lactic acid. Alpha hydroxy acid extracted from milk, although most forms used in cosmetics are synthetic. It exfoliates cells on the surface of skin by breaking down the material that holds skin cells together. It may irritate mucous membranes and cause irritation. See AHA.

Lactobacillus bifidus. Type of “friendly” bacteria found in the intestine that helps maintain a healthy natural flora in the large intestine by creating an environment that prevents potentially harmful bacteria from growing. Whether or not this has benefit when applied topically on skin is unknown.See bifidus extract.

lactobionate. Polysaccharide that has water-binding properties for skin.

lactobionic acid. See polyhydroxy acid.

lactoperoxidase. Enzyme derived from milk; it has antibacterial properties for skin and may be helpful for eliminating acne-causing bacteria (Sources: Journal of Experimental Therapeutics and Oncology, 2007, volume 6, issue 2, pages 89–106; and Journal of Applied Microbiology, May 2006, pages 1034–1042).

lady's mantle extract. See Alchemilla vulgaris.

lady's thistle extract. Extracts for which there is a great deal of research showing it has many medical health applications when taken orally. There is no research showing it to be beneficial for skin, though it may cause allergic reactions (Source: http://www.naturaldatabase.com).

Laminaria digitata. See algae.

Laminaria japonica. See algae.

Laminaria longicruris. See algae.

Laminaria saccharine. See algae.

lanolin. Emollient, very thick substance derived from the sebaceous glands of sheep. Lanolin has long been burdened with a reputation for being an allergen or sensitizing agent, which has always been a disappointment to formulators because lanolin is such an effective moisturizing agent for skin. A study in the British Journal of Dermatology (July 2001, pages 28–31) may change all that. The study concluded “that lanolin sensitization has remained at a relatively low and constant rate even in a high-risk population (i.e., patients with recent or active eczema).” Based on a review of 24,449 patients who were tested with varying forms of lanolin, it turned out that “The mean annual rate of sensitivity to this allergen was 1.7%”—and it was lower than that for a 50% concentration of lanolin. It looks like it’s time to restore lanolin’s good reputation. That’s a very good thing for someone with dry skin, though it can be a problem for someone with oily skin, because lanolin closely resembles the oil from human oil glands.

lanolin alcohol. Emollient derived from lanolin. See lanolin.

lappa extract. See burdock root.

Larrea divaricata extract. See chaparral extract.

Larrea tridentata. See chaparral extract.

L-ascorbic acid. Form of vitamin C that is a potent antioxidant and anti-inflammatory agent (Sources: Bioelectrochemistry and Bioenergetics, May 1999, pages 453–461; and International Journal of Radiation Biology, June 1999, pages 747–755). However, claims that it can eliminate or prevent wrinkles when applied topically are not substantiated in any published studies. In addition, it is stable only in a formulation with a low pH, and that is potentially irritating for skin (Source: Dermatologic Surgery, February 2001, pages 137–142).

lauramphocarboxyglycinate. Mild detergent cleansing agent. See surfactant.

laureth-23. Derived from lauryl alcohol and used either as a surfactant or emulsifier (or, in many cases, as both). See surfactant.

laureth-4. Derived from lauryl alcohol and used either as a surfactant or emulsifier (or, in many cases, as both). See surfactant.

laureths. Substances that in various combinations create a wide range of mild detergent cleansing agents called surfactants. See surfactant.

lauroyl lysine. Amino acid derivative that functions as a skin- and hair-conditioning agent. It also contributes to a product’s texture by helping to gel solvents, while also remaining stable under high heat conditions (Source: Organic and Molecular Biochemistry, November 2003, pages 4124–4131).

Laurus nobilis. See bay leaf oil.

lauryl alcohol. See surfactant.

lauryl glucoside. See surfactant.

lauryl lactate. Mixture of lauryl alcohol and lactic acid that functions as a skin-conditioning agent and emollient; also used to impart fragrance.

lavandin oil. Essential oil of the hybrid lavender plant Lavandula hybrida and used in cosmetics as a fragrance ingredient. Its irritant potential for skin is similar to that of lavender oil and camphor. Lavandin yields four times more oil per volume of plants than true lavender, but it is of inferior quality and has a distinct camphor scent. It is not used in perfumery, but instead to scent soaps, air fresheners, and similar items.

