The Journal of Science and Business Research

 

Seasonal Growth Rate Variation of Scalp

Hair in one Individual

 

P. B. Sindafin, MBA

 

Introduction 

            In 2004 an acquaintance became involved in an on-line discussion group concerning the care of long hair.  The association with the other members of this group was a natural extension of the fact that she has been allowing her hair to grow for many years, and the subject of how best to care for it was a natural extension.  One of the concepts often discussed on that board is seasonal growth variations, and when she mentioned that to me, she agreed to allow me to do a definitive study of her hair growth rate and patterns in order to confirm or dispel the notion of seasonal variations, and to quantify any variation found.  This article summarizes the findings of that study.

Overview

            A review of the literature shows that a great deal of work has been done concerning why individual hair follicles cycle from a period of growth (anagen phase) to no-growth (catagen phase.)  One particularly thorough article (Stenn, et. al., 2001) has an amazing bibliography of more than 500 references.  In brief, the control of hair growth varies by species, and is mediated by a veritable plethora of hormonal agents, many, but certainly not all of which are produced by, or otherwise controlled by the pituitary gland.  Concerning this, Jankovic (Jankovic, 1998) says “The hair follicle has a treasure of control mechanisms which influence its growth.  Many are under the influence of androgens, while the others are highly autonomous.”  Undoubtedly, this study of follicle cycling is important in certain industries, such as wool production, mink ranching, and the like.  For instance, Craven (Craven, et. al., 1994), and Parry (Parry, et. al., 1991) have related the daily length of the photo period (length of light in each day), to wool production in Wiltshire Horn sheep in New Zealand.  Foitzki (Foitzki, et. al., 2003) suggests that humans, unlike sheep, have seasonally-independent catagen - anagen cycling.

            Regrettably, however, there appears to be little previous research addressing the concept of seasonal growth rate differences in human hair, or the effects of photo period on the rate at which hair is produced during any single anagen growth phase.

 

Study Parameters

            The original objective of this study was to confirm or dispel the question of whether there is, in fact, a seasonal variation in the growth rate of human scalp hair, and if there is, to quantify that variation.  The main experimental data element, therefore, consisted of a weekly measurement of the subject’s hair length.  This basic data element was compared to a number of other elements and indices, to discern any influences on hair growth.  As the study progressed, additional observations became possible, such as the possibility of rapid growth after trimming and routine cyclical growth rate changes.

            For the purposes of this article the following comparisons have been made: 1) hair growth rate by season; 2) hair growth compared to sun angle; 3) hair growth compared to average temperature; 4) hair growth compared to the number of minutes of daylight; and 5) monthly hair growth.  The concept of hair growth compared to moon illumination is also occasionally mentioned in the discussion board.  This study demonstrates that there is no correlation, however, and that will be mentioned here, only in passing.

 

            This subject's hair type is graded as very fine in texture, with a definite wave pattern. 

 

Methods

            As mentioned above, the primary data element, or measured variable, was the subject’s actual, or absolute hair length.  These measurements were made with a single flexible plastic-coated fabric tape measure, marked in millimeters (mm), and all measurements were made and recorded in whole mm.  The very first measurement was, in fact, made with an inch-graduated tape measure (1/8 inch minimum gradations), and it was immediately obvious that such gradations were too large to be precise.  The millimeter tape was obtained before the second measurement.  The first measurement, therefore, represented a mathematical conversion of an inch-denominated length and was later discarded from the data set.  The millimeter tape was kept in its original plastic container between measurements, and not used for any other purpose.

            All measurements were made on sequential Sundays, at approximately the same time of day.  Notation was kept concerning what day the measurement was in the subject’s wash cycle.  During the first eight months of the study, the subject followed a 3-day wash cycle.  Approximately four months before the end of the study she switched to a 4-day cycle.  In one case (measurement #2) the measurement that occurred on a wash day was made after the wash, and this measurement was also later discarded from the data set.

            Measurements were made by placing the zero-end of the tape at a specific point on the subject’s forehead, and letting the tape then go directly over her head, and down her back.  Although it is certainly possible that the placement of the zero-point might have introduced some error into the measurement, this error would have been small, certainly no more than 1 mm, because of the fixed location of a specific freckle on the subject’s forehead that was used as the zero-point. Illustration 1 shows the zero-end tape placement.  In any case, any error introduced into the measurement because of zero-end placement variation would not have been additive.  With a large enough number of observations, any zero-end placement error would have resulted in the creation of a zero-end point that was actually a mean within a very narrow range, likely to be less than 0.5mm.  For all practical purposes then, the zero-end point was a statistically fixed point.

