Oily skin, with visible irregularities
In adolescence, oily skin can easily show irregularities. The biological cause of this change is an increase in the quantity of sebum, combined with a change in sebum quality.
Thickened skin, clogged pores, blocked sebum, bacterial growth... a vicious circle sets in and visible irregularities begin to show. These irregularities that appear can sometimes continue through to adulthood.
Environment and diet can also play a role.