Lavandula angustifolia. See lavender extract and oil.

Lavandula officinalis. See lavender extract and oil.

lavender extract and oil. Primarily a fragrance ingredient, although it may have antibacterial properties. There is no research showing it has any benefit for skin (Sources: Phytotherapy Research, June 2002, pages 301–308; and Healthnotes Review of Complementary and Integrative Medicine, http://www.healthwell.com/healthnotes/Herb/). It can be a skin irritant (Source: Contact Dermatitis, August 1999, page 111) and a photosensitizer (Source: Family Practice Notebook, http://www.fpnotebook.com/DER188.htm). Research also indicates that components of lavender, specifically linalool, can be cytotoxic, meaning that topical application causes skin-cell death (Source: Cell Proliferation, June 2004, pages 1365–2184).

L-carnitine. Also known as carboxylic acid, it is often erroneously labeled an amino acid (which it is not). It has been claimed to have miraculous (albeit unsubstantiated) properties for enhancing the metabolization of fat when taken orally. There is research in animal studies showing it has anti-aging benefits when taken orally (Source: Annals of the New York Academy of Sciences, April 2002, pages 133–166). However, there is no known benefit for skin when it is applied topically in skin-care products, though it may have antioxidant properties. See antioxidant.

L-cysteine. See antioxidant.

lecithin. Phospholipid found in egg yolks and the membranes of plant and animal cells. It is widely used in cosmetics as an emollient and water-binding agent. See natural moisturizing factor (NMF).

lemon. Potent skin sensitizer and irritant. Though it can have antibacterial properties, the irritation can hurt the skin’s immune response (Source: http://www.naturaldatabase.com).

lemon balm. See balm mint extract.

lemon oil. Can be a skin irritant, especially on abraded skin (Source: http://www.naturaldatabase.com).

lemongrass extract. Extract that can have antibacterial properties (Source: Journal of Applied Microbiology, 2000, volume 88, pages 308–316), but it also may be a skin irritant.

lemongrass oil. Also known as Oil of Verbena; can be effective as a mosquito repellent (Source: Phytomedicine, April 2002, pages 259–262). As a volatile fragrant oil, it contains compounds (including limonene and citral) that can cause irritation.

Lentinus edodes extract. Extract from the shiitake mushroom that may have antimicrobial and antibacterial properties, although it could be a potential skin irritant (Source: International Journal of Antimicrobial Agents, February 1999, pages 151–157). There is research showing it also has antitumor activity when taken orally (Source: Mutation Research, September 2001, pages 23–32).

Leptospermum scoparium oil. See manuka oil.

leucine. Amino acid. See amino acid and natural moisturizing factor (NMF).

Levisticum officinale root extract. See lovage root extract.

licorice extract. Extract that has anti-inflammatory properties (Source: Healthnotes Review of Complementary and Integrative Medicine, http://www.healthwell.com/healthnotes/Herb/). See glycyrrhetic acid.

licorice root. See licorice extract.

Lilium candidum bulb extract. Extract derived from the white lily bulb. There is no research showing this has any benefit for skin.

lime (oil or extract). Citrus fruit whose volatile compounds are skin irritants and photosensitizing (Source: http://www.naturaldatabase.com).

Limnanthes alba. Plant commonly known as meadowfoam; its seed oil is a non-volatile plant oil used as a skin-conditioning agent.

limonene. Chemical constituent of many fragrant natural ingredients, notably citrus oils (d-limonene) and pine trees or species of the mint family (l-limonene). Early research suggests that limonene may be a potential anti-cancer ingredient and immune stimulant when consumed orally, but other research suggests that limonene may promote the growth of tumors. Topically, limonene can cause contact dermatitis and is best avoided unless its presence in skin-care products is minuscule (Sources: http://www.naturaldatabase.com; and Journal of Occupational Health, November 2006, pages 480–486).

