MEASURING THE CHEMOSENSORY IRRITABILITY OF HUMAN SKIN

Contact with environmental chemicals can cause cutaneous sensations of irritation, such as burning, stinging, and itching, that are not alwa ys correlated with physical signs of irritation. This paper describes the development and evaluation of a psychophysical methodology that al lows for measurement of this form of chemical irritation in humans. In the first experiment a labeled magnitude scale (LMS) was used to asse ss the chemosensory irritation produced when single concentrations of lactic acid, capsaicin, and ethanol were repeatedly applied to the ski n of the face. The chemicals were applied on filter paper disks, occlu ded, and alternated at 3 min intervals with a water vehicle. The resul ts revealed large differences in reactivity to the three chemicals wit hin and between individuals. A second experiment assessed the reliabil ity of the method and found good reproducibility in a subgroup of subj ects 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 aci d, and indicated that for a given subject, sensitivity to one chemical did not always predict sensitivity to the other chemical. Overall, th e data demonstrate that chemosensory irritation varies significantly a cross both individuals and chemicals, and that this variability can be quantified by combining rigorous methods of stimulus control with mod ern methods of psychophysical scaling.