THE VISIOUS CIRCLE OF ACNE
(Note: this is a simplified explanation of a complex skin condition)
During puberty, the reproduction/sex hormones become active and the amount of androgen hormones strongly increase. Suddenly much sebum is produced. This leads to oily hair, oily skin and in many cases to acne.
On the picture a cross-section of the skin with a hair is shown. We have a lot of those hairs, about 20-200 per cm². On the picture it is shown that the hair is penetrating the "dead" skin, and generates an opening to the outside world through the hair shaft. To the hair shaft sebaceous glands are present. Within the glands sebum is produced. This sebum is transported to the skin surface via the hair. The sebum creeps against the hair to the surface, and is scattered there.
In order to reach the surface, the sebum must therefore have a good viscosity. If the viscosity is too high it will be more difficult for the sebum to get to the surface, and it will accumulate in the sebaceous glands and in the hair shaft where the hair is located: as result comedones (black- and white heads) are formed. Within the sebaceous gland there is also a regular skin bacterium, Propionibacterium acnes, which preferably lives from sebum.
Because of the accumulated sebum the hair shaft clogs and eventually breaks open. Result: inflammation and pimples...
Propionibacterium acnes is optional anaerobic. That is to say that the bacteria prefer to live in the absence of oxygen, but to a certain extent accepts oxygen. Due to the high viscosity and thus the absence of oxygen the total results in a vicious circle of further P. acnes proliferation leading to more congestion, more P. Acnes, more infections, etc.