In adolescence, oily skin can easily become acne-prone with blackheads and spots. The biological cause of this change is an increase in the quantity of sebum (hyperseborrhoea), combined with a change in sebum quality (dysseborrhoea).
Thickened skin, clogged pores, blocked sebum, bacterial growth... a vicious circle sets in and comedones turn into inflamed red spots.
Environment and diet also play a role in acne. The condition can also affect adults, with adult women suffering from acne more than men.