The Effects of Aging on Hair-More Than Just Amount
The Effects of Aging on Hair-More Than Just Amount
1 Chapter 14 The Effects of Aging on Hair—More Than Just Amount M.J. Flagler, J.R. Schwartz, C.R. Robbins, and T.L. Dawson The Procter & Gamble Company Introduction Most literature on how hair changes with age focuses on hair loss or alopecia, or changes in the aging hair follicle, the site where the hair is produced. While these are important areas of investigation that provide crucial insight into the biological mechanisms for some of the fundamental changes in hair as we age, it is equally evident that the hair fibers that emerge from our scalp exhibit significant changes as we age that have a great impact on the overall cosmetic properties of the hair.
We have chosen to focus this chapter on the changes that occur to the actual hair fibers with age, as it is these changes that will ultimately be experienced by consumers as they age. Along with a critical review of the existing literature on hair aging, highlighting changes in hair pigmentation (graying), diameter, curvature, ellipticity, structural properties (stretching, bending, torsional rigidity), and lipid composition, we highlight the central knowledge gaps that need to be addressed for each of these parameters with age and provide both data and rationale for how changes to these hair properties will impact consumer perception of their hair as they age.
In the final section of this chapter, we speculate with regard to the interdependence of these changes on the overall cosmetic properties of consumers’ hair. More work is required to determine
The Effects of Aging on Hair–More Than Just Amount 2 the relative contributions of these changes in hair with age on overall hair assembly properties and consumer perception; however, we propose that diameter changes with age are likely to impart a large impact on overall perception of hair aging (described in detail in text). The hope is that a better understanding of how hair changes with age will enable the design of new cosmetic treatments that combat or reverse these effects. Fiber Properties—Part I: Color Graying of hair with age: For scalp hair, graying (canities) generally begins in the temple region and it spreads to the vertex or crown and finally to the remainder of the scalp, usually affecting the occipital region last.
Graying is an expression of a gradual decrease in the function of the melanocytes,1 the pigment producing cells located near each hair bulb in the lower region of the follicle. A relatively small number of melanocytes can produce an intensely pigmented hair fiber of 1 meter or longer. These melanocytes function in 7 to 15 different hair cycles to produce pigmented hairs for up to four decades or longer.2 Each group of melanocytes in each follicle functions independently of similar groupings in neighboring hairs. Graying results from a decrease and the eventual termination of the activity of the enzyme tyrosinase in the lower bulb.3 This enzyme is involved in the reaction called Raper’s scheme for the formation of 5,6-dihydroxyphenylalanine, which is transformed into its corresponding quinone starting with the amino acid tyrosine.4 Subsequently, this species reacts further to produce the hair pigments.
Kukita3 has shown that the onset of tyrosinase activity coincides with the appearance of melanocytes during anagen and its activity increases rapidly with increasing numbers of melanocytes. Tobin et al.5 have summarized the regulation of coat color in hairy mammals by POMC-derived peptides (α-melanocyte stimulating hormone, adrenocorticotropic hormone, and β-endorphin). The expression of these peptides and the melanocyte-1 receptor (MC-1R) are confined to specific regions of the hair follicle and are adjusted or controlled
Chapter 14 3 in their effectiveness during the hair growth cycle.
Furthermore, β-endorphin was shown in this study5 to play a role in the regulation of human hair pigmentation. Data from two large studies on graying: Hair graying is closely related to chronological age, and within the bounds of today’s technology, the age of its onset is largely controlled by genetics. A review of the hair graying process by Tobin and Paus2 states that the average age of onset of gray hair for Caucasians is in the mid-30s, while for Asians it is in the late 30s and for those of African descent it occurs in the mid-40s.
One large study involving the incidence of graying was conducted by Keogh and Walsh in 1965 in Australia.6 This study included subjects 25–60+ years of age. Because Keogh and Walsh found no significant difference in the graying of hair of males versus females, they combined the male and female data and separated it into 5-year age increments. This study contained a total of 8,720 persons, including 6,653 men and 2,067 women. This paper did not specify racial characteristics of the group; however, since the data is from Victoria, Australia, it may be assumed that the data largely or totally represents Caucasians.
The study included persons who had dyed their hair and those who had not dyed it. The authors stated that when the hair had been dyed or suspected of being dyed, they relied on the subject’s own statement regarding the true color and degree of grayness prior to dyeing. The authors indicated further that preliminary trials showed the subjects to be in substantial agreement with ratings by trained observers. For those subjects who had not dyed their hair, these scientists first experimented with standards of cut hair, then with photographs, and finally chose to rely on the trained observers’ judgments regarding only two categories of graying: any gray, and complete or total gray.
The effect of hair color on the perception of graying: The afore- described study6 did not provide any significant difference in either percentage (%) of “any gray” or % “complete gray” of males versus females. However, significant differences were found in the ability to see graying as a function of the three hair color types described in Table 1.
The Effects of Aging on Hair–More Than Just Amount 4 The data of Table 1 summarizes that of Keogh and Walsh, comparing % any gray and % complete gray versus the “true color of the hair” categorized as Fair, Medium or Dark.
