## 4 Discussion
### Pulp Temperature Dynamics and Fluence Considerations in Dental Bleaching Procedures  
The duration of a temperature increase and the retention of heat that can potentially cause pulp damage are crucial factors to consider [91]. Indeed, a 5 \({}^{\circ}\)C increase with a duration of only 1 min is a critical threshold for maintaining pulp health [30; 105]. However, heat dissipation occurs rapidly according to the concept of Tissue Relaxation Time (TRT), which is directly proportional to the square of the diameter of the irradiated tissue, and inversely proportional to its diffusivity [15]. Although the heat transfer through blood flow within the pulp is very low [32], the flow within the periodontal ligament may play a crucial role [106]. Furthermore, a recent spectrophotometric investigation examined the light attenuation of various bleaching gels [15]. This study integrated theoretical calculations based on [PERSON]'s research [107], and included calculations of tissue relaxation time [108]. The findings from this investigation demonstrated that for a 1 mm layer of bleaching gel with light attenuation of approximately 80% and a 4 mm target area (calculated mean of pulp area) [109], the maximum acceptable fluence was determined to be 93.5 J/cm\({}^{2}\).  
Additionally, based on prior estimations of approximately 80% light attenuation within the bleaching gels, and the established threshold of 5-10 J/cm\({}^{2}\) required for photobiomodulation (PBM) [110], and specifically for pulp tissue exposed to HP [13; 19; 98], we may suggest that a fluence value of 25-50 J/cm\({}^{2}\) would be the preferable choice for dental bleaching. Under these conditions, concurrent PBM may potentially occur, which could lead to advantageous effects on the dental pulp. Nevertheless, this is a value which should be confirmed by further research.  
In the present systematic review, the fluences employed in the studies featured varied between 12.5 and 180 J/cm\({}^{2}\). Only one clinical and one in vitro study reached the upper limit of this threshold fluence. Importantly, it was evident that studies employing high fluence parameters exhibited more detrimental effects compared to those with lower parameters [54; 89]. Notably, only four of the included in vitro studies investigated pulp temperature increases, as shown in Table 7 [7; 84; 89; 91].  
[PERSON]'s study [76] utilized a 532 nm wavelength laser with a fluence of 84 J/cm\({}^{2}\) for 20 s, which resulted in a temperature increase of 5.13 \({}^{\circ}\)C. In contrast, [PERSON] [91] employed a fluence of 17.7 J/cm\({}^{2}\) at 980 nm for 20 s, leading to a 6.7 \({}^{\circ}\)C temperature rise. However, [PERSON]'s research [89] applied a fluence of 180 J/cm\({}^{2}\) at 980 nm for 30 s, resulting in a substantial 14.06 \({}^{\circ}\)C temperature increase. Notwithstanding, [PERSON]'s study [84] employed a fluence of 72 J/cm\({}^{2}\) at 980 nm for 30 s, which resulted in a more modest 2.63 \({}^{\circ}\)C temperature increase.  
The considerable variance in results can be attributed to various factors, including differences in parameters and methodology [111]. For example, exceptionally lower results may arise from the utilization of a water bath, while excessively higher results might be linked to the direct heat absorption by the thermocouple [112]. Interestingly, in one study ([PERSON] et al.) [84], the temperature was continuously monitored between two laser activation cycles, and it exhibited a decline back to the initial temperature after a resting period of 2.5 min.