If you’re like me, you enjoy browsing Pinterest and looking up some DIY. You may see some DIY recipes for making your own facial washes, cleansers, etc., and many of these DIY recipes probably use household ingredients. Among these household ingredients, you’ll often see things such as baking soda, lemon juice, or apple cider vinegar in many of the recipes. However, if you look at the chemistry of your skin, you’ll find that many of these household ingredients really shouldn’t be used near your skin, especially on the more sensitive skin of your face.
Your skin actually has a barrier that protects the underlying tissue from infection, dehydration, chemicals, and mechanical stress. This layer is known as the stratum corneum or “Horny Layer”. Within this layer are dead cells, or corneocytes, that are embedded within a lipid matrix. These lipids are cholesterol, fatty acids, and ceramides.
The stratum corneum contains flattened dead cells that are embedded in a lipid layer.
You may also see the top layer of your skin referred to as the “Acid Mantle.” This is because your skin is actually acid with a surface pH value of 5.4-5.9 (remember that pH below 7 is acidic and pH above 7 is basic). The formation of the lipids in your stratum corneum is reliant upon pH-dependent enzymes. For example, β-Glucocerebrosidase is responsible for synthesizing important ceramides in the layer and is most active at a pH of 5.6.
Knowing that skin has a surface pH of about 5.5, how does this impact the products we use on our face? One important thing to consider is that lipid organization and metabolism is dependent on an acidic pH, and alterations in these functions can disturb the barrier function of the stratum corneum. So let’s say we use baking soda, or sodium bicarbonate, as an ingredient in our DIY face wash. Baking soda has a pKa of 10.329, so if you made a typical solution with baking soda, it would have a pH of around 10. Since this is well above the optimum pH level of about 5.5, the barrier function of the stratum corneum will be impaired and the skin will be more susceptible to irritation and infection.
Now let’s look at the other end of the spectrum. Consider using lemon juice, which is around 5-6% citric acid with a pH of about 2.2. While this pH is acidic just like the skin surface, it is much more acidic than the optimum skin pH. This can cause damage to the skin barrier just like substances with too basic a pH. An important thing to note is that pH is on a logarithmic scale, meaning that an increase by one pH unit is actually indicative of a tenfold change in acidity. Therefore, lemon juice is actually about 3 pH units or 1000 times more acidic than the skin surface. In fact, you are actually in danger of getting chemical burns from lemon juice.
The pH scale runs from 0 to 14 with 0 being the most acidic, 14 being the most basic and 7 being neutral. Note that a healthy skin surface falls at a pH of about 5.5, which is weakly acidic.
Now just because your skin has a pH of 5.5 doesn’t mean your products have to strictly be around that pH. A popular class of skincare ingredients called Alpha Hydroxy Acids (AHAs) need to be at an acidic pH in order to pass through the acidic stratum corneum and act on the living cells in the skin layers below. A common AHA, glycolic acid, has a pH of 3.83 and can help to smooth skin and lessen the appearance of fine lines. However, these ingredients need to be diluted in order to avoid damaging the skin barrier, and, if used too frequently, can cause skin irritation.
What all this information means is that skin pH matters, and you want to take care not to use too acidic or basic ingredients to avoid damaging your skin. So if you see a Pinterest DIY that suggests using baking soda or lemon juice on your skin, stay away — your skin will thank you.
For more information about pH and your skin, check out these peer-reviewed scientific papers:
pH Induced Alterations in Stratum Corneum Properties: K. P. Ananthapadmanabhan et. al. International Journal of Cosmetic Science. (2003)
The pH of the Skin Surface and Its Impact on the Barrier Function: M. H. Schmidt-Wendtner and H. C. Korting. Skin Pharmacology and Physiology (2006)