 

Illustration 1.  Zero-end Placement

            Error might also have been introduced on the other end of the measurement, that being at the “hemline” of the subject’s hair.  It was quickly noted that the subject’s hairs grow at varying rates.  Early in the study, and then again each few weeks, micro-trims were performed to create a straight hemline of hair, all of the same length (to which measurement was made.)  Within no more than 4 weeks, the hemline was noticeably “ragged” however (in a mm sense) with a few hairs growing out faster than the bulk.  These faster-growing hairs were affectionately named “gallopers” and will be discussed separately later.  Illustration 2 shows a freshly-manicured hemline, one with some growth of gallopers, and another with significant gallopers.  It is indicated where the hair length was measured, as the hemline would become more ragged due to the galloper growth.  Measurement was always made to the imputed end of the hair bulk.  In any case, however, any error introduced in this measurement would have not been additive, as above.  The repeated measurements would, in fact, have taken on a mean measurement error.  Clearly, 46 inch hair is longer than 39 inch hair, and the observations here are validated, regardless of any measurement errors of mm dimensions.

            The hair, especially the ends, was regularly inspected with a ophthalmoscope (lighted magnifier) at 15 and 20 power magnification.  Throughout the duration of the study, the individual hair ends remained blunt with no tapering or splits.  This was due, no doubt, to the regular trims. 

 

 

Illustration 2.  Hemline Measurement

            Data from weekly measurements was maintained in a QuattroPro spread sheet (QuattroPro 10, Corel Corporation, © 2001).  Although actual hair length was reduced periodically because of occasional larger-than-micro trims, before and after measurements were made so that the larger trims could be quantified.  Data was therefore logged as both actual hair length (after trims) and what was termed “Growth-rate length” - the length the hair would have been had the trim not been performed.  This latter measurement was used in all cases to determine growth rate, to draw graphs, and for statistical calculations.

Definition of “Seasons” 

            The first measurement of this study was taken on May 22, 2005 (this was the original inch-denominated measurement.)  It was noted that the mid-summer equinox (June 21) occurred in the week after the measurement of June 19, four weeks after the original measurement.  Several different definitions of “seasons” were then arbitrarily made, generally attempting to use eleven-week periods, with two week intra-seasonal periods established between subsequent seasons, thus filling the entire 52 week year.

            For reasons that became apparent only well into the study, the four 11-week seasons were finally defined beginning on the four dates nearest the dates of equinox and solstice.  For the same reasons, seasons were later further defined as “early” and “late.”  Specific information concerning these seasonal definitions will be presented later.

Progression of the Study

            The study extended for 56 consecutive weeks, which represents something more than an entire yearly solar excursion cycle.  Data used in this study includes the “early summer” sub-season of one year, through the “late spring” sub-season of the following year.  This also allowed the first two (possibly spurious) measurements (the first inch-denominated measurement, and the second measurement made after a wash) to be discarded.

            Four non-variable data elements were determined for each measurement day - the maximum solar sun angle, the percent moon illumination, the number of minutes of sunlight, and the average temperature.  This information was taken from United States Naval Observatory (Navy) and the National Oceanic and Atmospheric Administration (NOAA) web sites.

            It should be noted that all measurements were made at essentially equivalent latitudes (within 200 miles north-south.)  In any case, the subject resided at the same latitude during the entire study, and departures from that latitude were brief.  The maximum sun angle was, therefore, determined by the subject’s fixed location, as were the length-of-day and mean outdoor temperature aspects of her exposure to any seasonal influences.

Findings and Discussion

 

Overall Hair Growth Rate

            In spite of the fact that there is scant literature concerning seasonal growth rate, one can find numerous citations concerning overall growth rate.  Some are more generalized, such as “. . . approximately 1- 2 centimeter a month. . .” (Baratz, 2001), (Gray, 2006), (Stephens, 2006), to a more specific “. . .hair grows 0.34mm per day. . .” (Keratin, 2006), and to a very precise “. . . The frontal growth rate (0.355 +/- 0.024 mm/day) was significantly lower than that of the occipital region (0.389 +/- 0.021 mm/day). . .” (Runne, 1986).  Our study is in agreement with all of these, demonstrating an overall yearly growth rate average of 0.35 mm/day.

Seasonal Hair Growth Rate

            This study clearly demonstrates that the subject’s hair growth rate does, in fact, vary over time, and in synchronization with the seasons.

