McGill University, Montreal, Québec, Canada.
Curr Probl Dermatol. 2021;55:374-384. doi: 10.1159/000517645. Epub 2021 Oct 25.
Infrared light (760 nm-1 mm) constitutes approximately 40% of the solar radiation reaching the ground at sea level. Shortest wavelength near-infrared (NIR) photons (NIR or IR-A: 760-1,400 nm) can penetrate the epidermis, dermis, and subcutaneous tissue with numerous biological effects. NIR used to have a bad reputation on the basis of past studies using high-intensity artificial light sources (above the solar IR-A irradiance threshold) at high doses leading to detrimental effects (i.e., upregulation of matrix metalloproteinase-1). However, when looking at the other side of the coin and what we can learn from the sun, NIR intensity matters. Hence, mimicking sunlight NIR intensity (30-35 mW/cm2) will rather trigger beneficial cutaneous effects. It is likely that intensity is more important than the fluence (dose) delivered. Moreover, the law of reciprocity (i.e., the biological effect is directly proportional to the total dose irrespective of intensity) does not always apply when considering tissue response in photobiology. In fact, the biphasic dose curve (Arndt-Schulz curve) of photobiomodulation establishes that if irradiance is lower than the physiological threshold value for a given target, it does not produce beneficial effects, even when -irradiation duration is extended. Also, photo-inhibitory deleterious effects may occur at higher irradiances. Remarkably, the beneficial "sweet spot" in between corresponds to the irradiance of the sun. NIR might even precondition the skin from an evolutionary standpoint as exposure to early morning NIR wavelengths in sunlight may prepare the skin for upcoming mid-day harmful UVR. Consequently, NIR light appears to be the solution, not the problem.
红外光（760nm-1mm）约占海平面到达地面的太阳辐射的 40%。最短波长的近红外光（NIR 或 IR-A：760-1400nm）可以穿透表皮、真皮和皮下组织，具有多种生物学效应。过去的研究使用高强度人工光源（高于太阳 IR-A 辐照度阈值）和高剂量的近红外光，导致了有害的影响（即基质金属蛋白酶-1 的上调），因此近红外光曾有过不良的声誉。然而，当我们从太阳的角度来看待另一面时，我们可以从中学到一些东西，那就是近红外光的强度很重要。因此，模拟阳光的近红外光强度（30-35mW/cm2）将更有可能引发有益的皮肤效应。很可能强度比剂量（剂量）更重要。此外，当考虑到光生物学中的组织反应时，reciprocity 法则（即生物效应与总剂量直接成正比，与强度无关）并不总是适用。事实上，光生物调节的双相剂量曲线（Arndt-Schulz 曲线）表明，如果辐照度低于给定目标的生理阈值，即使延长照射时间，也不会产生有益的效果。此外，在更高的辐照度下可能会发生光抑制有害的影响。值得注意的是，有益的“甜蜜点”正好对应于太阳的辐照度。从进化的角度来看，近红外光甚至可以使皮肤预先适应，因为暴露在清晨的阳光中的近红外波长可能会使皮肤为即将到来的中午有害的 UVR 做好准备。因此，近红外光似乎是解决方案，而不是问题。
