Blackheads known medically as comedones are caused when the pore is only partially blocked. It is a myth that the black colour that gives them their name is caused by dirt. This is actually caused by the skin's pigment, known as melanin, reacting with the oxygen in the air.
Blackheads are formed when a pore gets clogged with excess oil, known as "sebum", and skin debris, referred to as "keratin". Unlike whiteheads, which are "closed" in by skin, blackheads are open. Their black appearance is caused by oxidation rather than poor hygiene or dirt.
Blackheads most commonly appear on the face, particularly around the nose, but they can occur all over the body.
Anyone can develop blackheads.