 

 

 

Figure 1.  Overall Hair Growth

            Figure 1 shows the subject’s overall hair growth during the study period.  This represents the subject’s actual hair length with the trims added back in.  In other words, had there been no trims, this would have been the actual length.

            Linear regressions of the growth rate were calculated for the four 11-week seasons, as defined above.

In this case, the growth rate (regression coefficient) is expressed in inches per week.  The very high R² for each coefficient indicates that the reliability of these coefficients is very high (R² values range from zero to one.  The closer the value is to one, the higher the reliability.)

 

 

Figure 2.  Hair Growth and Seasonal Rates

 

            A graphical representation of this seasonal variation is demonstrated in Figure 2.  In this graph, the four seasonal growth rate coefficients are overlaid on the hair length.  The result is a representation of the growth rate throughout the year.  The steeper the coefficient line, the more rapid the growth, and the flatter the coefficient line, the slower the growth.

 

Figure 3.  Seasonal Growth Rate Coefficients

 

            Another way to visualize the differing growth rates is to demonstrate all four seasonal coefficients, as in Figure 3. Again, the flatter (winter) line indicates slower growth, and the steeper (spring) line indicates faster growth.  As indicated above, the concept of sub-season variation became obvious later in the study, and this will be discussed later.

 

 

Figure 4.  Weekly Hair Growth

 

            The weekly growth rate of the subject’s hair is demonstrated in Figure 4.  It is evident that growth rate follows a shorter-term cyclical pattern consisting of a spike in one week, and 3 or more weeks of decreasing growth rates, followed inexorably by another spike.  Exceptions to this were the period starting in mid-December (week 30) when the growth changed, and then settled into a 2-week cycle of spike-slow-spike-slow, etc. lasting for 12 weeks.  At the end of that period, a sudden and prolonged burst of growth was recorded.  Although both robust in actual growth rate, and protracted in duration, this (spring) spurt of growth, too, demonstrated the familiar cycle, as at the first of the study.  The origin of the demonstrated cyclical growth pattern is unknown, and deserves further study.  One would not be able to postulate a purely hormonal cause in this specific subject.

            As will be demonstrated later, this raw weekly growth rate data is difficult to correlate with other factors, and simple inspection shows only a rudimentary long-term seasonal variation.  In order to smooth this variation, 3, 4, and 5 week moving averages were calculated for each week, consisting of that week and the prior 2, 3 or 4 weeks.  Figure 5 represents the 4-week rolling average growth rate, by week.  Clearly a pattern emerges.  Keeping in mind that “winter” occupies approximately 1/4th of the graph, that being the 1/4th of the graph to the right of center (weeks 30 - 43 approximately), Figure 5 predicts that there must be a seasonal variation.

 

 

Figure 5.  4-Week Rolling Average of Hair Growth

 

 

Growth Rate Variation and Sun Angle

 

            The scalp hair growth rate of this subject varies directly with the sun angle.

 

 

Figure 6. Sun Angle Variation at Subject's Location
 

            Figure 6 demonstrates that the sun angle (elevation) varies between 21degrees and 68 degrees of elevation from the horizon during the year, at the subject’s latitude.  The high point, of course, marks the time known as the beginning of summer, and the low point marks the beginning of winter.

 

 

Figure 7. 4-week Moving Average Growth and Sun Angle
 

            Figure 7 demonstrates the sun angle (blue bars) overlaid by the 4-week moving average of hair growth.  The correlation is obvious.

 

 

Figure 8.  5-week Average Growth and Sun Angle

 

            As discussed above, further smoothing of the growth data was undertaken by computing the 5-week moving average.  Figure 8 shows this data overlaid onto the sun angle.  A careful consideration of both Figure 7 and Figure 8 should be made.

            Statistical analysis of this data reveals the following correlations:

	Sun Angle	4-week Avg Growth	5-week Avg Growth
Sun Angle		0.6696**	0.7706**
Avg Temp	0.7884	0.4579	0.5633
Minutes of Light	0.997	0.6612	0.7620
Moon Illumination		-0.0617	0.0380

** p < 0.01

There is a clear and statistically significant correlation between both the 4-week and the 5-week moving average growth rate, and the sun angle.  In particular, the correlation coefficient of 0.7706 (5-week Avg growth rate vs. sun angle) at the p<.01 significance is convincing.

