SCALP HAIR LENGTH 375 Table VI Calculations From the Data on Hair Length in USA and Larger Populations* % Population (site) Hair length (cm) Number of persons (calculated from equations A and B)** 12.04 (shoulder) 34 1.88 (shoulder blade) 53.5 0.281 (waist) 73.5 1.78 x 10 -2 (buttocks) 103 8.45 x 10 -4 (knees) 134.5 3.3 x 10 -5 (ankles) 168.5 9.6 x 10 -6 183 1.5 x 10 -7 305 6.0 x 10 -s 488 26.6 million USA 118 million developed world 4.2 million USA 18.6 million developed world 620,000 USA 2.75 million developed world 39,300 USA 175,000 developed world 1,900 USA 8,400 developed world 73 USA 324 developed world 1,500 world 21 USA 93 developed world 400 world 0 USA 1 developed world 7 world 0 USA and developed world 3 world * Population of USA = 270 million, but since approximately 82 percent of the USA population (15) are age 12 and above, use 221 million as the adult population for the USA. Since approximately 75 percent of the world's population (10) are 12 and above, use 4.5 billion as the adult population of the world. ** Numbers rounded off, except where fewer than 100. (1 1,12) or at about 25 to 76 cm (10 to 30 inches) in hair length (assuming a growth rate of 12.7 cm (five inches) per year) (13,14). As suggested above, the three graphical points (Figure 3) from the literature, for the very longest hair, most likely do not contain all the people in the world with those lengths of hair, making the number of people and our calculated percentages low for the very longest hair (beyond 183 cm). The difference between this literature value at 183 cm (2.9 x 10-7%) versus the point obtained by extrapolation of the line in Figure 2 (96 x 10-7%) differs by a factor of about 33, further suggesting that this conclusion (percentages from these literature values are low) is true. In the graph represented by Figure 3, two straight lines are formed intersecting at about 183 cm. The two very different slopes suggest a different mechanism or explanation for maximum hair lengths from 34 to 183 cm versus 183 to 488 cm. The equation de- scribing the latter three points of this line from the literature data is: Y = - 14.266 - 0.00473X (B) The data point at 183-cm hair length (from the literature) is close enough graphically to the point from the USA hair study (by extrapolation from equation A) to suggest that at least on a relative basis our assumptions are reasonable approximations. Nevertheless, the point at 183-cm length provides very different values for calculating the percentage and numbers of people at 183-cm length, i.e., two people in the USA from equation B versus 21 people from equation A, differing by a factor of about 10. We believe that the higher value from equation A is closer to the actual value, and we use that value in Table VI. The region and slope from 35 cm to 183 cm (one to six feet) hair length appears to be governed by a "normal" anagen to catagen to telogen transition, suggesting a normal anagen period of up to as long as 12 to 14 years. This is a different view from the past belief of about a six-year anagen period approaching the "normal" upper limit. Anagen periods are frequently cited as two to six years and several weeks for catagen and telogen however, few have actually attempted to measure anagen time periods and only on small populations (11,12). In addition, longer anagen periods than six years have been cited (11). As indicated, the slope in the longer-hair-length region (183-to-488 cm) might suggest a different mechanism to control this extra-long anagen

376 JOURNAL OF COSMETIC SCIENCE Table VII Very Long Hair From the Literature Hair length No. of persons and source 488 cm -+ 91 cm (16' _+ 3') 305 cm + 91 cm (10' _+ 3') 183 cm _+ 30 cm (6' _+ 1') Three persons (4,5) Mata Jagdamba of India, 423 cm Hoo Sateow, 513 cm, and his brother, 493 cm (Thailand) Six persons Diane Witt USA (1988) 244 cm (8) (1992) 305 cm (8) (2000) 386 cm (8) Georgia Sebrantke, 301 cm (6) Wang Li Juan, China estimate from photo, 244 cm (6) Dyq, China estimate from photo, 274 cm (6) Lu Seng La, Thailand, 387 cm (6) Hiroko Yamazaki, Japan, 231 cm (6) Thirteen persons Jin Weiqun, China, 183-213 cm, estimate (6) Irena Godyn, USA, 193 cm (6) Cindy Christian, 188 cm (6) Tien Chun Mei, China, -183 cm, estimate (6) Xu Huiqin, China, 175 cm (6) Sabine, Germany, 173 cm (6) Darla, USA, 153-183 cm, estimate (6) Alia Bokareva, Russia, 165 cm (6) Dhorie Geronimo, 165 cm (6) Susanne Kalb, 165 cm (6) Yi Tao La, Thailand, 165 cm (7) Suzy, Florida, USA, 153 cm, estimate (6) Crystal Gayle, USA, 153 cm (6) period, but more likely it involves a defect in the mechanism to terminate anagen and to provide for the transition to catagen and telogen. If this same slope from equation A (35-to-183 cm) is extrapolated to represent one person in 4.5 billion (current "adult" world population), it provides a maximum hair length of about 247 cm. The simple fact that there are three people in the world today with hair nearly twice that length (see Table VII) confirms that a change in slope is necessary to account for these people with the abnormal hair growth condition causing scalp hair lengths greater than 183 cm. As indicated earlier, equation A permits one to estimate the percentage and numbers of people in the USA with hair of any length between 35 and 183 cm (see Table VI). For the developed world, the USA numbers were simply multiplied by 4.44, the factor between the USA population and that of the developed world (2,10). We estimate about 1,500 people in the world with hair down to their ankles, and about 324 people in the developed world with hair of that length, but only about 73 people in the USA with ankle-length hair. On the other hand, this equation suggests approximately 1,900 people in the USA have hair reaching to their knees, but more than 8,000 in the developed world and more than five times the developed-world number for the entire world. For estimating the numbers of people in the world with hair lengths longer than 183 cm