The Effects of Aging on Hair-More Than Just Amount
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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
1The Effects of Aging on Hair–More Than Just Amount
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
2Chapter 14
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.
3The Effects of Aging on Hair–More Than Just Amount
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
4Chapter 14
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
5The Effects of Aging on Hair–More Than Just Amount
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
6Chapter 14
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
% Little % Moderate % Total % Any
Age
Gray Gray Gray(Eqn) 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.
7The Effects of Aging on Hair–More Than Just Amount 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, 8
Chapter 14
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
9The Effects of Aging on Hair–More Than Just Amount
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.
10Chapter 14
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
11The Effects of Aging on Hair–More Than Just Amount
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.
Figure 1.
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,
12Chapter 14
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
13The Effects of Aging on Hair–More Than Just Amount
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
14Chapter 14
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.
15The Effects of Aging on Hair–More Than Just Amount
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
16Chapter 14
(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.
17The Effects of Aging on Hair–More Than Just Amount
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.
18Chapter 14
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
19The Effects of Aging on Hair–More Than Just Amount
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
20Chapter 14
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
21The Effects of Aging on Hair–More Than Just Amount
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
22Chapter 14
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. stronger.
23The Effects of Aging on Hair–More Than Just Amount
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,
24Chapter 14
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
25The Effects of Aging on Hair–More Than Just Amount
cells. We have not been able to locate a comprehensive study of hair
lipid composition versus age on a large number of subjects; however,
there is literature that allows us to connect the dots and arrive at a few
useful conclusions. Nicolaides and Rothman51 determined that the
cholesterol content of the hair fat of boys 6–12 years of age (females
were not included in the study) is much higher than that of adults,
while the squalene content for this same group is only a fraction of
that of adults. In another paper, Nicolaides and Rothman52 showed
that the total free fatty acid content of the hair of young Caucasian
boys (ages 6–12) is lower than that of pooled hair of adults. These
effects may be attributed to the fact that the amount of lipids from the
sebaceous glands that are incorporated into the hair fiber are lower
before puberty and the sebaceous glands’ contributions are higher
after puberty. In support of this conclusion, upon reaching puberty
the sebaceous gland activity increases dramatically. The increased
sebaceous output increases the amount of squalene, fatty acids, and
wax esters in teenagers and adults hair compared with the hair of
children.51-53 Furthermore, Pochi et al.54 found that the ratio of wax
esters (sebaceous origin) to cholesterol and cholesterol esters (matrix
cell origin) increased from 0.35 in children to 7.47 in young adult
women and then decreased to 1.46 in post-menopausal women.
Pochi and Strauss determined sebum content from the foreheads
of males and females at different ages (see Figure 2).53-55
These same scientists also concluded that the amount of sebum
produced varies with the size of the sebaceous gland. The data of
Pochi and Strauss shows that sebum is very low before puberty, but
increases rapidly at puberty and remains at a high level until about
age 45–50 where it declines, and the decline with females is greater
than the decline for males. Interestingly, data for lipids on and in
hair by age, derived from unpublished data by the P&G/Wella Hair
Research Group, shows a corresponding relationship to that found
on foreheads (see Figure 3).
In this study, hair samples were collected from 51 Caucasian
females varying in age 7–88 years, extracted and analyzed by GC/
MS, at the German Wool Research Institute DWI, for lipids. The
26Chapter 14
data for total hair lipids versus age is summarized in Figure 3 and
compares favorably with the data of Pochi and Strauss shown in
Figure 2. A box plot revealed two outliers which when rejected
provided a normal distribution with a Shapiro-Wilk W of 0.978 and
a p = 0.5. These data when regressed versus age provided a quadratic
model with p < 0.0001, root mean square error of 5.623 and an r2
of 0.387. From the model equation, the maximum for hair lipids was
at age 45, corresponding to where the steep drop begins for sebum
production on the foreheads of women by Pochi and Strauss, as seen in
Figure 2. These effects lead into the effects of menopause on hair fibers.
