quaternium-15. Formaldehyde-releasing preservative used in cosmetics. It can be a skin sensitizer, as can all preservatives.

quaternium-18 hectorite. Used as a suspending agent and also has emulsifying properties.

quercetin. Flavonoid pigment that has antioxidant and anti-inflammatory properties. Quercetin has antihistamine properties when taken internally via foods (e.g., apples, red wine, and buckwheat natural sources) or supplement form (Source: www.naturaldatabase.com). See bioflavonoid.

quercus. See oak root extract.

Quercus infectoria extract. See oak root extract.

quillaja extract. Extract of the Chilean soap bark tree. It contains a good amount of saponins, which have cleansing, antimicrobial, and water-binding properties for skin. See saponin.

quince seed. Used as a thickening agent in cosmetics, but also has skin-constricting properties (Source: www.naturaldatabase.com) and may cause skin irritation.

quinoa oil. Derived from quinoa grain, it may have antifungal properties (Source: Journal of Agricultural Food Chemistry, May 2001, pages 2327–2332). It may also have emollient properties for skin, but there is little research showing this to be the case.