To maximize safety on Asian skin, a pulse width of approximately 20 milliseconds is used to exploit the difference in cooling rates between the skin and the hair follicle. This specific duration allows the laser to bypass the melanin in the skin's surface—preventing burns—while effectively heating the hair root to the point of destruction.
The Core Mechanism The 20ms setting relies on "Selective Photothermolysis." It strikes a precise balance where the laser pulse is slow enough to let the skin cool down, yet fast enough to cook the hair follicle before it can release its heat.
The Science of Thermal Relaxation Time (TRT)
To understand why 20ms is 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 Biological Discrepancy
Laser hair removal works because different structures in your skin cool down at different speeds.
According to clinical data, the epidermis (top layer of skin) has a very short TRT, typically between 3 to 10 milliseconds. It heats up fast but also cools down fast.
Conversely, hair follicles are larger (0.2 to 0.3 mm diameter) and retain heat much longer. Their TRT ranges from 40 to 100 milliseconds.
The "Safe Zone" Calculation
The 20ms pulse width is calculated to fit perfectly between these two figures.
Because 20ms is longer than the epidermis's TRT (3–10ms), the skin has enough time during the pulse to dissipate the heat into the surrounding tissue. This prevents the rapid temperature spikes that cause blistering.
However, 20ms is shorter or sufficiently intense relative to the follicle's capacity to hold heat. The follicle cannot cool down fast enough during this window, leading to the thermal damage required to stop hair growth.
Why This Matters for Asian Skin
Treating Asian skin requires navigating a specific physiological challenge: the presence of higher concentrations of melanin in the epidermis.
The Melanin Competitor
In laser treatments, melanin is the "target" (chromophore) that absorbs heat. In lighter skin, melanin is mostly in the hair. In Asian skin, melanin is present in both the hair and the epidermis.
This makes the epidermis a competing target. If the laser treats the skin like the hair, it will burn the surface.
Avoiding Surface Damage
By extending the pulse width to 20ms (and up to 40ms), clinicians ensure the epidermis is not overwhelmed.
Short pulses (such as those used in Q-switched modes) deliver energy too quickly for melanin-rich skin to handle. This causes immediate thermal injury. The 20ms pulse is essentially a "slower delivery" of energy that melanin-rich skin can tolerate safely.
Understanding the Trade-offs
While 20ms is a highly effective baseline, laser parameters are never "one size fits all." Understanding the limits is crucial for avoiding adverse effects.
The Risk of Going Too Short
If the pulse width is significantly shorter than 10ms, you risk falling inside the epidermis's TRT window.
In this scenario, the skin absorbs energy faster than it can release it. For Asian skin, this drastically increases the risk of burns, blistering, and post-inflammatory hyperpigmentation (dark spots left behind after healing).
The Risk of Going Too Long
While longer pulses are safer, there is a point of diminishing returns.
If the pulse width extends too far (e.g., beyond 100ms for finer hair), the hair follicle may have enough time to dissipate heat during the pulse. This results in sub-optimal heating, where the follicle is warmed but not destroyed, leading to ineffective treatment.
Making the Right Choice for Your Goal
The 20ms standard is a starting point, but optimal results depend on adjusting to the specific patient profile.
- If your primary focus is Safety on Darker Skin (Fitzpatrick IV-VI): Increase the pulse width toward 30ms or 40ms to provide the epidermis more time to cool, further reducing the risk of pigmentation changes.
- If your primary focus is Treating Very Dark Skin (Type VI): Consider ultra-long pulse widths (up to 100ms), which allow for higher energy densities to be delivered safely by significantly slowing the heating rate of epidermal melanin.
- If your primary focus is Efficacy on Standard Asian Skin: Maintain the 20ms setting to ensure the pulse remains aggressive enough to destroy the follicle while utilizing the natural cooling speed of the epidermis to protect the surface.
Ultimately, the 20ms pulse width is the engineered solution to the biological problem of destroying a target (hair) without harming the shield (skin) that covers it.
Summary Table:
| Factor | Epidermis (Skin Layer) | Hair Follicle | Optimal Setting |
|---|---|---|---|
| TRT (Cooling Time) | 3 – 10 milliseconds | 40 – 100 milliseconds | N/A |
| Thermal Behavior | Heats and cools rapidly | Retains heat longer | N/A |
| Pulse Width Action | Heat dissipates (No burn) | Heat accumulates (Destruction) | 20ms |
| Asian Skin Risk | High (Epidermal Melanin) | High (Target) | Balance Safety & Power |
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References
- Shiuh-Yen Lu, Yih-Ying Wu. Hair Removal by Long-Pulse Alexandrite Laser in Oriental Patients. DOI: 10.1097/00000637-200110000-00008
This article is also based on technical information from Belislaser Knowledge Base .
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