The Codex Alimentarius Commission (part of the World Health Organization and the Food and Agriculture Organization program of the United Nations and will be referred to as 'Codex' in this document) lists sulfites as one of the priority 'allergens'. Of course, sulfites do not cause allergic reactions but can elicit adverse reactions upon ingestion in a small percentage of consumers. Codex placed sulfites on the priority list to provide guidance to individual countries by indicating that the labeling of sulfites was important to protect sulfite-sensitive consumers.
Sulfites are used as food additives for a variety of applications. In most countries, sodium and potassium sulfite (NaHSO3, KHSO3), sodium and potassium metabisulfite (Na2S2O5, K2S2O5), sodium sulfite (Na2SO3) are allowed along with sulfur dioxide gas (SO2). In some countries, potassium sulfite (K2SO3) and sulfurous acid (H2SO3) are allowed. Some countries restrict certain uses of sulfite. The U.S. has a long-standing restriction on use of sulfites in meats, but this restriction does not exist in most other countries. The U.S. also prohibits the use of sulfites on fresh fruits and vegetables (salad bars, etc.) intended to be consumed raw.
Sulfites may also occur naturally in some foods especially fermented foods as some strains of yeast can produce sulfite from sulfate that exists in much larger quantities in nature. However, naturally occurring levels of sulfite are typically quite low (<10 ppm total SO2).
Labeling regulations for sulfites vary from one country to another. But, many countries require the declaration of the presence of sulfites in foods when residue levels exceed some specified concentration. Commonly, 10 ppm total SO2 is the specified concentration and this is the level specified in the U.S. Total SO2 can be determined by several methods but the optimized Monier-Williams distillation-titration procedure is the most widely accepted (Official Method 990.28 in Official Methods of Analysis of AOAC International, 17th edition. 2000. Dr. William Horowitz, Ed. Gaithersburg, MD USA).