them pH is greatly important, but only one among many. Biologically active ingredients, such as AHAs and BHAs, commonly have highly complex ways in which they interact with the skin and affect its functions. Sometimes these mechanisms are well understood, oftentimes they aren’t. Achieving great results with skin care products is possible, and things that fulfill this goal are proper formulations and proper use. Having some understanding of what goes on your skin is very helpful and it does give you a sense of control over your own health and beauty.
To your health and beauty,
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