How does laser hair removal really work? WHAT actually happens and WHY doesn’t hair grow back?

A detailed, scientific explanation of how laser & IPL hair removal works, and how at-home machines work in a similar way

I’m a curious little bunny. Always in the detail, I want to know why or how something happens. So, I’m unsatisfied by the multitude brief explanations about how laser & IPL hair removal work. Things like this from the Mayo Clinic:

“During laser hair removal, a laser emits a light that is absorbed by the pigment (melanin) in the hair. The light energy is converted to heat, which damages the tube-shaped sacs within the skin (hair follicles) that produce hairs. This damage inhibits or delays future hair growth.”

But HOW does it do it? WHAT actually happens to the hair and WHY doesn’t it grow back? I need more. It’s just not enough for me.

Maybe I’m alone in this. But I think if you’re considering laser hair removal at a clinic or buying a home use machine, you should know the detail. Right?

Sooo, anyways. I went on an interweb mission and thought I’d share my research findings. I hope you benefit from the hours I could have spent watching anything at all starring Chris Hemsworth.

Here’s what you’ll learn:

A brief recap

This is the simple explanation you’ll find most places.

Back on laser hair removal 101 we learnt the difference between laser & Intense Pulsed Light energies. We also learnt that different substances in our body absorb specific light wavelengths.  We call these substances chromophores. Chromophores are the dark pigment in hair (melanin), oxyhaemoglobin in blood, and water.

Laser & IPL energy is absorbed by the dark melanin pigment in your hair follicle.

For laser & IPL hair removal we care about melanin in our hair. Melanin absorbs light wavelengths between 600-1100 nm. So, the hair follicle cells (those with melanin in them) absorb laser or IPL pulses at these wavelengths. The energy converts to heat which damages the hair follicle. The hair then shuts down and enters a resting phase. So, you see slower growth and eventually, many fewer hairs growing; this is hair reduction.

The process is the same for both professional and at-home laser & IPL hair removal machines. They both use a process called selective photothermolysis.

And now for…

— The science bit —

What’s selective photothermolysis?

The melanin in your hair follicle (the bit below the skin surface) absorbs a pulse of optical energy (light). It converts to heat and damages the hair follicle compartments to stop hair growth.

The melanin ‘selectively’ absorbs the laser or IPL energy. This means it leaves the surrounding tissues undamaged.

'Selective' means the chromophore only and not the surrounding tissues. Photothermolysis: 'Photo' means melanin absorbs light photons, 'thermo' means the absorbed light transforms to heat, and 'lysis' means the heat destroys target cells.
Both IPL and laser hair removal use selective photothermolysis to reduce hair growth.

But there’s melanin in your skin too. Therefore, there must be a big contrast between your hair and skin colour so the laser targets the hair’s melanin and not the melanin in your skin.

That’s why the best candidates for laser and IPL are people with fair skin and black hair. And why you can’t treat dark skin tones with all types of laser and IPL (but you can with some).

Related: The best home IPL devices for dark & black skin

So, what exactly happens to the hair?

What happens to the hair?

This next section best explains what happens to your hair during professional, high-power laser and IPL hair removal. Home laser and IPL hair removal is a bit different and we’ll get to that shortly!

Successful hair reduction leaves the hair ‘miniaturised’ or halts the growth cycle.

Miniaturisation is when mature, dark, thick hair (called terminal hair) reverts to vellus hair. Vellus hair is very fine, soft, light and barely noticeable hair which develops during childhood and covers most of your body.

Close up photo of a young girls mouth and nose showing very fine vellus hairs.
Hard to see: vellus hairs cover most of your skin. Photo by Eric Muhr on Unsplash

So, how does laser and IPL do this? It does it by damaging specific zones and key cells in your hair follicle.

What are the hair follicle zones?

The hair follicle is a pocket in your skin which builds your hair shaft and controls your hair growth cycle.

The zones in your hair follicle are:

  • Dermal Papilla with fibroblast cells. At the follicle base and provides the cells with blood and nutrients
  • Hair matrix with Matrix keratinocytes and Melanocytes found in the hair bulb
  • Outer root sheath with epithelial stem cells, hair shaft keratinocytes, inner root shaft keratinocytes
  • Bulge with epithelial stem cells. A zone next to the hair bulb

Here’s a diagram:

Detailed diagram of a hair follicle. The dermal papilla is at the bottom of the bulb, then the hair matrix. The outer root sheath surrounds the hair shaft (hair fibres) and the bulge sits a small way up the outer root sheath next to the Arrector pili muscle which makes your hair stand up and causes sebum to exit the sebum gland.
The areas of the hair follicle treated during IPL & Laser hair removal are the dermal papilla, hair matrix, outer root sheath and the bulge.

Not all these zones and cells contain melanin. So, there’s a chain of events.

  1. The laser/IPL targets and heats the cells with dark pigment.
    These are the pigment producing melanocytes and matrix keratinocytes of the hair matrix, and the hair shaft keratinocytes.
  2. Then, heat moves from these areas to other non-pigmented cells.
    These non-pigmented cells are the dermal papilla fibroblasts, keratinocytes in the inner and outer root sheath and epithelial stem cells of the hair follicle bulge.

The variables and power of the light energy ultimately determine the amount of damage done to these key areas. Professional treatments are very efficient, and home devices less so (but still effective!)

Next, let’s understand the key cells and what role they play.

What are the key cells and what do they do?

The key cells to damage in laser and IPL hair reduction are:

  1. Dermal papilla fibroblasts
  2. Epithelial stem cells in the hair follicle bulge
  3. Matrix keratinocytes

Let’s look at 1 & 2 first and why they’re important.