The numbers of subjects are all relatively large, especially in the three lowest age groups which contain more than 300 subjects per group. The data of age groups 42.5 through 52.5 are all 205 subjects or larger; however, in age group 55+ the number of subjects drops to 137 or greater, while in age group 62.5 and above the number of subjects in each hair color group are only 26 or fewer. Therefore, the data of this oldest age group are the least reliable. Table 1. Age and graying of hair from the work of Keogh and Walsh6 % Any Gray % Completely Gray Age Fair Medium Dark Age Fair Medium Dark 27.5 3.4 9.4 27.7 27.5 0.4 0 0.0 32.5 19.4 34.0 47.4 32.5 0.6 0 0.5 37.5 39.2 54.1 63.9 37.5 0.9 2.2 0.54 42.5 60.1 70.0 77.3 42.5 4.6 3.3 2.6 47.5 79.1 82.1 87.2 47.5 8.9 7.0 7.4 52.5 93.6 90.7 94.1 52.5 23.9 17.4 13.8 57.5 100 96.1 97.7 57.5 42.3 30.1 18.6 62.5 97.8 98.6 98.2 62.5 50.0 23.3 16.8 Model equations using JMP statistical software were used to calculate the above values from the actual data of Keogh and Walsh.
In general, the calculated values are within ± 2% of the Keogh and Walsh values. All three % “any gray” models contain R2 = 0.99 or greater and p < 0.0001. For % “completely gray,” higher order polynomial models were used, but were not effective for extrapolating to zero gray. An effective model could not be constructed for fair hair, therefore the Keogh and Walsh data are listed above for that group; for percentage of “completely gray” for both medium and dark hair: R2 = 0.99 or better and p < 0.02 or better. These authors analyzed the data by logistic regression analysis and concluded that at approximately age 49 (48.6) about 50% of this population has 50% gray hair.
This statement has been widely misquoted to apply to all populations. It would appear that age 49 for 50% gray hair should apply to Caucasian populations, but, if we accept the conclusion of Tobin and Paus2 about graying in other
Chapter 14 5 geo-racial groups, we would expect it to be about five years later for Asians and 10 years later for Africans. (The term geo-racial is used instead of ethnic in this chapter because the genetic traits of these three groups was very much influenced by geography as two of the three names indicate.) The age that graying begins: Keogh and Walsh pointed out that the perception of graying for the percentage of “any gray” hair is higher for those with darkest hair and lowest for those with the lightest colored hair, as shown in Table 1. Chi Square analysis shows significant differences between fair and medium-to-dark haired groups.
To test at what ages the detection of graying begins, we examined the Keogh and Walsh data for the three classes of hair color because the data collection started at age 25, making the data acceptable for study. The models summarized in Table 1 were mathematically extrapolated to % “any gray” to determine the age at which graying begins for the three different hair color groups. This analysis suggests that the detection of any gray hair for the dark-haired persons begins at about age 21–22; for medium-color-haired persons it begins at about age 25; and for the fair-haired persons it begins at about age 26.
There is also evidence in the literature that graying occurs at ages earlier than those suggested.7 This “premature graying” has been defined in Caucasians as the onset of graying before 20 years of age,7 before 30 years of age in people of African descent,2,8 and before 25 years of age in Asians.2,7 This definition agrees well with our statistical models for the beginning of graying in Caucasians. The conditions of premature graying are generally related to diseases such as pernicious anemia, hyperthyroidism, and certain autoimmune diseases or even premature cardiovascular disease.8 In contrast to slight or moderate graying, complete graying appears to occur earlier in the fair-haired group than in the other two groups, especially after age 40.
The question then becomes, does this effect arise because light hair will appear to be more completely gray before dark hair, because dark hairs will stand out more readily
The Effects of Aging on Hair–More Than Just Amount 6 against a light background of gray hair than light hairs against a light gray background, or does this effect involve the cessation of production of pheomelanin (reddish-blonde pigment) versus eumelanin (brown-black pigment) in the melanocytes? We believe this effect is more likely due to perception, or the former effect rather than the latter. A second large study including graying of hair: The second large study on graying with respect to age is the Copenhagen city heart study by Schnohr et al.9 The authors of this study do not specify racial characteristics of their subjects; however, because of the location, it may be assumed that the subjects are largely or totally Caucasian.
One advantage to the Copenhagen study is that it contained large numbers (13,000 total subjects; 5,837 men and 7,163 women). The disadvantages of this study are that the age groups were in 10-year increments and did not include any subjects less than 30 years of age. In addition, the authors separated out those subjects who dyed their hair but did not determine the grayness of hair on those subjects. The Copenhagen study separated all subjects into those with no gray hair, those with little gray hair, moderate gray hair, and total gray/white hair, and also those with dyed hair and those with wigs.
To consider all of the data, we assumed that the percentage of gray-haired subjects in the dyed hair group was the same as in the non-dyed hair group and added the two together. With this assumption and combining the data of men and women, the data of % any gray and % total or complete gray correspond well with the data of Keogh and Walsh. The differences were generally within ± 2%. These data are summarized in Table 2 in terms of the percentages of little gray, moderate gray, and total gray. Best estimates on % little gray, % moderate gray, and % total gray in 5-year age increments: To see if the data from these two large studies could be combined and to look more carefully at graying in 5-year increments, in effort to derive more meaningful results, we examined the data of these two studies in tandem.
Plots were made
Chapter 14 7 of the mid-point age of each age group versus % any gray and of % total or complete gray. As before, these plots are clearly not linear, but display a distinct curvature. Data from all three hair color types of the Keogh and Walsh study were combined and the data of men and women in the Copenhagen study were combined. The Keogh and Walsh study went down to age 25, while the Copenhagen study stopped at age 35 (30 to 39). On the other hand, the Keogh and Walsh study used few subjects at age 55 and above, whereas the Copenhagen study used many more subjects in the higher age groups.
