Natural skin surface pH is on average below 5, which is beneficial for its resident flora

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 (N = 330), we have assessed the skin surface pH of the volar forearm before and after refraining from showering and cosmetic product application for 24 h. 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.

Validation of the VapoMeter, a closed unventilated chamber system to assess transepidermal water loss vs. the open chamber TewameterR

BACKGROUND/AIM

Transepidermal water loss (TEWL) is one of the most important biophysical parameters for evaluating the efficiency of the human skin water barrier. Different approaches exist to measure TEWL. The most commonly used methodology consists of the open chamber diffusion technique in which the water vapor pressure gradient is measured in g/h m2 according to Fick's law. A typical apparatus is the Tewameter. Recently, a portable device--the VapoMeter--became available with a humidity sensor in a closed chamber.

METHODS

In the present work, the closed chamber VapoMeter is compared with the open chamber Tewameter for its applicability to assess TEWL. A comparative study--including parallel in vivo measurements with both devices--was carried out on human forearm skin.

RESULTS

It could be concluded that both instruments are reliable tools. A good correlation between recordings (r=0.503-0.966) was found with a consistent feature of measuring higher TEWL values for the Tewameter than for the VapoMeter. Probe pressure, probe temperature and relative humidity were revealed to be important parameters inducing significant differences in data outcome.

CONCLUSIONS

From skin barrier damage experiments it became clear that the Tewameter is able to detect significantly smaller differences than the VapoMeter. In addition, the closed chamber device is currently not sensitive enough to discriminate for the effects induced by diurnal rhythm and fluctuations as a function of time. On the other hand, the small and handy VapoMeter allows more flexibility in measuring protocols and in in-use performance.