Because laser light energy is transformed into heat upon absorption by melanin, the change in tissue temperature, and the rate and time period of this temperature rise determines the tissue response. In a study using real-time thermal imaging, the temperature change in response to ruby laser was determined to be in a range of 5-10°C with heat dissipation occurring 2 s after exposure [13]. Cellular injury with subsequent inflammation and repair occurs after tissue temperature increases by only 5-10°C [55]. Deactivation of most cellular enzymes begins to occur at a temperature of 40- 45°C, with an initial reversible damage that can become irreversible with sustained exposure. Temperatures above 60°C lead to denaturation of most proteins, whereas a further increase above 70°C leads to dena – turation of DNA. Vaporization of tissue water with cell shrinkage, hyperchromasia, membrane rupture, protein denaturation, and collagen hyalinization occurs at temperatures of 60°C-140°C [56]. Though thermal injury is probably the predominant mechanism for hair reduction, a role of photomechanical damage has also been suggested to contribute to the same, whereby the laser-induced sharp temperature gradient between melanosomes and the surrounding structures are thought to lead to a thermal expansion and generation, and propagation of acoustic waves that can mechanically damage the melanosome-laden cells [57].
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