So, where possible, the Keogh and Walsh data were used for ages 20–30 and the Copenhagen study for ages 60–70. Means of both studies were averaged for ages 35–55. Table 2 summarizes the analysis. Table 2. Percent little gray, % moderate gray, and % total gray at different ages for female Caucasians, calculated from data of Keogh & Walsh6 and Schnohr et al.9 Age % Little Gray % Moderate Gray % Total Gray(Eqn) % Any Gray(Eqn) 20 0 0 0 0 25 6.3 0* 0 6.3 30 22.5 0.6* 0 23.1 35 43.5 3.7 0.5 42.8 40 52.3 9.3 2.7 61.4 45 54.7 16.4 6.0 76.2 50 52.2 22.7 10.3 86.0 55 46.2 30.1 15.7 90.9 60 38.0 33.7 22.2 92.4 65 29.1 36.8 29.8 93.3 70 20.8 37.5 38.5 98.0 % Little Gray is from a cubic equation for the Schnohr data of ages 35–70 and for ages 20–30 by subtraction.
For the cubic model R2 = 0.99, p = 0.0015 and the root mean square error = 2.157; % Moderate Gray was obtained by subtraction of % Total Gray + % Little Gray from % Any Gray; *this data point was from linear regression analysis and extrapolation from ages 35–70; % Total Gray is from a quadratic model of data combined from the Keogh and Walsh and Copenhagen studies where R2 = 0.994, p < 0.0001 and root mean square error is 1.167; % Any Gray is from a quadratic model from data of Ke- ogh and Walsh and the Copenhagen studies combined where R2 = 0.996 and p < 0.0001 and the root mean square error is 2.8799.
The Effects of Aging on Hair–More Than Just Amount 8 No comparable data on graying of hair versus age based on large numbers of Asians or Africans could be found in the literature. Therefore, assuming that the conclusions of Tobin and Paus are correct, that graying begins for Asians and Africans about 5 years and 10 years, respectively, later than for Caucasians and that once graying begins the rate of graying is the same for all three of these geo-racial groups, then one can approximate the incidence of graying versus age for Asians and Africans by examining the data of Table 2 and moving each graph point or data point back 5 years for Asians and 10 years for Africans.
Until sufficient data can be obtained for large numbers of people of Asian and African descent, these approximations should be useful.
Conflicting literature on interdependence of graying and hair curvature, diameter: There is conflicting literature as to whether gray hairs are coarser or finer than highly pigmented hairs. For example, Hollfelder et al.10 provided evidence from five Caucasians that gray hairs on the same person are coarser and wavier than highly pigmented hairs. This observation by Hollfelder et al. is consistent with observations by Yin et al.11 that fine Caucasian hair is straighter than coarse Caucasian hair. Similarly, Van Neste examined 60 hairs from each of three different scalp sites on 24 women, and a global comparison of all hairs (more than 3,300) showed that the average diameter of non-pigmented hairs exceed that of pigmented hairs by approximately 10 µm.63 Furthermore, both Van Neste and Hollfelder reported a more prominent medulla in non-pigmented compared to pigmented hairs.10,63 If this is the case, then the more prominent medulla would likely provide the appearance of whiter or grayer hair by scattering light and changing the refractive index at the hair-to-air interfaces of the medullary pores.
However, Gao and Bedell,12 studying gray hair and dark hairs from four persons plus one sample of pooled gray hair, measured cross-sectional parameters with a laser-scanning micrometer and found no significant differences in the maximum center diameter, center ellipticity, or cross-sectional areas; however,
Chapter 14 9 the center minimum diameter of the dark fibers were slightly larger than those of gray hairs. So whether gray hairs are coarser than highly pigmented hairs remains in question and needs further investigation. Knowledge gaps for hair graying and age: To date, we have not been able to find any literature on graying and scalp hair density versus age, therefore further investigation will be required to determine whether these factors are related. As stated above, there is conflicting evidence in the literature as to whether gray hairs are coarser or finer than highly pigmented hairs, and additional large studies are required to resolve this debate.
Another important knowledge gap for hair graying and age is the paucity of data for Asian and African populations, which precludes a direct comparison of the rate of graying versus age in these geo-ethnic groups with that of Caucasians.
Fiber Properties—Part II: Non-color Scalp hair diameter versus age: Women, men and ethnicity: Infancy through childhood for males and females: Trotter and Duggins,13 in their study of scalp hair from infancy through childhood, reported cross-sectional areas on the same 14 Caucasian subjects from 1 month through 10 years of age. We calculated mean fiber diameters from this study and arrived at data summarized in Table 3. We also analyzed this same study’s data by the matched pairs test showing highly significant differences at 1 month, 7 months, 2 years, and 3 years from all other ages. Several of the other pairs were not significantly different as indicated by the connecting lines in this table.
