Low-level laser therapy (LLLT) and photobiomodulation (PBM) are increasingly popular non-invasive treatments used in physiotherapy, sports medicine, and rehabilitation. A common question about their use is whether they cause significant heat on the skin. Understanding the thermal effects of therapeutic lasers is important for ensuring safety and optimizing treatment outcomes. This article reviews and explains the findings of three important studies that examined the effects of therapeutic lasers at different wavelengths and settings on human skin temperature. These studies used lasers and LEDs at wavelengths such as 635 nm, 808 nm, 810 nm, and 904 nm. Each investigated how these light sources influence skin temperature and whether the treatment causes any harmful heating effects.
Effects of 810 and 904 nm Lasers on Human Skin (Joensen et al., 2011)
Joensen and colleagues (2011) studied how therapeutic lasers at 810 nm (continuous wave) and 904 nm (super-pulsed) wavelengths affect human skin temperature. Their research involved healthy volunteers who were exposed to laser treatment on the forearm. The study found that the 810 nm laser increased skin temperature by approximately 2.1°C when used with continuous wave output. This increase was considered mild and safe, but noticeable. In contrast, the 904 nm super-pulsed laser caused a temperature rise of only 0.3°C, which is nearly negligible.
The difference in thermal effects is mostly due to the way energy is delivered. Continuous wave lasers emit a steady beam of energy, which can gradually heat the skin. Super-pulsed lasers, on the other hand, deliver high peak power in very short pulses with low average power, allowing the skin to cool down between pulses. This reduces the overall heating effect.
Importantly, Joensen et al. concluded that both laser types are safe for human use, with minimal risk of overheating or burns when used correctly. Their findings support the use of therapeutic lasers in clinical settings, especially when considering the balance between therapeutic effects and patient comfort.
Thermal Effects of Combined Laser and LED Therapy (Grandinetti et al., 2015)
Grandinétti and colleagues (2015) explored how combining super-pulsed lasers (904 nm) with red (640 nm) and infrared (875 nm) LEDs affects skin temperature. This kind of combination is often used in clinical phototherapy devices. The researchers found that during a 300-second application, the maximum skin temperature increase was about 3.0°C. This increase was still within safe limits and did not cause discomfort.
Like Joensen et al., they emphasized that super-pulsed lasers cause minimal heating. However, the addition of LEDs—especially the continuous red and infrared ones—contributed to the overall temperature rise. The LED components deliver continuous or quasi-continuous light, leading to slightly more heating than pulsed lasers alone.
Despite this, the study showed that the thermal impact of combined therapy was well tolerated by participants. The temperature stayed far below dangerous levels (below 41°C), and the researchers confirmed that such devices can be safely used in regular treatments without thermal injury.
Influence of Skin Color and Tissue Thickness on Thermal Effects (Souza-Barros et al., 2018)
Souza-Barros and colleagues (2018) studied how skin color and tissue thickness affect the way therapeutic lasers at 635 nm (red) and 808 nm (infrared) interact with skin. They found that skin with higher melanin (darker skin types) absorbed more light and thus showed a slightly greater increase in skin temperature. Thicker skin tissues also affected how much light penetrated, but the influence on temperature was less significant.
The temperature increase during exposure was still relatively low in all skin types, showing that both wavelengths are safe across diverse populations. However, the study suggests that clinicians may need to consider individual differences—like skin color and tissue thickness—when adjusting treatment parameters for comfort and safety.
Their study also measured how much light was transmitted and reflected. While these physical properties varied by skin type and thickness, the thermal effects remained within safe levels. This supports the general safety of low-intensity laser therapy even in people with darker skin or different tissue structures.
Conclusion
Together, these studies show that therapeutic lasers, even when used with continuous or combined light sources, do not cause dangerous heating of the skin. Continuous wave lasers like the 810 nm may produce moderate warmth, while super-pulsed lasers like the 904 nm cause minimal or no thermal effect. Adding infrared and red LEDs may increase skin temperature slightly more, but the increase remains safe and comfortable for most users. Skin color and thickness can slightly affect how much heat is generated, but this does not lead to harmful levels.
These findings reassure clinicians and patients that low-level laser therapy, when properly used, is not only effective but also safe in terms of skin heating. Adjustments in treatment settings can be made based on individual skin characteristics, but there is no strong thermal barrier to using these technologies across a wide range of people.
References:
Joensen J, Demmink JH, Johnson MI, Iversen VV, Lopes-Martins RÁ, Bjordal JM. (2011). The thermal effects of therapeutic lasers with 810 and 904 nm wavelengths on human skin. Photomed Laser Surg. 29(3):145-153. doi:10.1089/pho.2010.2793
Grandinétti Vdos S, Miranda EF, Johnson DS, de Paiva PR, Tomazoni SS, Vanin AA, Albuquerque-Pontes GM, Frigo L, Marcos RL, de Carvalho Pde T, Leal-Junior EC. (2015). The thermal impact of phototherapy with concurrent super-pulsed lasers and red and infrared LEDs on human skin. Lasers Med Sci. 30(5):1575-1581. doi:10.1007/s10103-015-1755-0
Souza-Barros L, Dhaidan G, Maunula M, Solomon V, Gabison S, Lilge L, Nussbaum EL. (2018). Skin color and tissue thickness effects on transmittance, reflectance, and skin temperature when using 635 and 808 nm lasers in low intensity therapeutics. Lasers Surg Med. 50(4):291-301. doi:10.1002/lsm.22760
