Beneficial Applications and Deleterious Effects of Near-Infrared from Biological and Medical Perspectives

Over half of solar energy consists of near-infrared and a wide range of preventative mechanisms have been evolutionarily maintained in organisms to protect against effects of near-infrared. However, the biological effects of near-infrared have not been investigated in detail. Despite the essential requirement of a water-filter to imitate solar near-infrared filtered by atmospheric water, previous studies used near-infrared resources without a water-filter or a cooling system. With these methods, near-infrared energy is primarily absorbed in the superficial tissues, thus these approaches are unable to sufficiently evaluate the biological effects of solar near-infrared that reaches human tissue. We have elucidated that near-infrared (1100 1800 nm together with a water-filter that excludes wavelengths 1400 1500 nm) non-thermally affects the skin into the deeper tissues. The biological effects of near-infrared have both beneficial applications and deleterious effects. Near-infrared induces collagen and elastin stimulation, which achieves skin rejuvenation and skin tightening, and induces long-lasting vasodilation that may prevent vasospasm and be beneficial for ischemic disorders. Near-infrared also relaxes and weakens dystonic and hypertrophic muscles to reduce wrinkles and myalgia. Nearinfrared is an essential tool in cancer detection and imaging, and induces drastic non-thermal DNA damage of mitotic cells, which may be beneficial for treating cancer. Activation of stem cells by near-infrared may be useful in regenerative medicine. However, continuous near-infrared exposure induces photoaging and potentially photocarcinogenesis. Humans have protective mechanisms against near-infrared on multiple levels, including perspiration, blisters, vasodilation, hair, skin, adipose tissue, and cotton or wool clothing. Further protection should be considered, as biological effects of near-infrared are significant, and standard sunscreens and glasses cannot sufficiently block near-infrared. This paper reviews the effects of near-infrared and introduces the new findings of near-infrared from a biological point of view.