Covered In What?

Did you know that the outermost layer of your skin is composed entirely of dead skin cells and oil? It's called the stratum corneum, and it protects the living skin cells underneath it by lubricating the skin and trapping in moisture [source: University of Iowa].

Oily Skin Causes

People with oily skin often wonder what they're doing to cause the problem. They may blame it on the greasy pizza they ate last night, but the truth is they should probably blame their genes. Diet can aggravate acne, but it doesn't cause oily skin [source: WebMD]. Oily skin is hereditary -- you can change your diet or change your lifestyle, but if your mom or dad has oily skin, chances are you will, too. However, if your oily skin comes and goes, it may have less to do with genes and more to do with hormonal shifts in your body.

Androgens are the hormones largely responsible for oil production, and sometimes they can get kicked up a notch -- particularly during puberty [source: Bouchez]. You go through a growth spurt, your voice changes, you start getting hair in weird places and, to top it all off, your body produces excess oil that clogs your pores. Similarly, androgen levels tend to increase in women just before a menstrual period, during pregnancy and during menopause [source: Poirot].

In a few cases, oily skin can be the result of your lifestyle. Taking oral contraceptives, for example, can alter hormone levels and increase oil production, and oily skin is one of several side effects associated with taking steroids [source: Reader's Digest]. Stress can also cause oily skin -- when you're tense, your body releases a hormone called cortisol, which can increase skin's oil production [source: WebMD].

Keep reading to learn what role pores play in oily skin.