Ilex paraguariensis. See yerba mate extract.

Illicium vernum. See anise.

imidazolidinyl urea. Formaldehyde-releasing preservative (Source: Contact Dermatitis,
December 2000, pages 339–343). See formaldehyde-releasing preservative.

inactive ingredient. The list of inactive ingredients is the part of an ingredient label that is not regulated by the FDA other than the requirement that it be a complete list of the contents in descending order of concentration; that is, the ingredient with the largest concentration is listed first, then the next largest, and so forth. Thousands and thousands of inactive ingredients are used in cosmetics, and there is controversy about how truly inactive these substances are in regard to safety as well as about their long-term or short-term effects on skin or the human body.

inositol. Major component of lecithin that may have water-binding properties for skin. It is not a vitamin, although it is sometimes mistakenly thought of as a B vitamin.

insulinlike growth factor (IGF). Stimulates fat cells and connective tissue cells. See human growth factor.

intercellular matrix. “Mortar” that holds layers of skin cells together, creating a contiguous natural, external barrier. Preserving the intercellular layer intact keeps bacteria out, moisture in, and the skin’s surface smooth. Skin’s intercellular matrix (also referred to in this book as skin-identical ingredients) includes ceramides, hyaluronic acid, vitamin C, glycerin, cholesterol, and free fatty acids. See natural moisturizing factor (NMF).

interleukin (IL). Stimulates growth of white blood cells. See human growth factor.

Inula helenium. See elecampane.

iodopropynyl butylcarbamate. Used as a preservative in cosmetics. See preservatives.

Iris florentina extract. See orris root.

Irish moss extract. Type of red algae. See algae.

iron oxides. Compounds of iron that are used as colorings in some cosmetics. They also are used as a metal polish called jewelers’ rouge, and are well-known in their crude form as rust.

isobutyl acetate. See solvent.

isobutylparaben. See parabens.

isocetyl salicylate. See sodium salicylate.

isododecane. Hydrocarbon ingredient used as a solvent. Isododecane enhances the spreadability of products and has a weightless feel on skin. All hydrocarbons used in cosmetics help prevent the evaporation of water from the skin.

isoflavone. Plant estrogen with potent antioxidant properties (Source: Free Radical Biology and Medicine, December 2001, pages 1570–1581).

isohexadecane. Used as a detergent cleansing agent, emulsifier, and thickening agent in cosmetics.

isoleucine. See amino acid.

isoparaffin. See paraffin.

isopropyl alcohol. See alcohol.

isopropyl lanolate. Derived from lanolin, it is used in cosmetics as a thickening agent and emollient.

isopropyl myristate. Used in cosmetics as a thickening agent and emollient. Historically, animal testing has shown it causes clogged pores (Source: Archives of Dermatology, June 1986, pages 660–665). Results derived from animal testing were eventually considered unreliable, however, and there is no subsequent research showing this ingredient is any more of a problem for skin than other emollient, waxy, thickening ingredients used in cosmetics.

isopropyl palmitate. Used in cosmetics as a thickening agent and emollient. As is true for any emollient or thickening agents, it can potentially clog pores, depending on the amount in the product and your skin’s response.

isostearamide DEA. Used as a surfactant, water-binding agent, and thickening agent. See surfactant, water-binding agent, and thickening agent.

isostearic acid. Fatty acid used as a binding agent and thickener. See fatty acid.

isotretinoin. See Accutane.

ivy extract. See English ivy extract.