The principles used in toxicological risk assessment, are: (i) hazard identification, (ii) dose-response, and (iii) exposure. In considering the hazard identification for laser devices, the mechanism or mode of action is particularly important. Specifically, by knowing the mode/mechanism of action and information around dose-responsivity, the adverse event profile can be predicted. In this regard, the work of Anderson et al. [40,41] which introduced the concept of selective photothermolysis as the mechanism of action of lasers, is central to our understanding of such devices and their therapeutic/adverse event profiles. Selective photothermolysis is quite straightforward, to the extent that a chromophore target is identified and its absorption profile understood. Based on this information, a device can be selected which emits a monochromatic wavelength of light to specifically target the chromophore. The aim of photothermolysis is to produce thermal damage in the target, while minimizing damage to the surrounding tissue. Nearly every laser treatment modality has a component of photothermolysis as part of its therapeutic benefit [42-54].
A component of photothermolysis is the thermal relaxation time [55]. As stated by Choi and Welch [55], the concept of thermal relaxation time is used to determine the pulse-width for laser light in such a way that the heat generated within the target structure due to absorption of photon energy produces maximum damage without damaging the surrounding tissue. In other words, to limit damage beyond the target, the laser “dose” (i. e., energy + duration of exposure) must be sufficiently short so that absorption is limited to the chromophore at the site or target, and not the surrounding tissue. By and large, the dermal safety of laser devices commonly used in dermatology is based on photothermolysis/thermal relaxation time.