Thick and thin hair – how much light energy do they actually absorb during photothermal treatments?

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 = pi x (radius)2] are between 0.0000023 and 0.00026 square centimetres!! Hairs are very small…

Body Area%Resting Hairs Telogen%Growing Hair AnagenDuration of TelogenDuration of AnagenNo. of follicles per square cm
Scalp15853 – 4 months2 – 6 years350
Cheeks30 – 5050 – 70880
30708.5 months1 year500
Moustache(Upper Lip)35651.5 months4 months500
Arm Pits70302 months4 months65
Pubic Area70303 months6 months70
Arms80204.5 months13 weeks80
Legs & Thighs80206 months4 months60
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:

SiteNumber of hairs% of 1 cm2
Upper lip5000.1112.9
Arm pits650.011.7
Table 2: Percentage of hair coverage in various body sites

Table 2 shows the range of area hair can cover in a number of body sites, depending on whether the hair is thin or thick. So, for example, chin hair can range anywhere from 0.11% to nearly 13% coverage, depending on the hair diameters.

This is quite remarkable, and counter-intuitive. I’m sure we all imagine that the hair coverage is significantly greater than these figures, yet, we are wrong! Unless the data in Table 1 is completely wrong, these are the facts!

What does this mean for light-based treatments?

This is an interesting question. We set up our lasers and IPLs to deliver a certain amount of energy per square centimetre (the ‘fluence’ – see my wee video on this topic). We choose certain wavelengths (yet another wee video) that we know will be absorbed preferentially by the chromophores in the targets (melanin in hair, haemoglobin in blood etc).

But how much of this energy is actually absorbed by the hair? Well, going from the calculations in Table 2, it appears very little is absorbed, since the targets areas are so small!! (To be more precise, I’d have to do a volume calculation. This would involve calculating the energy distribution in the skin, which depends on the absorption and scattering coefficients of each constituent. This approach would give a more accurate set of answers, but I suspect the actual numbers would be very similar to the above, much simpler, calculation. Plus, I can’t be bothered doing that particular calculation at the moment…)

So, let’s look at the maximum amount of light energy that may be absorbed in the hairs. From Table 2, it appears that the hair in the chin and upper lip will absorb around 13% of the incident light energy. But in the armpits, pubic region and legs this drops to less than 2%.

This is very surprising!!

What this tells us is that the vast majority of the light energy is not doing the job we are attempting. In fact, Monte Carlo calculations by my Swedish colleague, PA Torstensson, reveal that much of this light energy is either reflected or back-scattered out of the skin altogether. This can be more than 50% of the incident energy, depending on the wavelength.

Of the remaining 50%, a relatively small portion is absorbed in the hair melanin. Most of the rest of it will be absorbed in the dermis where it will be converted into heat energy. This is not good! This will cause the temperatures in the dermis to rise, potentially damaging those tissues, and may lead to scarring.

“Cooling is critical”

What this means in reality is that we must mitigate against all this excess heat energy in the dermis, before, during and after treatments. We do this by applying a proper level of cooling – contact cooling is the most efficient. Air cooling is fine but surface contact cooling with ice packs is at least four times more efficient at removing the excess heat energy.

Proper cooling will not only minimise any potential tissue damage but it will also improve the comfort level for the patient. In addition, it allows for higher fluences to be used, which may be required in certain procedures (such as for deeper follicles).

We need to realise that we are deliberately over-heating the skin in all of these photothermal treatments – so we must balance that over-heating with plenty of cooling. The calculations above reveal that when we treat hair (and, it turns out, blood vessels too!!) we are mostly heating the skin – the treatment of the hair is almost a “by-product”!

So, be sure you apply plenty of cooling to your patients’ skin – they will certainly appreciate it.

I hope this surprised you – it certainly did me!

Ciao for now,


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