Sweat electrolyte concentrations obtained from within occlusive coverings are falsely high because sweat itself leaches skin electrolytes.

D. B. DILL’S (12) 1930s observations of humans working in desert heat made him skeptical of the high sweat chloride concentrations reported in the literature. At fault, he surmised, might be methods of sweat collection that “restricted opportunity for evaporation” (p. 763). For this reason, he developed whole body washdown (WBW) methods, which became de rigueur in his, and later in D. L. Costill’s, work (7). In 1953, Kleeman et al. (20) pursued this issue by simultaneously determining sweat electrolyte concentrations in arm bags and as obtained via WBW. At their disposal was the recently developed flame photometer, which added the capability for measuring sodium and potassium concentration to that for chloride. Just as Dill had suspected, Kleeman et al. found that the arm bag method, in which evaporation was prevented, always gave significantly higher concentrations of all three ions than those obtained via WBW. Thinking along Dill’s thread, they suggested that wet skin alters the composition of sweat. In this note I hypothesize that aqueous solutions remaining on skin, including sweat, rapidly leach electrolytes, including sodium, potassium, and chloride, from its outermost layer, the stratum corneum (SC). Consequently, concentrations of sweat electrolytes obtained from within an occlusive covering are artifactually high. The fluid within the covering becomes something other than the sweat that emerged from the sweat duct and was deposited on the skin. It will therefore be referred to as “faux sweat.” Three lines of evidence support this leaching hypothesis: 1) anomalously high concentrations of potassium in small volumes of faux sweat (similar data exist for sodium); 2) a direct demonstration of the leaching of urocanic acid into aqueous solutions on the surface of skin; and 3) current models of SC behavior as a function of its water content.