When a hair follicle gets clogged with sebum, bacteria and dead skin cells it forms a comedo. Sebum is the oily, waxy material secreted by the sebaceous glands connected to the hair follicles. Naturally this sebum is white, but if the comedo content reaches the surface of the skin, the bump appears black and is called a “blackhead” or “open comedo”. The black colour is a result of the sebum oxidizing when getting in contact with the air. Therefore, the dark colour is not caused by dirt, as many people think.