Setting the pulse duration in the millisecond range is effective because it precisely matches the thermal relaxation time of the hair follicle. This specific timing allows the laser to deliver lethal heat to the hair root while simultaneously giving the surrounding skin enough time to cool down, preventing burns.
Core Insight The effectiveness of millisecond pulses lies in the principle of selective photothermolysis. By aligning the pulse width with the hair follicle's natural cooling rate (10–100 ms), energy accumulates sufficiently to destroy the hair papilla, while the faster-cooling epidermis dissipates heat safely.
The Mechanism of Thermal Relaxation
To understand why milliseconds are the standard, you must understand Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to dissipate 50% of the heat it has absorbed.
The Disparity in Cooling Rates
Different structures in the skin cool down at drastically different speeds based on their size.
The epidermis (specifically the melanosomes within it) is very small. Its TRT is extremely short, approximately 0.3 milliseconds. It heats up fast but also cools down almost instantly.
The hair follicle, conversely, is a much larger structure. Its TRT is significantly longer, typically ranging from 10 to 100 milliseconds. It holds onto heat for a much longer period.
Leveraging the Time Gap
Laser hair removal exploits this time gap.
If you use a nanosecond pulse (like in tattoo removal), the energy is delivered so quickly that even the tiny melanosomes in the skin cannot cool down, leading to surface damage.
By stretching the pulse to milliseconds, you exceed the TRT of the epidermis. This allows the skin to dissipate heat during the laser pulse itself, keeping it safe.
However, because the pulse is still within the TRT of the hair follicle, the follicle cannot cool down fast enough. The heat accumulates, leading to thermal destruction.
Deep Follicle Destruction
The goal of permanent hair reduction is not just to burn the hair shaft, but to destroy the hair papilla and stem cells.
Thermal Conduction
Destruction of the regenerative structures requires heat to travel from the melanin-rich hair shaft to the surrounding follicle walls and papilla.
This process, known as thermal conduction, is not instantaneous. It requires sustained heating over milliseconds.
Controlled Heat Accumulation
A millisecond-range pulse maintains the temperature of the hair shaft long enough for this conduction to occur.
If the pulse were too short, the shaft would vaporize without transferring enough lethal heat to the surrounding stem cells.
Understanding the Trade-offs
While millisecond pulses are the gold standard, improper calibration within this range can lead to treatment failure or adverse effects.
The Risk of Pulses Being Too Short
If the pulse width is set too short (closer to the microseconds), you encroach upon the TRT of the epidermis.
This reduces the skin's ability to dissipate heat, significantly increasing the risk of burns or hyperpigmentation, particularly in darker skin tones where epidermal melanin absorbs more energy.
The Risk of Pulses Being Too Long
If the pulse width is extended too far beyond the follicle's TRT (e.g., excessively long pulses on fine hair), the follicle will begin to cool down during the pulse.
This prevents the target from reaching the critical temperature required for destruction. The heat may also diffuse too broadly into the surrounding dermis, causing non-specific pain or inflammation without destroying the hair.
Making the Right Choice for Your Goal
The exact millisecond setting should be dynamic, adjusted based on the patient's physiology.
- If your primary focus is treating lighter skin (Fitzpatrick I-II): Use shorter pulse widths (e.g., 6 to 20 ms) to generate rapid, intense heat that effectively destroys the follicle.
- If your primary focus is treating darker skin (Fitzpatrick V-VI): Extend the pulse width (e.g., 15 to 34 ms or longer); this slower energy delivery allows the melanin-rich epidermis to cool while still heating the follicle.
- If your primary focus is treating coarse, thick hair: Use longer pulse widths to account for the larger volume of the hair, allowing sufficient time for heat to conduct through the entire structure.
Success relies on balancing the thermal retention of the follicle against the thermal dissipation of the skin.
Summary Table:
| Feature | Epidermis (Skin) | Hair Follicle | Long-Pulse Laser (ms) |
|---|---|---|---|
| Thermal Relaxation Time | ~0.3 ms (Very Short) | 10–100 ms (Long) | Optimized for Follicle TRT |
| Heat Reaction | Cools down almost instantly | Retains and accumulates heat | Allows skin cooling / Follicle damage |
| Mechanism | Protects from surface burns | Destroys papilla and stem cells | Controlled thermal conduction |
| Clinical Result | High safety profile | Permanent hair reduction | High efficacy for all skin types |
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References
- Paraskevas Kontoes, Savva Myrto. Hair induction after laser-assisted hair removal and its treatment. DOI: 10.1016/j.jaad.2005.09.034
This article is also based on technical information from Belislaser Knowledge Base .
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