Sodium hydroxide is a highly alkaline ingredient that is used in small amounts in cosmetics and personal care products to establish and hold the pH of a product.
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations Na+ and hydroxide anions OH−. Sodium hydroxide has a pH value of 13, which means that it is highly basic, or alkaline. It is highly soluble in water, and readily absorbs moisture and carbon dioxide from the air.
In addition to its presence in cosmetics and personal care products, sodium hydroxide is used in many different industries, for instance, in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents, and as a drain cleaner. Worldwide production in 2004 was approximately 60 million tons, while demand was 51 million tons.
Sodium hydroxide was historically used in the formulation of soaps, but is currently seen in a variety of formulas, including bath products, cleansing products, fragrances, foot powders, hair dyes and colors, makeup, nail products, personal cleanliness products, shampoos, shaving products, depilatories, skin care products.
The main function of sodium hydroxide in cosmetics and personal care products is to establish and hold the pH of a product. In chemistry, pH stands for ‘potential hydrogen’ and it refers to the level of acidity or alkalinity in a given solution. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.
The skin’s normal pH is slightly acidic (typically between 4 and 6). This acidity of the skin is termed the “acid mantle” and is maintained by sebaceous glands, sweat glands, and normal skin flora. The acid mantle provides a film of amino/lactic acids and oils that effectively protect skin from environmental factors (bacteria, pollutants) that contribute to premature aging and irritation.
The pH of cosmetics and skin care products is important to maintain the skin’s normal pH as closely as possible. On one hand, if a product is too acidic it may irritate the skin or cause a stinging sensation. On the other hand, a product that is too alkaline is detrimental because it depletes your skin of vital, natural fats (or “lipids”). Additionally, a disrupted acid mantle will not allow for products to absorb into the skin as well.
Even though typical formulations should only contain small amount of sodium hydroxide in order to adjust the pH, if a product is too alkaline it can damage your skin. In fact, a study on skin products found that using an alkaline cleanser, even once, can do significant damage to the skin. According to FutureDerm, an alkaline cleanser disrupts the skin’s acid mantle. Consequently, this alters the bacterial flora composition on the skin and the activity of the enzymes in the upper layers of skin, as these have an optimal pH level. And the damage is cumulative: The longer you use it, the more damage it does to your skin.
The United States Food and Drug Administration (FDA) has approved sodium hydroxide as GRAS (Generally Recognized as Safe) as a direct food additive. However, it is primarily used in the washing and chemical peeling of produce.
Concentrated sodium hydroxide is a strong irritant and corrosive to the skin, eyes, respiratory tract, and gastrointestinal system if ingested. The severity of effects caused by sodium hydroxide is a function of the concentration, the pH, the length of tissue contact time, and local conditions and skin type.
Sodium hydroxide is approved for use in cosmetics and personal care products in varying concentrations: 5% by weight in nail cuticle solvents, 2% by weight in hair straighteners for general use, 4.5% by weight in hair straighteners for professional use, up to a pH 12.7 in depilatories, and up to pH 11 in other uses as a pH adjuster.
According to EWG, sodium hydroxide has received a rating of 3 to 4 on a 1 to 10 scale, with 1 being the lowest health hazard and 10 being the highest.
References: Wikipedia, “Sodium Hydroxide”, Truth In Aging, “Sodium Hydroxide”, Chemists Corner, “The Importance of pH in Cosmetic Formulation”, FutureDerm, “Is Your Skin’s pH Level Making Your Skincare Less Effective?”, 2016, SebaMed, “Too Acidic, Too Alkaline: The Different Dangers”, 2016, Dermatology 1997;195:258–262, Cosmetics Info, “Sodium Hydroxide”, EWG’s Skin Deep Cosmetic Database, “Sodium Hydroxide”.