## 4 Discussion
A previous systematic review (He 2012) (He 2012) concluded that light increases the risk of tooth sensitivity during in-office bleaching, and light may not improve the bleaching effect when high concentrations of HP (25-35% (\(w/v\))) are employed, but with lower concentrations the effect is significantly favorable for the activated groups (He 2012). This review and meta-analysis included 11 studies, of which three applied a hybrid LED/laser system, and only one study used a laser as the activation source. All other studies applied LED or halogen lamps. As stated above, in our analysis the control group would require a 24.15% higher HP level to achieve results similar to the laser-activated group. Additionally, and in contrast to the review of He (He 2012), there was no significant change in sensitivity arising from laser activation. Again, [PERSON] et al. in 2020 conducted another systematic review which compared high and low hydrogen peroxide (HP) concentration bleaching gels ([PERSON] 2019). Notwithstanding, it was highlighted that various aspects of the bleaching procedure were not taken into account in the analysis. These aspects included: (1) the duration of gel contact with dental tissue, (2) the number and frequency of bleaching applications, (3) the duration of each whitening session, (4) the utilization of light activation, (5) the age of the study population, (6) the pH of the products, and (7) the inclusion of additional substances in the whitening gels ([PERSON] 2019). Hence, in view of this high level of heterogeneity, results arising therefrom cannot be compared to the present study which also showed similar variations. Nevertheless, here the only two significant factors influencing the outcome significantly were HP concentration and the bleaching gel contact time. A further meta-analysis by SoutoMaior including 21 articles (out of which 11 used a LED/laser combination, only two used a laser in the test groups, and 14 used LED or halogen lamps amongst the test groups) concluded that light activation did not increase the bleaching efficacy in immediate, short, or medium terms of application ([PERSON] 2019). Unfortunately, the analysis did not provide any distinction between the types of light applied in the studies included. Once again, a high heterogeneity of data was recorded concerning activation sources and exposure time to the bleaching agent. Sensitivity incidence was not significantly different between light vs. non-light activated procedures. However, in terms of sensitivity intensity, light-activated methods demonstrated a significantly lower intensity of tooth sensitivity than that of the non-light systems. The authors attributed this finding to the use of the LED/laser HL (hybrid light), and by its potential analgesic and anti-inflammatory effects. Additionally, the reduced contact time was found to play a contributory role towards this favorable result. In our study, no significant differences in color outcome or sensitivity were found between irradiated vs. non-irradiated groups, but similarly to SoutoMaior ([PERSON] 2019), a significantly reduced contact time (of ca. 10 min) was revealed for the test groups. _Limitations of the Study:_  
One limitation of the study conducted in the current paper is the overall small number of \"qualifying\" studies that could be subjected to statistical analysis, both parametric and non-parametric. Indeed, for the clinical and in vitro studies explored, only n = 16/19 and 10/20 of these could be included, respectively, in the final statistical analysis conducted for color outcome (a), whereas for option (b) (tooth sensitivity and morphological alterations for clinical and in vitro studies respectively), only 13/19 and 8/20 could be assessed, respectively. These restrictions were applied in view of the availability of variable parameters and criteria for these evaluations, a sizeable proportion of which were lacking, most especially for the in vitro investigations.