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

Over half of the solar energy consists of near-infrared, and in addition to natural near-infrared, humans are increasingly exposed to artificial near-infrared from electrical appliances. Thus, we are exposed to tremendous amounts of near-infrared. Despite the wide prevalence of a variety of ultraviolet blocking materials, the necessity to protect against near-infrared has not been well recognized. To clarify the necessity to protect against near-infrared, we assessed cell viability of human fibroblast cells after water-filtered broad-spectrum near-infrared (1100 -1800 nm together with a water-filter that excludes wavelengths 1400 -1500 nm) treatment using 2 sets of transparent polycarbonate plates, one to block ultraviolet and the other to block both ultraviolet and near-infrared. The cell viability was significantly decreased after 10 rounds of near-infrared irradiation at 20 J⁄cm 2 in near-infrared treated cells without a protective polycarbonate plate and near-infrared treated cells using the polycarbonate plate to block only ultraviolet. Assuming that the cell viability of the non-irradiated control to be 100, the cell viability of the near-infrared treated cells without any protection was 0.2. The cell viability of the near-infrared treated cells with the polycarbonate plate to block only ultraviolet was 0.3, whereas both ultraviolet and nearinfrared protected cells retained a viability of 85.1. The results of this study indicate that protection from not only ultraviolet but also near-infrared should be considered to prevent skin damage.