 

 

Figure 9.  5-week Average Growth Rate vs. Sun Angle
 

            Additional graphical representations of the relationship between sun angle and hair growth can be made.  For instance, Figure 9 demonstrates the 5-week average growth plotted against sun angle.

Hair Growth and Temperature

            Referring back to the chart of correlations above, it is noted that the hair growth is poorly related to the actual average temperature.  Whatever correlation there is, is related to the fact that temperature increases and decreases roughly follow the sun angle with a lag of approximately 6 weeks.  That is to say, the period of highest (and lowest) temperatures occurs approximately 6 weeks after the start of summer (and winter).

The 40-Degree-Sun-Angle Effect

            Figures 7, 8, and 9, all taken together demonstrate fairly convincingly that some unusual event happens at a sun angle of approximately 40 degrees.  During the fall of the year, the subject’s growth rate fell until the point where the sun angle was approximately 40 degrees, and then remained steady throughout the winter.  In the following spring, when the sun angle reached 40 degrees (on its way back up), the spring growth spurt occurred.

            What is it about a sun angle of 40 degrees that triggers a sudden change in growth rate?  Definitively, we do not know.  What is interesting, however, is that a sun angle of 40 degrees occurs one week from the date of maximum sun angle change in the fall, and two weeks from the date of maximum sun angle change in the spring.  Mathematically, the point of maximum sun angle change is the first derivative of the sun angle.  Simply put, from the sun’s lowest point it rises, and does so more and more rapidly, until it is one-half way to its highest point.  From that half-way point to the point of highest angle, the change occurs more slowly.  It is likely that some innate body mechanism senses the days becoming longer and shorter, and when the relative increase (or decrease) of the change of day length is noted, the body reacts. 

 

 

Figure 10.  5-week Average Growth and Sun Angle Rate-of-Change

 

            Figure 10 above shows the 5-week average growth rate together with the rate of change in day length.  The relationship of change in hair growth rate at the 40-degree sun angle clearly is also related to the times when the sun-angle change, and thus the rate of change in the length of day, is greatest. 

 

 

Figure 11.  5-week Rolling Average Growth Rate per Minute of Daylight

The unit of growth is 10^-3mm per minute

 

            Figure 11 shows the 5-week moving average of growth rate per minute of daylight.  Of note is the positive hump in the mid-winter.  This hump comes from the fact that the hair continued to grow at a steady rate throughout the period, while the length of day fell to the minimum and came back up.  Additionally, the extremely strong and extended spring growth spike shows increased growth/minute rates.  Otherwise, the growth rate centers around 0.003 mm/minute of daylight.

Sub-seasonal Growth Rate Variation

            Noting the very definite change of growth rate at the 40-degree sun angle, we then further divided the seasons into sub-seasons, in order to more precisely discern when growth speeds up and slows down.  The process started by dividing late summer from early fall, at week 20.  Early fall was carried forward for 6 weeks, and late fall for 5, to end on the actual first week of winter.

            Similarly, late winter was divided from early spring at week 42.  Late winter was carried back 6 weeks and early winter back another 6 weeks, to join late fall.  Early spring was carried forward 7 weeks, and late spring carried forward another 7 weeks, ending on the actual first week of summer.  Finally, early summer was carried forward 8 weeks, and late summer 7 weeks, to meet early fall, as described above.

            Having these definitions in hand, linear regressions were calculated for each sub-season.

 

 

 

 

Figure 12.  Graphical Representation of Sub-seasonal Growth Rates

 

            Figure 12 shows a comparative representation of the 8 sub-seasonal growth rates.  Note that the growth rate representations for late summer and early spring (pink "+" and light green "x"), and late fall and early winter (brown "+" and light blue "x") overlie each other.  The fastest growth is in the late spring, and the slowest in late winter.

 

 

Monthly Growth Rates

            Finally, monthly growth rates were determined by subtracting the last actual length of one month from the last actual length of the previous month.  Although this is interesting, in as much as “monthly growth” is a common topic of conversation on Internet discussion boards, it is of little value to this study for two reasons: first, of course, is that four months each year have five weeks, instead of four, and secondly the use of only twelve data points would make the findings almost meaningless.

Actual Hair Length and Rapid Growth after Trimming 

 

 

Figure 13.  Subject's Actual Hair Length, Including Trims
 

            Figure 13 demonstrates the subject’s actual hair length during the study period.  Of note is the fact that on several occasions hair “trims” were performed (weeks 23, 28, 41 and 51).  In 3 of the 4 instances of trimming, the entire length of the trim was re-grown within two weeks.  For instance, a 6mm trim performed on week 41 was replaced by 8mm of growth within the next two weeks.

