Page 65 - Blog Posts v1

A21 - Thick and thin hair – how much light energy do they absorb?
How thick is a hair? Well, according to some research by physicist, Brian Ley (they do all the best research!!!), it ranges between 17 and 181 microns in diameter (see his results here). That is between 0.017 and 0.18 mm.
So, that means that their cross-sectional areas [A = 𝜋 x (radius)2] are between 0.0000023 and 0.00026 square centimetres!! Hairs are very small...
Approx. Depth of Terminal Anagen Follicles
Body Area
% Resting Hairs Telogen
% Growing Hair Anagen
Duration of Telogen
Duration of Anagen
# of Follicles per sq.cm.
Daily Grow Rate
#of Total Follicl es
3–4 months
2–6 years
0.35 mm
3 –5 mm
Scalp
15
85
350
3 months
4–8 weeks
0.16 mm
1 million
total for all of Head
& scalp
2 –.5 mm
Eyebrow
90
10
3 months
4–8 weeks
Ear
85
15
0.32 mm
Cheeks
30 - 50
50-70
880
2-4 mm
10 weeks
0.38 mm
Beard, Chin
30
70
1 year
500
2-4mm
Moustache (UpperLip)
35
65
6 weeks
16 weeks
500
1-2.5 mm
0.30 mm
3.5-4.5 mm
Axillae
70
30
4 month
65
0.30 mm
Trunk
3 month
70
425,000
2–4.5 mm
3.5– 4.75 mm
Pubic Area
70
30
12 weeks
6 month
70
18 weeks
0.30 mm
Arms
80
20
13 weeks
80
220,000
Legs & Thighs
80
20
24 weeks
16 weeks
60
0.21 mm
370,000
2.5-4 mm
0.35
Breast
70
30
65
mm
Table 1: Data from Richards & Merhag
According to the data in Table 1 above, the number of hairs per square centimetre varies enormously across the body, from as low as 60 on the legs and thighs to around 880 on the cheeks.
If we multiply those numbers with the cross-sectional areas of the hairs, we can find how much hair (by area) there is in each square centimetre:
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