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 cause spots and even acne in genetically predisposed families.
Many cosmetics also contain these kinds of acids such as stearic acid and esters of fatty acids such as isopropyl myristate or butylstearate which are more potent even than our own fatty acids in the productionof some acne lesions.
One of the worse offenders called isopropyl myristate,which is absorbed readily by the skin, helps cosmetics apply more smoothly and gives them a slicker, sheer feel. Isopropyl myristate, an alcohol used in hair rinses, hand lotions and fragrances, is also a solvent that dries the skin and hair, and creates cracks and fissures in the skin, which encourage bacterial growth.