Sebum production on forehead
3
2.4
mg lipid/10 sq cm/3 hr
Sebum production
1.8
Males
1.2 Females
0.6
0
8 10 10 20 30 40 60 80
Age in years
Figure 2
The effects of menopause on hair fiber lipids: Wills et al.56
showed that wax esters and squalene content (both primarily from
sebum) were significantly lower in post-menopausal than pre-
menopausal women. These same scientists noted that their analytical
procedure could not distinguish between wax esters and cholesterol
esters; however, the wax ester levels are higher in adult human hair
than cholesterol esters as shown by the work of Pochi et al.54
27The Effects of Aging on Hair–More Than Just Amount
Figure 3
Wills et al. also found that the hair of pre-menopausal women (N
= 80) was greasier than the hair of post-menopausal women (N = 47)
by expert visual assessment. In addition, the amount of lipid found
on the forehead of these same subjects was significantly higher in
the pre-menopausal group than either post-menopausal groups
receiving or not receiving hormone replacement therapy, and the
post-menopausal group not on hormone replacement therapy had
the lowest amount of lipid (-57% versus the pre-menopausal group).
The ages of these three groups were: pre-menopausal, mean age 30,
age range 24–34; post-menopausal, mean age 60, age range 50–76;
and post-menopausal with hormone replacement therapy, mean
age 57, range 48–68. So, the prime variables are menopause and
hormone replacement therapy; however, there is also an additional
factor of age especially between pre- and post-menopausal groups.
Consumer Perception of Changes in Fiber Properties
with Age and Important Knowledge Gaps
This section deals only with those fiber properties that have
been shown to change with age and the consumer hair assembly
properties that have been shown to be or likely to be affected by
28Chapter 14
such changes. For more complete and more speculative discussion of
these effects, see the attached references.45,46,57,58
Hair from different scalp sites is an important knowledge gap:
Changes in hair diameter and density with age, two key parameters
involved in the perception of scalp coverage, have been studied
on a limited number of scalp sites.15-24 Mirmirani and Dawson et
al.24 reported a study on 44 Caucasian women and concluded that
mean hair fiber diameters in the frontal site of these women was
higher on pre-menopausal versus post-menopausal women, but they
observed no significant difference in the occipital region. This study
demonstrates the importance of determining the effects of age on
hair from different scalp sites, especially during the pre-menopausal
versus post-menopausal years. Since the onset of menopause on
average for Caucasians and Japanese is age 51,59,60 we would expect
similar effects among the different geo-racial groups. Studies on
pre-teenage versus post-teenage years would also provide important
information because of the large hormonal changes at puberty and
the effects on hair diameter.
The impact of diameter and hair density on hair coverage and
its perception: Robbins and Dawson et al.16 described the effects
of diameter and hair density on age and proposed a new metric
“relative scalp coverage” for the perception of the amount of hair
on one’s head. When considering only diameter and density, this
metric is defined as a two-dimensional parameter as: the average
fiber cross-sectional area multiplied by the number of hair fibers
per square centimeter. Considering only diameter and density,
relative scalp coverage was calculated to peak at age 35. This peak
age is produced because hair diameter increases until about age 45
but density peaks in the late twenties; therefore, the combination of
these two important contributors to scalp coverage provides a peak
for relative scalp coverage at age 35.
Robbins and Dawson proposed that when other important
parameters are considered for relative scalp coverage, it will provide
a three-dimensional parameter involving diameter, density, fiber
29The Effects of Aging on Hair–More Than Just Amount curvature, fiber length, color (hair and scalp) and style, and relates very much to hair body or the amount of hair. And since Mirmirami and Dawson have shown scalp site effects on diameter and density, scalp site will also have to be considered for a more complete model. The impact of hair color on the perception of graying and relative scalp coverage: The occurrence or incidence of any gray (look back to Table 1’s data from Keogh and Walsh6) are higher in the darkest hair and lowest in the lightest colored hair. Keogh and Walsh pointed out that lighter gray hairs can be observed more readily against a dark hair background than light hairs against a light hair background. But this effect is reversed for the incidence of completely gray hair which is perceived to occur earlier for the lightest hair color and last for the darkest hair. This effect of hair color relative to its background is why Robbins et al.16 suggested a related effect in the perception of relative scalp coverage which should be studied and determined. Trotter and Dawson19 determined that the hair of children tends to be lighter in color than adult hair as observed in 310 French Canadians, and summarized in Table 11. In this study, for adults, if any gray hairs were present they were removed and only pigmented hairs were included in the comparisons. Trotter and Dawson20 also examined hair of 340 American Caucasians in a similar manner and arrived at the same conclusion regarding age and hair color. Therefore, for these two Caucasian groupings it would appear that from childhood through the early teenage years hair tends to become darker and then sometime in the twenties and beyond, as hair becomes grayer, the overall color of Caucasian hair becomes lighter. For relative scalp coverage, as the color of hair changes relative to the scalp, the perception of scalp coverage will change, and determination of the quantitative relationship of this variable to relative scalp coverage remains an area for future investigation. 30
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