On top of genetics and underlying health conditions, there are a few external factors that can cause darkening in the area. Deodorants and antiperspirants have ingredients that can irritate the skin, and any inflammation can lead to a thickening — and darkening — of the skin over time. "Many antiperspirants use aluminum as an active ingredient," says Dr. Solomon. "It clogs up sweat ducts and, in the process, irritates the skin." Other ingredients to look out for are fragrance and alcohol, both of which can be harsh on sensitive skin types. Another source of irritation: your razor. Shaving, especially on dry skin or with a dull blade, can cause the skin to darken and thicken over time.