            This phenomenon has been previously reported by numerous observers in various hair-related Internet forums, and (for male facial hair) is both supported  by a University of Maryland document (Maryland, 1937) and refuted by Beardguy (Beardguy, 2006).  It has also been supported (for scalp hair) in an unsubstantiated and anecdotal way by Alag (Alag, 2006), and refuted, also in an unsubstantiated and anecdotal way by Balasaraswathy (Balasaraswathy, 2005).   It is noted that some posit that this after-cut-growth-spurt happens only in the spring.  Our study demonstrated this phenomenon in October, November, and February, and in May (although the trim in May was not replaced within 2 weeks, it was almost doubled within 4.)  Clearly, therefore, this is not a seasonally related process in our case.

            A critical look at our data reveals that the average growth in the two weeks preceding a trim was 4mm, that the average growth in the two weeks immediately following a trim was 5.75mm, and that the average growth in the two weeks following that, was 5.50mm.  The average 4-week post-trim growth rate over all four periods, then was 2.81mm/week.

            For a control, four 6-week periods were defined, during which there were no trims.  The results are shown below.  In these control periods, the average 4-week post no-trim growth was 2.50mm/ week, or 11.1% slower than the periods with the trims.

           It is noted that three of the four trims were preformed when the sun angle was less than 40 degrees, and it was in these instances that length after a trim was recovered within two weeks.  In the single instance when the trim was performed when the sun angle was greater than 40 degrees, the trim was not recovered within 2 weeks, but the overall growth rate was much higher in the 4 weeks after the trim.

            Quite clearly, our study was not designed to test this.  However, this data would seem to indicate the presence of some kind of increased growth-rate phenomenon.  A 12.5% faster overall growth rate should at the least prompt further study.  This subject will almost assuredly survive to be argued another day, but without data from a specifically designed study, we will simply say - there very well may be some kind of increased-growth-rate-after-trimming effect. 

Conclusions

            Our study clearly demonstrates that there is a distinct seasonal difference in the scalp hair growth rate of the individual studied.  Further, the growth rate varies in synchronization with the maximum sun angle at the subject's location.  Hair growth is most rapid in the spring, very slightly slower in the summer, markedly slower in the fall, and slowest in the winter.  Except in the low-sun-angle winter, the growth follows a peculiar pattern of 4 - 6 week cycles, consisting of a rapid growth period, followed by a period of decreasing growth rate.  The cause of this cyclical pattern is unknown. 

 

            Alag (Alag, 1999) suggests that it is the direct presence of sunshine on the hair that causes growth.  Our study did not directly test this proposition, in that no allowance for the actual time spent out of doors, or for the percentage of cloud cover was made, but the data intuitively suggests that it is not direct sunshine on the hair that causes growth, rather it is the duration of sunlight in the air (or sunlight in the eyes.)

 

            More specifically, our study suggests that the rate of change in day length is a primary determinant of the actual (instantaneous) daily rate of hair growth.  We have shown that the first derivative of the sun angle defines a point at which growth rate dramatically changes for this subject, at the latitude of residence.

 

            Hair growth rate is also shown to vary as the temperature varies, but the correlation is poor, and this is undoubtedly a result of the temperature following the sun-angle.  In this case, the temperature maximums and minimums, and average temperature curve followed the sun-angle curve by approximately 6 weeks.  The 5-week rolling average hair growth rate, however, tracked with the sun-angle curve, and not the temperature curve.

 

            Hair growth rate does not vary with the percentage of moon illumination.

 

            Although this study was not designed to test such, the data allowed a cursory examination of the phenomenon of very rapid hair growth after trimming.  Our data is not definitive, but indicates that such a phenomenon likely exists.

            Further study is warranted in two areas.  Firstly, this same subject should reside at a lower latitude (where the photo period is more nearly constant) for a period of time and the hair growth measured to discern whether the seasonal growth rate can be influenced simply by manipulating the photo period.  Secondly, this same study should be repeated using a large number of individuals, allowing matching of such factors as hair type, fitness level, diet, age, sex, etc.

