Immediate contact reactions to benzoic acid and the sodium salt of pyrrolidone carboxylic acid. Comparison of various skin sites.

Benzoic acid (BA) and the sodium salt of pyrrolidone carboxylic acid (NaPCA) were tested in 13 healthy persons to obtain information about, firstly, the irritant properties of NaPCA and, secondly, the reactivity of various skin sites. BA at 16, 8, and 4 mM pet., and NaPCA at 50, 25, 12.5, and 6.25% in water, were applied to the forehead, cheek, neck and upper back. Erythema reactions were observed visually, and the changes in the skin blood flow were monitored using a laser-Doppler flowmeter. BA at 16 mM increased blood flow on the cheek of 12 test subjects, and on the neck, forehead and upper back of 6 subjects, but 8 and 4 mM BA elicited reactions only on the cheek. NaPCA caused reactions on the upper back of 3 test subjects only, but not on other test sites.