Clearly, the diameters are smallest at 1 month. They are also smaller at 7 months, 2 years, and 3 years compared with all other ages. Larger changes in diameter occur between 1–7 months and 7 months and 2 years compared to the total change that occurs between 4–7 years of age. These dimensional changes in the hair fiber correspond to the generalized description by Furdon and Clark14 that the fine hair of infants tends to be lost by about the
The Effects of Aging on Hair–More Than Just Amount 10 seventh month and is replaced by a coarser and longer hair which is generally replaced by an even coarser hair at about 2–3 years of age. But the data also suggests that the hair becomes coarser as the child continues to age. Table 3. Cross-sectional area and the age of Caucasian children from Trotter13 Age Statistics Calculated Average Diameter (µm)* 1 month ** 31 7 months ** 35 2 years ** 50 3 ** 55 4 59 5 60 6 63 7 64 8 65 9 66 10 66 *Diameters calculated assuming circularity; **Significantly different from all other values. Lines indicate those values that are not significantly different from each other by the matched pairs test.
Puberty through Adulthood for Females: The study by Otsuka and Nemoto15 on the hair of approximately 18,000 Japanese females ages 10–60 is the largest published study containing data on hair diameter and age. However, this study does not provide any experimental details, including the method for collecting hair samples, the site from which hairs were taken, nor the method of measurement. For women, this study shows that hair diameter versus age is not a linear relationship, but rather that it displays curvature increasing to a maximum near the age of 40 and then it decreases thereafter, as shown in Table 4.
Chapter 14 11 Table 4. Hair Fiber Diameters versus Age for Japanese men and women15 * Predicted Diameter(µm) Calculated Area of Cross-section (µm2) Age Men Women Men Women 15 84 79 5542 4902 20 84 80 5542 5027 25 83 82 5411 5281 30 81 82 5153 5281 35 79 82 4902 5281 40 76 82 4536 5281 45 72 81 4072 5153 50 68 78 3632 4778 55 63 75 3117 4418 *Data points were read from a graph in this paper by Otsuka and Nemoto and predic- tion equations calculated. The data points for diameters of this table are from the pre- diction equations which were all within ± 1% of the graph data points, which were then rounded off to the nearest micrometer.
Cross-sectional areas were calculated from the predicted diameters assuming circularity and average diameters. The second largest study on age versus diameter was by Robbins and Dawson et al.16 who measured average optical fiber diameter on 250–400 hairs in each of two sites in the parietal region of 1,099 Caucasian females ages 18–66 with self-perceived hair loss. The age for maximum diameter for these Caucasian females was calculated from both quadratic and cubic models to be 43–46 years of age, as seen in Figure 1. This peak in fiber diameter with age is in reasonable agreement with the study by Otsuka and Nemoto.
Several smaller studies on age versus fiber diameter were conducted by Tajima et al.17 , Nagase et al.18 , Trotter and Dawson19,20 , Ebling21 , Jackson et al.22 , and Birch et al.23 The data by Tajima et al. examined hair 40 mm from the coronal midline on the scalp to the ear. When modeled by a cubic equation, this study shows the maximum diameter for 113 Japanese females, ages 14–68, with little to no hair loss, to be in the low-forties (age 43). A graph in the Tajima paper shows the maximum diameter for these 113 women
The Effects of Aging on Hair–More Than Just Amount 12 to peak in the forties, but for 46 women, ages 38–68, who displayed hair loss, the peak was in the fifties. The study by Nagase et al. on 132 Japanese females, ages 10–70, shows the minor axis diameter of hair fibers taken from the “top of the head” to peak near age 40. Trotter and Dawson conducted two studies on hair fiber diameter versus age.19, 20 One study19 on hair taken from the vertex of 132 French-Canadian females, ages 0–89, shows an increase in mean fiber diameter in those in their teens through to those in their late 30s (from a quadratic model of their data on hair diameters) and a decrease thereafter.
The second study20 was on hairs taken from the vertex of 211 American females. These females showed an increase in fiber diameter from childhood up through the early 40s (from a quadratic model of their data on linear densities). A study by Jackson, Church and Ebling21 used the same data in a publication by Ebling.22 Hair fiber diameter was measured on 20 hairs plucked from the vertex from 125 female Caucasians, Figure 1.
Chapter 14 13 ages 13–72. The data was separated into three groups: those with diffuse thinning (n = 40), those with hypothyroidism (n = 14), and a “normal” group (n = 71). For each group the data was analyzed by linear regression analysis, despite the fact that graphs of the data appeared to display curvature with the highest data points in the 30s and 40s, and the data for the hypothyroid and normal group failed to reach significance. Therefore, no conclusion can be drawn with respect to the peak age for maximum diameter from this study. The first four smaller studies17-20 are in reasonable agreement with the conclusions from the two larger ones, indicating that the age for maximum hair diameter for females is near the age of 40.
One exception is the study by Birch et al.23 on more than 300 Caucasian females, providing the conclusion that the age for maximum diameter was ~ 30.
Mirmirani and Dawson et al.24 have shown that post-menopausal women have significantly lower hair fiber diameters, lower frontal scalp hair density, and lower growth rates than pre-menopausal women. Optical fiber diameter and phototrichogram methods were used to quantitate these hair parameters. Two studies were conducted by these scientists. An initial study included 44 women, 20 in the post-menopausal group and 24 in the pre-menopausal group. The second study included 177 women (ages 40–60) with 54 in the pre-menopausal, 33 in a peri-menopausal group (irregular periods or cessation of periods for less than 12 months), and 90 in the post-menopausal group.
