Measuring the Chemosensory Irritability of Human Skin

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Abstract
Contact with environmental chemicals can cause cutaneous sensations of irritation, such as burning, stinging, and itching, that are not always correlated with physical signs of irritation. This paper describes the development and evaluation of a psychophysical methodology that allows for measurement of this form of chemical irritation in humans. In the first experiment a labeled magnitude scale (LMS) was used to assess the chemosensory irritation produced when single concentrations of lactic acid, capsaicin, and ethanol were repeatedly applied to the skin of the face. The chemicals were applied on filter paper disks, occluded, and alternated at 3 min intervals with a water vehicle. The results revealed large differences in reactivity to the three chemicals within and between individuals. A second experiment assessed the reliability of the method and found good reproducibility in a subgroup of subjects from experiment 1. The final experiment tested a modification of the method that employed an ascending concentration series to estimate the minimal concentration necessary for detection of irritation. The results showed a wide range of sensitivities to ethanol and lactic acid, and indicated that for a given subject, sensitivity to one chemical did not always predict sensitivity to the other chemical. Overall, the data demonstrate that chemosensory irritation varies significantly across both individuals and chemicals, and that this variability can be quantified by combining rigorous methods of stimulus control with modern methods of psychophysical scaling.