An open comedo (single form of comedone, also known as a blackhead) is a clogged pore in the skin that is open to the air. Keratin (skin protein) and sebum (oil) combine to block the pore. They are often found on the face, but can be found anywhere on the body. Blackheads are actually not clogged with dirt, but exposure to the air causes oxidation, turning the internal contents black. Typically, they can be extracted using a comedone extractor.