Average fiber diameters were significantly higher in pre-menopausal versus post-menopausal women in the frontal site, but not in the occipital site. The data also suggested that this fiber diameter effect was independent of age. In the expanded study by Mirmirani and Dawson, average fiber diameters on the frontal site were significantly higher in pre- menopausal versus post-menopausal and peri-menopausal women. However, there was no significant difference in hair fiber diameters in peri-menopausal and post-menopausal women.
In the Robbins et al. study,16 the authors measured mean hair fiber diameters on 250–400 hairs from each of left and right
The Effects of Aging on Hair–More Than Just Amount 14 parietal sites on more than 1,000 female Caucasians, ages 18–66. In addition, they measured hair densities on analogous sites on these same women. They concluded that hair diameter in the parietal region increases until approximately 45 years of age and decreases with increasing age thereafter. The age for maximum diameter is clearly higher than for maximum density, and menopause with its physiological changes, including estrogen changes, plays a more important role in hair diameter.
On the other hand, the study by Otsuka and Nemoto15 indicated maximum hair diameter at around age 40. Since this study was done on more than 18,000 Japanese females, ages 10–60, it has to be considered relevant despite the fact that no additional experimental details were provided in that paper. If the menopause and its consequent physiologic changes are involved in the age for maximum diameter and subsequent changes, we would expect the peak to be closer to age 45 than age 40. But the difference between these two studies might also be explained by a geo-racial or population effect. Additional studies will be required to resolve this issue, however, since the median age for menopause occurs at approximately age 50 in women of most industrialized countries, including Japan and the United States,17 we lean toward the effects of menopause being critically involved and therefore suggest that the age range for maximum diameter for both populations is likely to be closer to the mid-forties.
Puberty through adulthood for men: For Japanese males, the study by Otsuka and Nemoto shows that scalp hair fiber diameter increases to a maximum in the late teenage years and then decreases relatively rapidly with increasing age. This study suggests a larger effect of age on hair fiber diameter for men than for women (Table 4). The study by Trotter and Dawson on only 82 male French Canadians shows a similar effect qualitatively: a peak in diameter in the late teens for men and a decline after that.19 The work of Courtois et al.25 on the same 10 French male subjects over a period of several years provides support for a decrease in hair diameter of male Caucasians ranging in age from the mid
Chapter 14 15 20s to the late 40s. Courtois et al. studied 10 Caucasian adult male subjects (ages 25–49) by making observations periodically over a 14-year period. These scientists demonstrated that the diameter of hair shafts decreased with increasing age beginning at age 25. Correspondingly, a reduction in the duration of the growth period also occurred. In addition, the time interval separating the loss of a hair in telogen and when a replacement hair appeared in anagen also increased. This study shows that hairs on the same male Caucasian after age 25 become finer during the next 14 years in agreement with the conclusions of Otsuka and Nemoto15 on age and fiber diameter on different Japanese males.
This effort by Courtois et al. on adult Caucasian males also supports our conclusions on the effects of age on the diameter of scalp hair of male Caucasians for the French Canadian data of Trotter and Dawson. Table 5 summarizes calculated rates of diameter decrease per year over 10-year periods from the data of Otsuka and Nemoto on Japanese men’s and women’s hair. This data clearly shows the effects of aging in a practical way. However, it is unfortunate that we have not been able to find larger studies on Caucasian and African men analogous to Otsuka and Nemoto’s on Japanese hair, so that we could be more quantitative in our conclusions on the effects of age on the scalp hair of men.
Table 5. Instantaneous rates for estimation of diameter changes per year at 10-year intervals Age Males15 Females15 Females16 35 -0.53 -0.02 +0.20 45 -0.80 -0.35 -0.043 55 -1.07 -0.80 -0.29 The instantaneous rates for Robbins et al. data16 were calculated from the first derivative of the quadratic model equation by substituting the mean ages 35, 45, and 55. The quadratic model was also used for the male data for the Japanese study15 while a cubic model was used for the females of the Japanese study, once again using the first derivatives of the model equations to calculate the instantaneous rates.
The Effects of Aging on Hair–More Than Just Amount 16 Knowledge gaps for diameter and age: From existing data on hair diameter and age, it would appear that the age for maximum diameter for females in the parietal region of the scalp is in the mid-forties. A more accurate determination of the true average peak age for female hair diameter awaits larger studies; however, equally important are questions about age and scalp site variation and whether there are geo-ethnic differences, because there is no data for diameter versus age for those of African descent. Determination of the age for maximum diameter for males in all scalp sites also requires additional investigation, as there are no large studies of this type from infancy to advanced age.
In addition, the rate of diameter decrease for women and men with advanced age requires additional large studies to more accurately elucidate. The biological causes of the peak age for scalp hair fiber diameter also remain an important knowledge gap in our understanding of the mechanisms underlying changes in hair fiber diameter with age.
Hair fiber curvature and age: Nagase et al.18 studied hair curvature from the hair of 132 Japanese females, ages 10 to 70, and reported an increase in curvature with age. In this paper, the curl radius was measured and regressed against age, providing a statistically significant increase in hair fiber curvature with increasing age. The index of determination (r2 ) for the linear regression was 0.185, indicating that about 19% of the total variation in hair curvature among Japanese women’s hair within the studied age range can be explained by age. The curl radius values for the total subjects fell from approximately 10 cm to 1.25 cm, while the curl radius values on the regression line of curvature versus age fell from about 4.5 to 2.9.
