What causes adult acne? Acne is directly caused when follicles become blocked due to an over production of sebum - that’s the oil content in the skin on your face. The excess sebum mixes with dead skin cells and debris, clogging up the follicles and, in turn, bacteria that naturally live on the skin contaminate these clogged follicles, and out come papules, pustules, nodules or cysts – whatever name you like best. It’s the hormone fluctuations that cause this oil production to accelerate – whether the imbalance is brought on by lifestyle, PMS, pregnancy, or even polycystic ovary syndrome (PCOS), the result is the same. And it’s not pretty. But before you jump to the latter conclusions, first take into consideration the following: stress, pollution and diet which can all lead to hormonal imbalance, the main cause of acne in adults.