Dermatologist Maurice Sullivan discusses the benefits of sunlight in treating some diseases; however, he notes that many diseases are caused by or aggravated by the sun. He shows a light spectrum chart and draws a diagram of the layers of the skin as they would appear under a microscope and then explains the effects different wavelengths have on the biologic activity of the skin. Ultraviolet rays, not filtered out by the stratum corneum, create the first stage of sunburn. Dr. Sullivan explains how tanning occurs then compares the stratum corneum depth and pigment of five studio models with different skin types. He shows a graph of major U.S. cities and skin cancer cases, which parallel the hours of sunlight and exposure in different geographic locations. To protect against sun damage, Dr. Sullivan recommends limiting exposure to the sun; using drugs such as quinine, zinc oxide, etc.; reapplying protective creams; and being aware of environmental conditions that filter or reflect the sun's rays.