Nagase et al., in this same paper, also measured hair luster using both a contrast ratio for luster and a sensory method. Both measures showed a decrease in luster with increasing age. The regression line for the effect of hair luster and age among Japanese women shows that age accounts for about 18% of the total variation in luster among these 132 women (r2 = 0.18). These scientists also plotted hair luster versus the curl radius and found a significant linear regression
Chapter 14 17 (r2 = 0.18). Therefore, for Japanese women there is an increase in hair fiber curvature with increasing age which has a negative effect on hair luster, especially at advanced age.
In a different publication by these same authors,26 frizziness was explained as a lack of synchronization in the curvature of neighboring hair fibers in an assembly of hair. Therefore, this increase in hair fiber curvature with age could create the appearance of frizziness with increasing age, which should be explored. We would expect a similar effect of age on hair fiber curvature among Caucasian women and men; however, this hypothesis awaits empirical determination. Also, the effect of age on hair fiber curvature among those of African descent requires additional study. In addition, the effects of a loss in hair luster and an increase in frizziness with increasing age remain to be examined among Caucasian and African groups.
We know that hair curvature has an exceedingly important effect on almost every important cosmetic hair property; therefore, we believe that the effects of hair curvature versus age and all cosmetic hair assembly properties is a major gap for future study.
Ellipticity and age: We have located five studies of hair fiber ellipticity versus age.13, 18-20, 26 Three of these papers are relatively large studies. In the earliest paper, Trotter20 took hair fibers from the vertex of 340 Caucasian (American) males and females at different ages, measured the maximum and minimum diameters, and calculated both cross-sectional sizes and ellipticity. Trotter initially separated the groups into males versus females and measured ellipticity for age groups set at every 10 years from age 0 to 79. One concern with the data of that study was the small and varied sample sizes (only 10 hairs per subject, and the number of subjects per age group varied from as low as 1 to as high as 45).
Trotter next examined hair from the vertex of 300 French Canadians measuring maximum and minimum diameters and reported hair shaft indices (Dmin /Dmax x 100) and cross-sectional areas by age group separating the data into hair of males and females.19 Table 6 summarizes the data from both Trotter studies.
The Effects of Aging on Hair–More Than Just Amount 18 Table 6. Hair ellipticity by age for two groups of Caucasians19, 20 French Canadians American Caucasians Age Group Ellipticity Ellipticity 0 to 9 1.36 1.34 10 to 19 1.37 1.36 20 to 29 1.39 1.32 30 to 49 1.38 1.34 50 to 89 1.40 1.33 0 to 89 1.38 300 = N 1.34 340 = N The solid line indicates that those ellipticities are not significantly different for those groups. The data from these two studies were analyzed by ANOVA and by the matched pairs test. Both statistical tests show that ellipticity is different between French Canadians and American Caucasians, but there is no significant difference between ellipticity of the different age groups.
Even if there were differences among the age groups, the difference would be relatively small, since both groups varied by less than ± 2% among age groups for each group of Caucasians. Another study on ellipticity versus age over a similar age range is shown in the aforementioned study by Nagase et al. on Japanese hair taken from the “top of the head.”18 This paper described ellipticity in terms of the ratio of maximum to minimum diameters among 132 Japanese females, ages 10 to 70. The authors found no statistically significant effect of ellipticity with age over this age range. The average ellipticity was 1.28 and the r2 = 0.0001 showing that the variation in ellipticity by age among Japanese women was neither meaningful nor significant.
This study suggests there is no effect of age on hair fiber ellipticity for Japanese females between the ages of 10 and 70. Another paper by Nagase et al.26 reported ellipticity determinations on what can be presumed to be the same study data: 132 Japanese females, ages 10 to 70. Although the average ellipticity over the total group was 1.28 ± 0.15, the total variation among the 8,926 individual hair fibers was 1.02–2.19, testifying to the necessity for measuring a large number of hairs to obtain meaningful data for ellipticity for individuals or for groups of subjects.
Chapter 14 19 Trotter and Duggins13 ran a sensitive study among Caucasian children by having hair sent to them periodically at 1-year intervals starting with infants through puberty.
This study was discontinued after 17 years because of dropouts. These scientists started with 15 infants each at 1 month (50 hairs) and 7 months (50 hairs) and summed these two data points to represent 100 hairs at age 1 (closer to one-half year). Then they measured 100 hairs taken from the vertex of each of these same 15 subjects at 2 years of age, with one additional child at age 2, and continued with these 16 children once per year until age 17. However, due to the high dropout rate beyond age 10 (only 10 children remained in the study), we have reservations about the conclusions that can be drawn beyond this threshold.
This data up to 10 years of age are summarized in Table 7. Table 7. Average indices of 100 hairs at ~ 1-year intervals on the same 14 subjects for the first ten years of life (letters corresponding to respective subjects)13 Age A B C D E F G H I J K L M N 1** 74 66 77 74 84 75 85 80 77 77 70 70 84 76 2 61 62 71 65 73 66 76 79 71 70 65 66 73 73 3 64 65 67 65 76 66 82 77 74 72 63 63 69 72 4 70 67 70 60 76 69 78 77 73 76 63 63 78 73 5 70 67 69 65 73 70 81 74 75 77 58 59 80 72 6 71 67 75 64 75 70 81 75 82 74 59 63 79 74 7 68 63 74 62 75 70 85 79 85 80 62 62 78 80 8 68 62 74 65 79 66 80 81 85 73 63 63 83 81 9 67 66 74 63 81 67 80 81 84 77 61 67 83 73 10 67 61 75 59 78 71 80 81 83 81 58 61 84 73 **One year measurements consist of 50 hairs taken at 1 month and another 50 at 7 months from each subject.
