For years I have been teaching people to pre-cool the skin surface before firing laser or IPL energy into the skin when treating hair. And I’ve always suggested pre-cooling for between 2 and 5 seconds. My 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 I was 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 I 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!!
As I posted previously, 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), the melanin in the hair will absorb much more energy than the epidermis – up to 20 times more!
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 C, then if the absorbed fluence raised the temperature to 40ºC, the patient 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.
The reason is simply because darker hairs have a much higher concentration of melanin in them, compared with the skin.
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 probably a 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 – Obvioulsy, don’t induce ‘freeze burns’ or ‘froste bite’ or anything similar, with too much cooling!!
I am happy to announce that our third eBook chapter on laser/IPL hair removal has just been completed and will be available very soon.
Hope this helps,
Mike.
Mike Murphy's Blog 