 

 

References

Alag, S. S. S.,  Hair Power, The Sikh Cultural Center, 1999.  http://www.sikh.net/SIKHISM/W/Hair1.htm

Balasaraswathy, P.,  A Dermatologist’s Web Site, Light House Poly Clinic, 2005.

http://www.skincarencure.com/hair.htm

 

Baratz, R,  Hair Analysis: Exploring the State of the Science, Proceedings of the Symposium on Hair Analysis,

            Agency for Toxic Substances and Disease Registry, CDC, 2001.

            http://www.atsdr.cdc.gov/HAC/hair_analysis/appendix_c_robert.html

           

Beardguy,  All About Beards, 2006. http://members.aol.com/beardguy/bfaq.htm

 

Craven, A. J., et. al.,  Prolactin Signaling Influences the Timing Mechanism of the Hair Follicle: Analysis of Hair Growth Cycles in Prolactin Receptor Knockout Mice, Endocrinology, 2001, Vol. 142, (6) 2533-2539. http://endo.endojournals.org/cgi/content/full/142/6/2533

Craven, A. J., et. al.,  The Effect of Long-day Photoperiod Treatments on Plasma Prolactin and Wool Follicle

            Activity in New Zealand Wiltshire Sheep, Proceedings of the New Zealand Society of Animal  

            Production,1994, (54) 135-138. http://nzsap.org.nz/proc/1994/ab94035.txt

 

Foitzik, Kerstin, et. al.,  Prolactin and Its Receptor Are Expressed in Murine Hair Follicle Epithelium, Show Hair Cycle-Dependent Expression, and Induce Catagen, American Journal of Pathology, 2003, (162) 1611-1621. http://ajp.amjpathol.org/cgi/content/full/162/5/1611

Gray, John, Statement about hair growth rate, Proctor and Gamble web site, 2006.

http://www.pg.com/science/haircare/hair_twh_23.htm

 

Hoffman, Rolf,  TrichoScan: Combining Epiluminescence Microscopy with Digital Image Analysis for the Measurement of Hair Growth In Vivo, European Journal of Dermatology, 2001, (11) 362-368. http://www.tricholog.de/download/PDF.paper.pdf

 

Jankovic, Slobodan, and Jankovic, Snezana,  The control of hair growth, Dermatology Online Journal, 1998, Vol 4, (1): 2. http://dermatology.cdlib.org/DOJvol4num1/original/jankovi.html

Keratin, Statement concerning hair growth rate from a commercial hair product advertiser, 2006.

http://www.keratin.com/aa/aa011.shtml

 

Maryland, University of,  The Science of Shaving, Hygiene, 1937.

http://otal.umd.edu/~vg/amst205.F96/vj68/documents.html

Navy, United States Naval Observatory web sites. http://aa.usno.navy.mil/data/docs/AltAz.html

            http://aa.usno.navy.mil/data/docs/RS_OneDay.html

NOAA, National Oceanic and Atmospheric Administration web site.

http://newweb.wrh.noaa.gov/mesonet/

 

Ohnemus, Ulrich, et. al.,  Hair Cycle Control by Estrogens: Catagen Induction via Estrogen Receptor (ER)-{alpha} Is Checked by ERß Signaling, Endocrinology, 2005, Vol. 146, (3) 1214-1225. http://endo.endojournals.org/cgi/content/full/146/3/1214

Parry, A. L., et. al.,  An evaluation of the New Zealand Wiltshire sheep as a model for studies on the physiology

            of fibre growth, Proceedings of the New Zealand Society of Animal Production, 1991, (51) 365-370.

            http://nzsap.org.nz/proc/1991/ab91068.txt

Randall V. A., Ebling, F. J.,  Seasonal changes in human hair growth, British Journal of Dermatology, 1991,

            Vol. 124, (2) 146-151.

 

Runne U, and Martin, H,  Changes in telogen rate, hair density, hair diameter and rate of growth in androgenetic alopecia of the male. Simultaneously a contribution to the significance of dysplastic hair, Hautarzt, 1986, Vol. 37, (4) 198-204. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3700106&dopt=Abstract

 

Stenn, K. S., et. al.,  Controls of Hair Follicle Cycling, Physiological Reviews, 2001, Vol. 8, (1) 449-494. http://physrev.physiology.org/cgi/content/full/81/1/449

Stevens, Claire, Hair - An Introduction, The Trichology Society, 2006. http://www.hairscientists.org/hair.htm

 

 

 

About the Author:

 

            P. B. Sindafin holds a MBA, but his undergraduate degree was a BS in Biology and Chemistry.  Although he has worked in the business world for most of his professional career, he notes that "My heart belongs to science."  He may be contacted at:  hairarticle@sbrjournal.net

 

       

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