The dermal papilla fibroblasts and epithelial stem cells communicate together and trigger your hair follicle’s cycle of growth. There are 3 stages in the hair growth cycle:

Illustration of human hair in the 3 growth stages, 1 Anagen growth stage, 2 Catagen growth stops and follicle and hair retract, 3 Telogen follicle inactivity and hair falls out, back to early Anagen where the follicle connects to the dermla papilla again and hair grows.
Our hair growth stages. Each cycle duration differs by body area.

Stage #1: Anagen (active growth stage), the dermal papilla attaches to the hair follicle and the hair shaft builds as cells divide in the hair bulb

Stage #2: Catagen (cells stop growth) the follicle and hair fibres retract from the dermal papilla and stop growing

Stage #3: Telogen (follicle inactivity) the hair follicle is inactive and the hair shaft eventually falls out (or is pushed out by new Anagen hair)

If we damage the dermal papilla fibroblasts and/or epithelial stem cells with enough heat, they chemically breakdown. They can’t communicate to start a new hair growth cycle. The damage is (in most cases) irreversible and the result is the hair follicles miniaturise or they remain dormant in the Telogen stage.

Therefore, smooth fuzz-free skin. Or just teeny vellus hairs. And these are so fine and light it seems as if nothing is growing at all.

And let’s not forget cell type number 3, the Matrix keratinocytes.

Damaging the Matrix keratinocytes causes the hair to jump to the Catagen stage. So, the hairs stop growing and can fall out over the course of a few weeks. This is what gives quick, smooth-skin results during professional and home sessions. 

But, if damage is to these matrix keratinocyte cells only, hair still grows back as thick and dark as before. That’s because the intact dermal papilla and hair bulge cells can still trigger terminal hair growth in the next Anagen stage.

So, successful IPL and laser hair removal damages all 3 key cells for long-lasting hair reduction.

It’s also important which growth stage your hairs are in when zapped.

Must treat Anagen hairs

For laser and IPL hair removal to be successful you must zap hairs in the Anagen growth stage.

This is when the follicle rapidly builds the hair and there’s lots of melanin-rich cells. Also, the dermal papilla attaches to the follicle only in the Anagen stage. So, it’s the optimum time for efficient heat transfer from the melanin-rich cells to the non-pigmented target cells and dermal papilla.

Hairs across your body all follow the 3 growth stages, but the cycle duration is different across sites. Therefore, you need several IPL and laser sessions at regular intervals to zap all the hairs during the Anagen stage.

Alrighty. If you’re still here, you’re probably wondering what’s on TV? And possibly, is at-home laser and IPL hair removal any different?

Well, yes. Yes it is. I shall explain.

How does at home laser & IPL hair removal work?

Both professional and home IPL / laser hair removal work by the same principle of selective photothermolysis. That is, the dark melanin pigment in your hair absorbs the light energy. It heats up and destroys key areas in the hair follicle to prevent regrowth.

Professional treatments are powerful and destroy these areas quickly and efficiently. But you do need multiple sessions (usually 4 weeks apart) to catch hairs as they cycle into the Anagen growth stage.

Close up photo of a powerful professional  laser treating the surface of skin
Powerful professional laser & IPL does max damage to Anagen hair key follicle areas

Home use devices are gentler and slower, and need several more sessions for a compound effect. Therefore, there’s a high effort start-up schedule followed by less regualr maintenance sessions.

Less powerful home devices

Close up photo of the leading homelaser & IPL hair removal machines
Home devices are scaled down in size and power for safe use at home.

Home devices are much less powerful than professional machines. A recent study explains this means a different ‘biological mode of action’ in home devices.

Remember those 3 key areas of the hair follicle?

  1. Dermal papilla fibroblasts – damage halts the hair growth cycle
  2. Epithelial stem cells in the hair follicle bulge – damage halts the hair growth cycle
  3. Matrix keratinocytes – damage gives temporary hair removal

The study explains the light energy is powerful enough to initially damage the Matrix Keratinocytes. This causes the hair to jump to the Catagen growth stage. So, the hairs stop growing and fall out over the course of a few weeks.

This is what gives you quick, noticeable hair reduction results during the start-up stage, usually after just a few sessions.

BUT…

Message on a brown furry background saying "STOP TOO SOON & THE HAIR GROWSBACK!"
Keep going: several maintenance sessions give long-lasting results.

If you don’t continue treatments the hair eventually starts a new Anagen stage and hair grows back just as before.

Effects build over time

However, keep going with the treatments and you get different results.

Close up photo of the treatment schedule printed on the Philips Lumea Prestige box
Follow the treatment schedule AND do touch-ups for at least 1 year.

That’s because each session also does a little damage to the dermal papilla fibroblasts and the stem cells in the hair follicle bulge. Remember, these cells trigger a new growth cycle. Repeated sessions build damage until it breaks the hair growth cycle and the follicle can’t grow new, thick hair.

The study suggests, stopping at 4 or less sessions means all your hair eventually grows back. 😲

You must continue for 1 year of less frequent sessions every 6 weeks or so. Then you’ll achieve high and long-lasting hair reduction (up to 85%) even 1 year after stopping the sessions.

So, top up sessions are SUPER IMPORTANT. And your results get better and longer-lasting the longer you use the home-use device.

Most devices suggest an initial start-up stage of sessions either weekly or every 2 weeks. The start-up stage lasts between 6 to 12 weeks They then suggest monthly top up sessions, or as you need them. So, based on this and the results of the latest study above, you’ll likely need between 12 and 20 sessions in your first year of home-use.

So, now you know how it all works, and why home laser hair removal needs more Chris Hemsworth sessions to get the same results.

Which are you most interested in – professional or at-home laser & IPL hair removal? See how Thor compare here:

Related: how do professional vs at-home laser hair removal compare?


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