The purpose Tooth whitening is becoming one of the most popular esthetic and corrective treatments for discolored teeth. Bleaching can be performed internally on non-vital teeth or externally on vital teeth by applying hydrogen peroxide, sodium perborate or carbamide peroxide, the most common agents used for bleaching. For acceleration or more effective tooth whitening, different light sources may be used. When the bleaching agent is activated under the influence of light, some amount of light is absorbed and the resulting energy is converted into heat. This can be perceived as a possible side effect during this type of tooth whitening. Therefore, light sources can have photothermal effects which are then associated with the chemical effect of the bleaching materials. In light-activated tooth bleaching procedures, there is a great concern about the heat generated by the light source, which may cause pulp irritation or severe damage such as necrosis. One of the purposes of the present study was to evaluate the surface and intrapulpal temperature changes after bleaching treatment with different gels of hydrogen peroxide (in further text HP) and carbamide peroxide (in further text CP) subjected to different sources of light activation (LED405, OLED, focused and unfocused femtosecond laser and ZOOM2). Also, one of the aims was to evaluate the influence of five bleaching agents on surface microstructure, change in chemical structure and microhardness of human tooth enamel and dentine as well as the change in color of stained specimens before and after the bleaching with different bleaching agents supported by new light sources. Finally, a possible genotoxic effect of two bleaching gels with high concentration of hydrogen peroxide on oral mucosa was investigated. Materials and methods Light sources used in this study were: LED405, OLED, femtosecond laser and ZOOM2. Each experimental group was treated with one of the following: 25% and 38% HP gel and 10%, 16% and 30% CP bleaching gels. For intrapulpal and surface temperature measurements, K-type thermocouple and infrared thermometer were used to repeatedly measure the temperature increase. Tooth surface was treated with five bleaching agents and vaseline which served as a control. For temperature measurements, we used extracted human maxillary central incisors and canines. Vickers microhardness was measured with a load of 100 g for 10 seconds at the baseline, after the last bleaching treatment and after 2 weeks storage in artificial saliva and surface treatment with amorphous calcium phosphate gel (in further text ACP) or 2 weeks storage in deionized water. Enamel and dentine surface morphology was observed under scanning electron microscopy (SEM) while structural and chemical changes were evaluated using energy-dispersive X-ray spectroscopy (EDS). For microhardness as well as SEM and EDS measurements, extracted human third molars were used. Change in color was determined by RGB colorimeter and UV VIS NIS spectrophotometer. For color measurement, specially made pastilles of hydroxyapatite were used. Genotoxic effect of two bleaching agents was analyzed using micronucleus test and the research was conducted on 22 human subjects. Results The average increases in the pulp chamber and tooth surface temperatures for LED405, OLED and unfocused femtosecond laser were below the critical temperature threshold of 5.5C, contrary to the treatments involving ZOOM2 and focused femtosecond laser, regardless of bleaching gel application. All bleaching agents showed significant reduction in surface microhardness. ZOOM2, which had the lower pH value (pH=3.20), showed larger decrease in surface microhardness compared to BOOST (pH=6.75) and 30 %, 16% and 10 % CP (pH=7.0). The treatment with ACP and artificial saliva can increase microhardness and reduce the mineral loss after bleaching treatment. After the bleaching procedure with highly concentrated HP and CP gels, enamel and dentine surface microstructure showed mild or slight alterations with no loss of superficial structure, while low concentration CP gels show no change in morphological structure of tested hard dental tissue. A statistically significant increase in Ca and F ions was found after the bleaching treatment and after the additional treatment with ACP and artificial saliva. The change in color before and after the whitening treatment with different light sources was measured using RGB and UV VIS NIR spectral analysis. After 30 minutes of bleaching treatment with LED405, there was a statistically significant increase in RGB index in comparison to OLED and without light activation for 10% CP, 16% CP, 30% CP, 25 % HP and 38 % HP gels. For femtosecond laser, a statistically significant increase in RGB index was found only when 16 % CP and 38 % HP were used. Finally, the difference between LED405 and femtosecond laser was statistically significant only when 25 % HP gel was used. Both of the tested bleaching agents (25% HP and 38 % HP) demonstrated a potential genotoxic effect. Statistically significant increase of genotoxicity markers was relatively small in amount. Significance Focused femtosecond laser and ZOOM2 produced a large temperature increase in the pulp chamber and at the tooth surface. Caution is advised when using these types of light activation, whereas LED405, OLED and unfocused femtosecond laser could be safely used. Although the application of the bleaching agents reduced temperature variability, the type of the bleaching agent applied was largely irrelevant, indicating similar insulating properties of different gels of HP and CP. All of the used bleaching agents resulted in a reduction of surface enamel and dentine microhardness. The application of ACP remineralizing agents in combination with artificial saliva can cause an increase in surface microhardness. A change in chemical structure of enamel and dentine after bleaching with different bleaching gels and after ACP treatment was found for Ca and F ions which can be a sign of possible remineralization. In combination with the used bleaching agents, LED 405 showed the best effect in color change, followed by femtosecond laser. OLED light showed the weakest effect in color change. Regarding the genotoxic effect of bleaching agents used in this study, it is important to say that oral mucosa cells have a short lifespan and a one-time exposure to such mild genotoxic noxa has probably a negligible carcinogenic potential. Therefore, bleaching can affect the genome of mucosal cells to a certain extent; however, it is difficult to assess clinical significance of these findings.