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 the 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 a 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
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 the skin’s pH, but fall into an uncommon
occurrence in daily life apart from acid-based personal care
Except for oils, every skin care product has its own
pH. This opens up two perspectives: the stability and
efficacy of the product depends 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
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
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 each 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 the 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
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
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 a 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
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
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.
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