As the follicles are obstructed, dead skin does not get expelled, which causes a whitehead. In contrast to blackheads, whiteheads are closed comedones, as the air cannot reach the follicle and it remains white. They are smaller in size than the blackheads. The reason why they are seen mostly during the teenage is that the sebaceous glands, which produce sebum, are most active during this period. Over-active sebaceous glands can also increase the level of the hormone androgen, which in turn increases the production of sebum.