The 1064nm wavelength exhibits relatively low absorption by chromophores such as melanin, enabling deep penetration into the dermis. While this characteristic is advantageous for treating deep-seated pigmentation, it also introduces potential safety concerns:

When treating light-colored or superficially located pigmented lesions, only a fraction of the laser energy is absorbed and disrupts the target pigment. A significant portion of the energy continues to penetrate into deeper tissue layers, potentially causing unintended photothermal or photomechanical effects along its path.

Consequently, if the same site is repeatedly exposed to high-fluence irradiation—especially with a spot size exceeding 4mm—there is a risk of "through-and-through" injury deep within the dermis, leading to severe cutaneous adverse events.

This is particularly critical in the treatment of benign pigmentary disorders such as melasma, solar lentigines, light freckles, and superficial seborrheic keratosis. Using high-fluence 1064nm settings may cause pinpoint bleeding or epidermal disruption, which can trigger post-inflammatory hyperpigmentation (PIH), exacerbate melasma, expand the lesion area, and provoke deep inflammatory pigmentation.

Special Considerations for Picosecond Lasers

Picosecond lasers, characterized by ultrashort pulse durations and extremely high peak powers, predominantly induce microscopic "cavitation effects" that produce subtle and rapid cellular disruptions. However, they also exhibit a more pronounced temporal lag effect—clinically visible inflammatory responses often take 3 to 5 minutes to manifest after irradiation.

This delayed response poses a significant risk: without appropriate dose adjustment based on the lag effect, overtreatment can easily occur, leading to secondary injuries including aggravated PIH, skin ulceration, or even tissue loss. This is the fundamental reason for the high incidence of adverse reactions observed in melasma treatments using picosecond lasers. The shorter the pulse duration, the more noticeable the lag effect in observing "normal" therapeutic endpoints.

Reference for Normal Treatment Response

Under standard safe operating parameters, the "frosting" phenomenon—immediate skin whitening—is typically observed only when treating dark tattoos or deeply pigmented dermal/epidermal lesions such as dark Ota nevi or lentigo simplex. In these cases, the skin immediately turns grayish-white and may subsequently exhibit mild pinpoint bleeding, which represents an expected therapeutic endpoint rather than an adverse event.

Understanding the boundaries of laser-tissue interactions and exercising prudent dose control are fundamental to achieving both efficacy and safety in clinical practice.