linalool. Fragrant component of lavender and coriander that can be a potent skin irritant, allergen, or sensitizer once it is exposed to air (Sources: Contact Dermatitis, May 2002, pages 267–272, and June 2005, pages 320–328). Current research indicates that this component of lavender can be cytotoxic (meaning toxic to skin cells) (Source: Cell Proliferation, June 2004, pages 1365–2184).

linden flower extract. Major active constituents in linden are flavonoids and glycosides. Flavonoids are potent antioxidants and glycosides are monosaccharides that have water-binding properties (Source: Healthnotes Review of Complementary and Integrative Medicine, http://www.healthwell.com/healthnotes/Herb/).

linoleic acid. Unsaturated fatty acid used as an emollient and thickening agent in cosmetics. There is some research showing it to be effective in cell regulation and skin-barrier repair, as well as an antioxidant and an anti-inflammatory (Sources: Archives of Dermatological Research, July 1998, pages 375–381; Clinical and Experimental Dermatology, March 1998, pages 56–58; Journal of Investigative Dermatology, May 1996, pages 1096–1101; and Seminars in Dermatology, June 1992, pages 169–175). See fatty acid and natural moisturizing factor (NMF).

linseed oil. Non-volatile plant oil. Linoleic acid is a component of linseed oil. See linoleic acid.

Linum usitatissimum extract. See linseed oil.

lipid. Wide range of ingredients found in plants, animals, and human skin. Lipids include fatty acids, sebum, and fats. In skin-care products, these are emollients and thickening agents. See fatty acid and natural moisturizing factor (NMF).

liposomes. Delivery system (not an ingredient) capable of holding other ingredients and releasing them after the liposome is absorbed into the skin. Liposomes are microscopic lipid (fat) sacs that are widely used as a way to deliver other ingredients into skin (Source: Journal of Pharmaceutical Sciences, March 2002, pages 615–622).

lithium magnesium sodium silicate. Synthetic silica-based clay composed of lithium, sodium, and magnesium. Used as a thickening agent and an absorbent.

Litsea cubeba. See lemongrass oil.

locust bean. See gums.

Lonicera japonica. Honeysuckle extract. A plant with soothing properties for skin.

loquat extract. Extract derived from a subtropical flower that has antioxidant and antitumor properties similar to those of green tea (Sources: Journal of Agricultural Food Chemistry, April 2002, pages 2400–2403; and Phytochemistry, February 2002, pages 315–323).

lotus seed extract. Extract that can have anti-inflammatory and antioxidant properties (Sources: Planta Medica, August 1997, pages 367–369; and Journal of Plant Physiology, May 2001, pages 39–46).

lovage root extract. Extract that is administered orally as a diuretic. In cosmetics, it is used as a fragrance. Theoretically, it can cause phototoxic reactions, including photosensitivity dermatitis (Source: http://www.naturaldatabase.com).

Luffa cylindrica seed oil or extract. Components of a plant that have antifungal properties (Source: Peptides, June 2002, pages 1019–1024) and antitumor properties, by preventing synthesis of certain proteins (Source: Life Sciences, January 2002, pages 899–906). They also have anti-inflammatory properties (Source: http://www.naturaldatabase.com). They may also be toxic to skin-cancer cells (Source: Melanoma Research, October 1998, pages 465–467). When the fruit from the luffa plant is dried it is used as an abrasive sponge.

lupine. Legume that is a source of isoflavones, a form of plant estrogen that has antioxidant properties (Sources: Phytochemistry, January 2001, pages 77–85; and Bioscience, Biotechnology, and Biochemistry, June 2000, pages 1118–1125). See isoflavone.

lupine oil. Extract of Lupinus albus, a legume; it has emollient and antioxidant properties, though it may also have significant allergen or skin-sensitizing potential. See lupine.

Lupinus albus extract. Species of legume. See lupine and lupine oil.

lutein. Carotenoid that has potent antioxidant properties (Source: Photochemistry and Photobiology, May 2002, pages 503–596).

lycopene. Carotenoid pigment that has potent antioxidant properties (Source: Photochemistry and Photobiology, May 2002, pages 503–596).

lye. See potassium hydroxide and sodium hydroxide.

lysine. Amino acid. See amino acid.