Infrared Light: Its Significance within the Solar Spectrum and its Influence on Human Physiology and Sun Exposure?

The infrared wavelength of the solar spectrum constitutes approximately 40% of solar radiation reaching the ground at sea level. The shortest wavelength, near infrared (NIR or IRA) 760-1400nm penetrates epidermis, dermis and subcutaneous tissue with biological effects. High intensity artificial sources of infrared light can have deleterious effects with upregulation of matrix mellatoproteinase-1. However, these wavelengths when mimicking sunlight intensity (30-35mW/cm2) can have beneficial cutaneous effects, corresponding to the irradiance of sunlight. Humans have evolved with exposure to the potentially carcinogenic effects of sun exposure successfully until the last century. We need to consider the factors that have changed to where skin cancer is one of the most frequent cancers afflicting modern Caucasian man with melanoma being responsible for a relatively high death rate amongst young adults. NIR might precondition the skin with early morning sun exposure preparing the skin for midday exposure with its higher levels of the harmful shorter wavelengths of UVA and B. With the sun low to the horizon at dawn, sunlight must pass through a thicker layer of atmosphere to reach the skin. This tends to attenuate the shorter wavelengths (UVR) that have higher intensity but less penetration, and accentuate the longer wavelengths that have less intensity but greater penetration. Thus, the red highlights in the sky and clouds seen at twilight, red light being at the longer end of the visible spectrum and potentially providing twilight protective and reparative solar radiation essential to maintain a balance between harm and beneficial effects of solar exposure.