These two measurements were averaged together. One year represents the hair closer to 4 months than to a full year. All other measurements were taken on 100 hairs per subject at the designated age. Trotter listed more subjects who dropped out at different times and have been deleted from this table. Not counting the 1 month and 7 month sampling, Trotter and Duggins had collected yearly data points measuring 100 hairs from each of 14 subjects from age “1” through age 10, commenting on
The Effects of Aging on Hair–More Than Just Amount 20 the small difference found between males and females. However, since so few males and females were used in this study (6 males; 8 females), even if there is a difference the sample size is so small that one cannot conclude that a difference exists between the ellipticity of the hair of males versus females. We analyzed the data differently from Trotter and Duggins by not combining the 1 month and 7 month data and we used the Wilcoxon signed rank non-parametric test for paired observations. The results, summarized in Table 8, show that the largest change occurs after 1 month when the hair is more circular in the earliest stage of infancy.
The 1 month data is significantly different from all other ages, including the 7 month measurement (P < 0.0001). It would also appear that another change occurs a few years later near the 5–6-year age range.
Table 8. Means of the data representing the effects of age on the hair fiber ellipticity from approximately 1 month through age 10 for the same 14 Caucasian children Age Statistics Maximum/Minimum Diameter (Ellipticity) 1 month ** 1.26 2 years 1.44 3 years 1.44 4 years 1.41 5 years 1.41 6 years 1.39 7 years 1.37 8 years 1.37 9 years 1.37 10 years 1.38 7 months 1.36 Data were evaluated by the Wilcoxon signed rank test for paired observations, ** indi- cates the values that are significantly different from all others. The black lines indicate values not significantly different from each other.
Knowledge gaps for hair fiber ellipticity and age: Although reported mean ellipticity values for African hair types are clearly higher than mean values for Asian and Caucasian hair, we could
Chapter 14 21 not find data for ellipticity versus age for African type hair. Current studies show no effects of age on ellipticity of the scalp hair for Japanese females, ages 10 to 70, and no effects for Caucasian males and females over a similar age range; however, we would not speculate on what to find among those of African descent because of the very high ellipticity among that geo-ethnic group. Stretching, bending and torsional properties of hair fibers and age: Equations for the moduli for elastic stretching, bending, and torsional shear all suggest that the stresses to deform human hair fibers increase with hair fiber diameter: • Es = H g L /A ΔL where Es is the elastic modulus for stretching; H is the Hookean slope; g is the gravitational constant; A is the fiber cross-sectional area (increases with fiber diameter); and ΔL isthe distance the fiber is stretched; • Eb = 64/ ∏ D4 where Eb is the bending modulus and D is the fiber diameter; and • Et = 128 ∏ I L/ P2 D4 where Et is the torsional modulus; I is the moment of inertia of the pendulum; L is the fiber length; P is the period of oscillation of the pendulum; and D is the fiber diameter.
Robbins and Scott27 have shown that the Hookean slope and the dry breaking stress determined by tensile elongation, and the bending stiffness index28 are proportional to hair fiber diameter. Also, tensile properties have been shown by Robbins and Crawford29 to be controlled primarily by the cortex, and thus the outer layers of the fiber do not provide a disproportionate contribution to the stretching and bending properties of the fiber as they do with torsion.30 Therefore, changes in hair fiber diameter will provide changes in tensile, bending, and torsional resistance or stiffness of hairs as diameter changes with age.
For Caucasian and Asian women who do not damage their hair, tensile stresses and bending stiffness will on average increase with age through the late 30s, but for men the available data on hair diameter versus age suggests that the peak in
The Effects of Aging on Hair–More Than Just Amount 22 these mechanical properties should occur near the late teens. Then with additional increase in age, these mechanical properties of hair fibers should decrease. Torsional rigidity will be somewhat different, as Persaud and Kamath30 have shown that the cuticle in the outer fiber layers plays a more significant role in torsional rigidity than in stretching behavior. Thus, Persaud and Kamath have shown that the shear modulus decreases with hair fiber diameter because of the higher ratio of cuticle to cortex with decreasing fiber diameter. Therefore torsional rigidity will decrease for the scalp hair of Caucasian and Asian women until near age 40 and for men until the late teens; then torsional stiffness will increase with advancing age.
Hair breakage is a multifactorial phenomenon involving bending, stretching and torsion deformations and includes: • Tangle formation with hair fibers looped over other hairs with severe bending deformations as shown by Brown and Swift31 • Knots that form more in hair with high curvature and are easily fractured32 • Treatments and weathering: Chemical damage increases breakage and conditioners decrease breakage33-38 • Relative humidity (RH) or water content of the hair: Highly coiled hair breaks more by dry state grooming, while straight to wavy hair provides more short segment breaks (< 2.5 cm) when dry, but more long segment breaks when wet38, 39 • Impact breakage or pulling a comb or brush through a tangle with breakage40 • Physical damage or wear by abrasion from specific grooming devices such as combs, picks or brushes and to some extent a fatiguing action.38, 40-42 Accumulated data from conditioned versus shampooed hair, bleached versus unbleached hair, and comb stroke length comparing broken hairs versus combing loads shows that hair breakage increases with combing forces.38, 43 Spearman’s non-parametric correlation test of these data provides a significant correlation for
Chapter 14 23 this effect. Furthermore, Kamath and Weigmann44 observed that wetting and combing hair tresses provides a large increase in the mid-length force and at the same time a decrease in the end peak force compared with combing dry hair (65% RH). A few years later, Robbins and Kamath38 observed more short segment breaks in dry versus wet combing, but more long segment breaks in wet combing. Furthermore, cross-cutting hair and combing it dry versus a tapered cut provides even higher end peak forces and more short segment breaks.45 These results confirm that combing forces correlate with hair breakage, but more importantly the location on the fiber where hair damage and breaks occur actually corresponds to where higher combing forces occur on combing force curves.
