240 JOURNAL OF COSMETIC SCIENCE 180 (a) lOO 120 20 o -2o 60 min 0 25 S0 75 100 125 150 175 Distance (mm) (b) 800 700 ß 600, 500. 400, 300. 200 ß 100, 0 10 I 1/2" Diameter Iron 1" Diameter Iron 20 30 40 50 60 70 Time (min) Figure 10. (a) Combing curve differences for light-brown fibers monitored after 15, 30, 45, and 60 min of curling iron application (164øC). (b) Combing work difference for light-brown fibers as a function of thermal exposure time reported for two curling irons with different barrel diameters. CONCLUSIONS The thermal treatment of hair with curling irons, at temperatures ranging from 130øC to 170øC, was found to result in significant damage to a variety of fiber types, including white (completely unpigmented), Piedmont, Caucasian light-brown, and Asian hair. The extent of damage was quantified in terms of (a) hair chromophore decomposition determined by fluorescence spectroscopy and Hunter colorimetry, and (b) hair surface modification determined by combing measurements. Both Trp and its oxidation prod- ucts (kynurenines) were affected. As judged by a reduction of its emission intensity, Trp was found to undergo decomposition in all types of hair, with the first-order kinetic rates

EFFECT OF CURLING IRONS 241 250 200 150 i5 •oo E (,3 50 -50 135 ß I nterm ittent [] Continuous ! , , 145 155 165 175 185 Temperature (øC) Figure 11. Combing work difference as function of temperature for fibers subjected to 3 min of thermal treatment using intermittent (15-s intervals) and continuous modes of treatment. in the range of 0.03 to 0.12 (min -1) and an estimated activation energy of 6.6 kcal/mol. The products of Trp oxidation, which emit light in the visible range (400-500 nm), were found to undergo decomposition in unpigmented types of hair (white, Piedmont, and bleached). Thermal treatments were also found to produce discoloration of hair, especially unpigmented hair, which becomes yellow (white or Piedmont hair) or dark-