The fluence must be chosen to kill the germ cells in the target follicles. Those are the real targets – not the melanin in the hair shaft. The melanin can be viewed as a surrogate which we can heat using light energy. But, the germ cells do not have a chromophore which we can exploit using light.
So, to kill the follicles we must heat the hair shaft melanin sufficiently to denature the germ cells irreversibly.
The skin colour is completely irrelevant in this consideration! It doesn’t matter what the skin colour is – only the hair colour. The skin colour (in the basal layer of the epidermis is simply a barrier we must surmount).
Let’s consider the skin colour. There are two important issues we should think about:
- The melanin in the basal layer will absorb light energy, inducing a temperature rise in that layer. This heat will diffuse towards the thermal pain receptors (just below the basal layer) and trigger them. If the temperature is high (above 45ºC), then pain will be felt.
- Any fluence absorbed in the basal layer will not be available to the deeper melanin in the hair shaft. So, darker skins will result in lower fluences reaching the hair melanin.
So we have a conundrum…
To successfully kill the hair follicles we must apply a certain minimum fluence at the skin surface.
But the skin’s natural colour will absorb some of this fluence, leaving less to reach the hair melanin.
This suggests that darker skins will require higher fluences than lighter-coloured skins…
Most laser/IPL devices have a “Skin Type” selection option – usually when a higher skin type is chosen, the fluence automatically decreases.
This is entirely WRONG!!!! For two reasons:
Firstly, the darker skin will ‘lose’ more fluence because of the higher absorption in the basal melanin;
and secondly, applying a lower fluence at the surface will reduce the likelihood of a successful treatment!
In reality, we should consider the following:
The fluence MUST be chosen to match the colour and depth of the hair;
the skin colour MUST dictate the amount of surface cooling!
The fluence applied to the skin surface should NEVER be set according to the skin colour – that defeats the whole purpose of the treatment. And yet, many of today’s devices are programmed to do precisely this.
It’s because the manufacturers want to reduce the risk of you damaging your patient’s skin – which is fair enough. But, by reducing the fluence, they are also reducing the chances of success!
What they should be telling you is that darker skins require much more cooling than lighter skins – before, during and after the treatment.
I calculated the temperature in light (SC1) and very dark (SC6) skins when hit with a 20 J/cm2 fluence at the surface. I looked the difference between the alexandrite, diode and Nd:YAG lasers.
Notably, maximum temperature achieved for the palest skin colour (SC1 with a melanin concentration of less than 10%) is marginally above the background dermal temperature of 37ºC.
However, in very dark skin (SC6 with a concentration of >43%) the temperatures range from 54.4ºC for the Nd:YAG laser to 184ºC for the alexandrite laser! It was 133ºC for diode lasers (see figure 1).
Obviously, all the other skin colours are between these two extremes.
So it is very clear that while all the current hair removal lasers are perfectly fine when treating very pale skins, only the Nd:YAG should really be consider safe when treating black skin.
Using an IPL with the wrong filters will also induce very high temperature in darker skins.
What these calculations show is that surface skin cooling is critical as the skin colours darkens – whether it’s a ‘natural’ skin colour or a tan. It is an unfortunate fact of physics that a small increase in fluence results in a large (exponential) increase in tissue damage – which is great when trying to destroy germ cells in follicles, but not so good when we consider the epidermis.
The bottom line is that we are trying to remove hair permanently from people’s skin – across a wide range of skin colours. The science shows that to do this we MUST apply the correct fluence, which depends on the hair colour and depth. It does NOT depend on the skin colour – that is completely unrelated.
But the skin colour is directly related to epidermal damage and pain. To alleviate the pain sensations we must apply plenty of cooling – before, during and after the treatment. This will suck much of the excess heat energy located in the basal layer (which is not that far from the surface!), resulting in much lower temperatures there.
I know this will go ‘against the grain’ of current thinking, but the physics is very simple. Reducing the fluence to ‘compensate’ for skin colour is quite wrong!!
I will be discussing this in more detail in our MasterClass in September 2022.
Ciao for now,