Page 140 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
This is perfectly fine and is true. But, where we went wrong was thinking that lowering the hair temperature, at the same time and by the same amount, would significantly affect the temperature rise there. It turns it that it doesn’t!!
The temperature rise in tissues (hair and skin) depends on the amount of fluence applied and the absorption coefficient of those tissues. These determine the amount of light energy which may be absorbed by those tissues. The higher the absorption coefficient, the higher the induced temperature.
So, if we fire some light energy at a black hair (with a high absorption coefficient) in a pale- coloured skin (low absorption coefficient in the epidermis, SC1), the melanin in the hair will absorb much more energy than the epidermis – up to 20 times more! See the absorption coefficients in Table 7 - Ratio of hair melanin concentration to skin melanin concentration’.
This means that the resultant temperature rise in the stem cells will be much greater compared with the epidermal melanin, depending on the skin colour and hair depth.
What this means is that if we were to cool the skin surface down to zero degrees Celsius, then if the absorbed fluence raised the temperature back up to 40oC, the patient/client wouldn’t feel any pain at all (since pain is triggered at 45oC). But the hair follicle temperature will easily reach over 80oC in the stem cells, which will lead to their destruction in a relatively short time. (Remember, the Arrhenius Equation tells us that the cells denature much, much faster at high temperatures).
The reason is simply because darker hairs have a much higher concentration of melanin in them, compared with the skin (Table 7).
Clearly, as the skin colours darkens, the difference in temperature rise between the skin and the hair reduces. This strongly suggests that we must apply more cooling on darker coloured skins. But it also indicates that there is a real limit on the skin colour we can safely treat, in terms of surface cooling.
So, the main point to take away from this is that we can happily cool the skin until the cows come home, and it should not affect the treatment outcomes at all – except it’ll make the treatments more comfortable and reduce the chances of unwanted skin damage.
CAVEAT – Obviously, do not induce frost bite or similar with too much skin cooling!! Be careful! Success vs fluence
The point of these treatments is to kill off the hair follicles with light energy. If we increase that energy to a level where it can successfully achieve this goal, then we will always get a good result. The reality is that higher fluences will always generate higher temperatures in the stem cells, leading to a higher probability of death.
________________________________________________________________________ 140 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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