I've found such an assortment of really interesting papers on the pH of our skin, I had to share them with you! The writing in purple is the text from the study with my comments before and after.
What is the proper pH of our skin? As I've written in the past, the general range is considered around pH 5.4 to 5.9, but this is based on the surface of the lower arm of the Caucasian male. There are variations depending upon skin type, skin colour, gender, and so on.
This paper notes: A statistically significant difference in skin pH between men (mean pH= 5.80) and women (mean pH=5.54) was found, with women being more acidic than men (P<0.01)...Spontaneous skin surface pH was found to be significantly lower in women, as compared to men - albeit, the difference was small and of unknown relevance.
So women tend to be slightly more acidic than men, but it doesn't seem to be a big deal. Okay, great. Let's take a look at another study about the average pH of our skin!
Variable skin pH values are being reported in literature, all in the acidic range but with a broad range from pH 4.0 to 7.0. In a multicentre study, we have assessed the skin surface pH of the volar forearm before and after refraining from showering and cosmetic product application for 24h. The average pH dropped from 5.12 ± 0.56 to 4.93 ± 0.45. On the basis of this pH drop, it is estimated that the ‘natural’ skin surface pH is on average 4.7, i.e. below 5. This is in line with existing literature, where a relatively large number of reports (c. 50%) actually describes pH values below 5.0; this is in contrast to the general assumption, that skin surface pH is on average between 5.0 and 6.0. Not only prior use of cosmetic products, especially soaps, have profound influence on skin surface pH, but the use of plain tap water, in Europe with a pH value generally around 8.0, will increase skin pH up to 6 h after application before returning to its ‘natural’ value of on average below 5.0. It is demonstrated that skin with pH values below 5.0 is in a better condition than skin with pH values above 5.0, as shown by measuring the biophysical parameters of barrier function, moisturization and scaling. The effect of pH on adhesion of resident skin microflora was also assessed; an acid skin pH (4–4.5) keeps the resident bacterial flora attached to the skin, whereas an alkaline pH (8–9) promotes the dispersal from the skin.
(From this paper.)
The people studied were from The Netherlands: 167, Germany: 87, The Philippines: 40, Spain: 36. In my humble opinion, this isn't really enough of a cross section of the different skin types around the world to make a definitive statement. If you read further into the paper, you'll see they address this issue.
Their conclusion is that our skin is about pH 4.7, which is more acidic than we thought! The concept of skin having a pH of 5.4 to 5.9 seems to come from skin that has been exposed to skin care products.
So why do we care about the pH of our skin?
The surface of the skin is covered with a protective acidic hydrolipid film with pH 4.5-5.5. This is the emulsion of substances dissolved in water, composed of sebaceous and sweat gland secretion and products of decomposed corneocytes, whose bactericidal and fungicidal properties are based on mild acidity. Additionally, this emulsion prevents evaporation of water from the surface of the skin, which contributes to the maintenance of stratum corneum hydration...pH values of the skin surface significantly influence microflora and, by protecting from penetration of microorganisms into the body, also influence the skin barrier function. Increased pH values reduce the antibacterial and antimycotic properties of the skin surface, which enables more frequent occurrence of infections. (From this paper.)
In other words, if the pH of our skin gets higher than normal, the ability of our skin to protect us from bacteria and yeast will be reduced and we'll get more of those kinds of infections.
[S]ubjects with skin pH < 5.0 show statistically significant less scaling and higher hydration levels than subjects with skin pH > 5.0 (this paper).
In other words, people with less acidic skin tend towards what we would call dry skin with more scaling (skin flakes) and lower hydration levels.
It is interesting to mention in this respect that competent lipid barrier formation in neonatal skin and barrier repair of damaged skin are delayed at neutral pH conditions. Furthermore, regeneration of barrier function after damage with acetone or extensive tape stripping proceeds significantly faster when the skin is exposed to acidic pH (5.5) than neutral pH (c. 7.2), indicating that barrier formation and restoration (both in mice and in humans) is a process stimulated by low pH as well as a steep pH gradient. (From this paper)
In other words, acidic pH levels our skin good, neutral possibly not great, alkaline bad.
So why do we care about any of this? Well, that seems like a silly question considering we're making our products to help with things like dry skin, that feeling of tightness after washing one's hands, and other things of that nature. We can alter the pH of our skin by using various products...which sounds like an interesting post for tomorrow!