Page 56 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
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”!
Applying good skin surface cooling will reduce the temperatures of those nerve sensors and so reduce the pain sensation felt. So, be sure you apply plenty of cooling to your patients’ skin – they will certainly appreciate it.
“Over-cooling” the treatment area is virtually impossible!!
For years we have been teaching people to pre-cool the skin surface before firing laser or IPL energy into the skin when treating hair. And we’ve always suggested pre-cooling for between 2 and 5 seconds. Our thinking was that we should not “over-cool” the skin because that might lower the temperature of the hair follicle too much.
It turns out we were wrong!!
The whole reason for cooling the skin is to lower the temperature of the thermal pain nerves, located just below the epidermal/dermal junction. By doing so, they will not trigger until their temperatures is raised above 45oC – this will require more fluence if they are pre-cooled by 10 or 15oCelsius.
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 24 - Ratio of hair melanin concentration to skin melanin concentration’.
This means that the resultant temperature rise in the germ 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 germ cells, which will lead to their destruction.
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