In other words, mid-length combing forces correspond to long segment breakage and the end peak force corresponds to short segment breakage. Changes in hair fiber properties as individuals age likely impact hair breakage. Robbins and Reich46 have shown that combing forces are related to hair fiber properties in the following manner. Combing forces increase with fiber curvature (quadratic relationship) and with fiber friction (linear relationship), but they decrease with fiber stiffness (linear relationship which is collinear with diameter). Smaller diameter hairs break more readily than larger diameter hairs because they are less stiff and they bend and tangle more readily.
Therefore, for many female adults, hair breakage concerns should increase when they reach their forties when fiber diameter begins to decrease.15-20 Changing straight to wavy hair has little effect on combing forces, which are determined primarily by fiber friction and diameter in that curvature range. However, as hair waviness increases to curly, combing forces increase and hair breakage increases even more.43 Since most current cosmetic hair treatments produce negligible effects on fiber diameter and stiffness, combing forces for straight to wavy hair are dominated by fiber friction. This is why hair conditioners are so effective in reducing combing forces and breakage for Caucasian type hair, and in that manner hair conditioners make hair more resistant to grooming actions, i.e.
The Effects of Aging on Hair–More Than Just Amount 24 Changes in fiber properties with age also influence hair styles/ types, which in turn have an impact on hair breakage. More young adults keep their hair longer partly because it can grow coarser and longer at that age, due to the longer time span of anagen up to about age 45 for women. In most Western societies as women age beyond 45, their hair grows finer and cannot grow as long. Therefore, many of these women tend to move to shorter hair styles. For younger women with longer hair styles both wet and dry hair conditioning is important.
However, damage to the ends can be very high because of the longer residence time (4 years for 20–24 inch tips) and more short segment breakage that occurs during dry combing and brushing. These actions provide more damage to the ends of the hair. Therefore, dry hair conditioning, especially at the tip ends, is very important to these younger women. For women in their fifties and beyond with shorter hair styles, hair breakage is more of a problem if their hair is curly. However, for all women with curly hair and shorter hair styles, mid-length combing forces are high especially for wet hair, and therefore wet hair conditioning is very important.
Knowledge gaps for stretching, bending and torsional properties of hair with age: The effects of changes in hair curvature with age have not been examined with respect to age and hair breakage for Caucasians and Africans, and would provide important insight into the relationship between these properties as a function of aging. The effects of changes in ellipticity with age remain to be examined for African hair types. If ellipticity changes with age in African hair, subsequent analysis of the impact of changes in ellipticity on hair breakage for African type hair would provide additional insight into the impact of age-related changes in fiber properties on hair breakage.
Hair Lipids and Age The two major sources of hair lipids are the hair matrix cells and the sebaceous glands that exist inside hair follicles. Masukawa et al.47 have provided some evidence that cholesterol, its esters,
Chapter 14 25 and its sulfate and ceramides originate from the hair matrix cells. Masukawa et al. have also concluded with others that 18-methyl eicosanoic acid (18-MEA) also originates from the matrix cells. On the other hand, these scientists concluded that although some fatty acids likely originate from hair matrix cells, most fatty acids, triglycerides, wax esters and squalene originate from the sebaceous glands, but the source of the hydrocarbons is not known at this time.
Masukawa et al. and Wertz and Downing48 have described the different lipid types and levels in adult hair. The results of these studies are summarized in Table 9.
Table 9. Lipids in human hair from Masukawa et al.47 and Wertz and Downing48 Source Type of lipid mg/gm hair A Squalene 0.7 A Wax esters 4.9 A Triglycerides 0.5 A Total fatty acids 14.4 B Cholesterol 1.3 (0.6)* B Cholesterol sulfate n/a (2.9)* B Ceramides 0.29 (0.5)* B Covalent fatty acids n/a (4.0)* B 18-MEA 0.30 (1.6)* C Hydrocarbons 2.4 TOTALS 24.79 *Data in parenthesis by Wertz and Downing,48,49 not in parenthesis by Masukawa et al.47 Masukawa et al. isolated lipids from the root sections of the hair of 44 Japanese females varying in age 1–81 years. Wertz and Downing48 examined four different Caucasian hu- man hair samples (three from individuals and one pooled hair sample of presumably Caucasian hair).
In another paper Wertz and Downing49 cited 4.3 mg/gm total integral (covalently bound) fatty acids with 40.5% as 18-MEA for human hair, or 1.7mg/gm 18-MEA (hair from three individuals and one pooled hair sample).
Of the different types of lipids found in human hair described in Table 9, those labeled “A” originate primarily from sebaceous glands and those labeled “B” originate mainly from hair matrix