Pearl Weena Marie E. Sabido, MD, FPDS
The sun emits electromagnetic radiation, of which the portions of concern to us are ultraviolet radiation (UVR, 200-400nm), visible light (400-700nm), and infrared (700nm-1mm). UVR is broken down into UVC (200-290nm), which is largely filtered out by the ozone layer and does not reach the earth’s surface; UVB (290-320nm); and UVA, which is further broken down into UVA II (320-340nm) and UVA I, or long-wave UVA (340-400nm) (Fig. 1). UVA, which comprises 95 percent of UVR that reaches the earth, is relatively stable throughout the day. Being longer than UVB, it penetrates deeper into the skin, down to the deep dermis. On the other hand, UVB makes up the remaining 5 percent of UVR that reaches the earth’s surface, and its strength peaks between 10am and 2pm. It has a shorter penetration into the skin, generally reaching down to the basal layer, or at best to the upper dermis.