Sodium sulfite is an antioxidant preservative that is used in cosmetics and personal care products to extend the shelf-life of products. It also functions as a hair-waving/straightening agent.
Sodium sulfite is a soluble sodium salt of sulfurous acid (sulfite) with the chemical formula Na2SO3. It can be prepared by reacting sulfur dioxide, soda ash (sodium carbonate), and water. The product of this reaction is sodium bisulfite (NaHSO3), which is then treated with excess soda ash to obtain sodium sulfite. The compound can also be obtained as a byproduct in the preparation of phenol. Sodium sulfite is a white powder or crystalline solid with no odor but a slightly salty taste. The compound is stable in dry air, but tends to decompose in moist air to produce sulfur dioxide and sodium hydroxide.
In addition to use in the cosmetic industry, sodium sulfite is used as a food preservative to prevent dried fruit from discoloring and for preserving meats. However, sodium sulfite destroys both vitamins B1 and E, which means that it cannot be added to foods that contain these vitamins. Additionally, sodium sulfite is an essential chemical in the pulp and paper industry. In fact, more than half of all the sodium sulfite made in the United States is used by the pulp and paper industry. Sodium sulfite is also used in water and wastewater treatment plants.
In cosmetics and personal care products, sodium sulfite functions as an antioxidant preservative and a hair-waving/straightening agent.
As an antioxidant preservative, sodium sulfite works by protecting the other ingredients in a formulation from oxidation, which is the loss of electrons. For example, when products are exposed to air, oxygen from the air is reduced, gaining electrons from ingredients such as fats and oils in the formulation. This is why sodium sulfite can be referred to as a reducing agent; it donates a hydrogen atom to the oxygen in order to protect the other ingredients. Oxidation results in rancidity, color changes, viscosity changes, and deterioration of active ingredients in finished products. Adding an antioxidant preservative like sodium sulfite will make the product less susceptible to degradation when exposed to environmental elements.
In addition to use as an antioxidant preservative, sodium sulfite functions as a hair-waving/straightening agent. It is often used in combination with sodium metabisulfite and sodium sulfate in products used to straighten the hair and in permanent waves. Other hair care products that use sodium sulfite include hair bleaches, hair dyes, colors and tints.
The United States Food and Drug Administration (FDA) includes sodium sulfite on its list of substances considered Generally Recognized As Safe (GRAS) for use as chemical preservatives in food. It is not to be used in foods recognized as sources of vitamin B1, or on raw fruits and vegetables. Sodium sulfite must be clearly labeled on the food packaging because some people are sensitive to sulfites.
The safety of sodium sulfite has been assessed by the Cosmetic Ingredient Review (CIR) Expert Panel. It was noted that the highly charged nature of this ingredient results in relatively low dermal penetration. After evaluating the scientific data, the CIR Expert Panel concluded that sodium sulfite was safe as used in cosmetics and personal care products.
Despite the approval of sodium sulfite by the CIR Expert Panel, it should be noted that a significant number of people are allergic to sodium sulfite. It can also be an irritant to the skin, eyes, and respiratory tract. With the low level of sodium sulfite used in cosmetics and personal care products, however, it poses little threat to a person’s health.
References: Wikipedia “Sodium Sulfite”, Encyclopedia.com “Sodium Sulfite”, Int J Toxicol 2003;22 Suppl 2:63-88. Cosmetics Info “Sodium Sulfite”