Which of the following causes blackheads?
Accumulation of sebum, followed by oxidation
The sebaceous glands of the skin give out an oily secretion called sebum. An accumulation of sebum in the pores of the skin, followed by oxidation by exposure to the air, can result in blackheads. The black colour is given by melanin and the accumulated sebum. In cold and dry weather, the sebaceous glands typically don't secrete enough sebum, leaving the skin dry and perhaps flaky. Hormonal imbalances such as those in adolescents, can result in acne.