It is often claimed that Asian skin is different from Black or Caucasian skin. This claim is so persistent throughout Asia, particularly East Asia, that it leads one to think there must be a physiological basis for it. Reviews such as the keynote presentation by Tony Rawlings at the 2005 IFSCC Conference provide a good overview of potential differences, but before discussing these, it would be beneficial to compare human skin with animal skin first.
Pig (ear) skin is generally believed to be the best model for human skin since its skin penetration characteristics and many other physiological parameters are roughly the same, or at least not very different. Based on this work, the authors concluded that “the results obtained (on pig ear skin) are similar to those of human skin, indicating the suitability of this porcine tissue as a model for human skin.”
Of course, skin is more than a series of external observations. The skin lipid composition (qualitative and quantitative) and its packing are important for optimal barrier function. Early work by Bouwstra et al. has indicated that pig skin exhibits liquid and hexagonal packing, yet the orthorhombic stratum corneum (SC) lipid packing phase has been identified as being critical to human skin barrier function. Nevertheless the barrier function of the two has been described as quite similar. In a review, Barbero and Frasch found that when the in vitro permeability of pig and human skin were compared, the correlation coefficient r was 0.88 (p < 0.0001) for the 41 compounds studied. They also found that the coefficient of variation of skin permeability (i.e., standard deviation divided by the average) was 21% for pig and 35% for human skin.
Within human subjects, percutaneous absorption studies of different racial skin types have also been conducted. These studies require many subjects to be statistically significant due to the high coefficient of variation in the permeability of human skin. In addition, different chemicals penetrate skin via different routes. Therefore, another way to characterize skin barrier function is to measure transepidermal water loss (TEWL) but this is not exactly the same. Percutaneous absorption relates to the outside-in barrier and applies to all chemicals, whereas the TEWL relates to the inside-out barrier and applies only to water. A third approach is to examine differences in clinical effects of the same application among different races; for instance, after applying an irritant. In the present paper, the author addresses these three possible methods to identify the existence of racial influences on human barrier function.
Race and Percutaneous Absorption
The subject of race and percutaneous absorption was popular in the 1980s but few papers have since emerged in the scientific literature. It is also highly unusual that most investigations of this aspect of skin penetration were performed in vivo. The oldest reference relating to this subject is from December 1978, when the percutaneous penetration of tritium-labeled diflorasone diacetate was examined in three Black and three Caucasian subjects. In the end, no marked differences were found.