Sebum, produced naturally by the skin, making it feel oily, is composed of six or seven principle ingredients, including triglycerides and squalene, a precursor of cholesterol. The triglycerides are broken down on the surface of the skin by bacteria to free fatty acids, which can clog your pores and cause spots and even acne in genetically predisposed families.
Many cosmetics also contain esters of fatty acids, such as isopropyl myristate or butylstearate, which are even more potent than the fatty acids naturally produced by our skin in encouraging bacteria growth that leads to outbreaks.
One of the worst offenders is isopropyl myristate, used in hair rinses, hand lotions and fragrances to give them a slicker, sheer feel. A solvent that is readily absorbed by your skin, with overuse it can dry the skin and clog pores, which encourages bacterial growth and exacerbates acne.