An important fact about iodine and iron is that although they are needed in tiny amounts, they are required every day, by everyone. If given as supplements, this would involve taking it daily for the rest of our lives. Salt, however, is something that is consumed by everyone, everyday. On an average, the same amount of salt (8 to 12 grams) is consumed every day per person. If this salt is fortified with iodine and iron, then the entire population automatically gets the required amount of iodine and iron without any changes in food habits or eating patterns.
