Blackheads form when hair follicles (pores) on the skin become clogged with excess oil (sebum) and dead skin cells. Sebaceous glands produce sebum to keep skin soft, but sometimes they produce too much, causing the excess oil to combine with dead skin cells and clog the opening of the follicle. When this clog remains open to the air, the material inside oxidizes and turns black, forming a blackhead. This is why blackheads get their dark appearance—it is not dirt but the oxidized sebum and skin cells exposed to air. Factors contributing to blackhead formation include overproduction of sebum, hormonal changes (such as puberty), irritation, excess dead skin cells, certain medications, skin trauma from squeezing, and stress. Blackheads commonly appear on the face, chest, back, neck, and shoulders where pores are larger or more numerous.