Sulfates are an additive to cleaning products that causes the foaming action commonly associated with soaps. The most common sulfate is sodium lauryl sulfate (SLS), found in most shampoos and conditioners. Used primarily as a foaming agent, a sulfate combines with water to emulsify grease, dispersing it into the water so that it can be washed away. Sulfates also act as a surfactant, a substance that reduces the surface tension of water, helping your shampoo loosen the grease and sebum from your hair and scalp.
However, sulfates can be harmful. Sulfates are also used for cleaning products like household soaps and detergents. If you have particularly sensitive skin, sulfates may increase your sensitivity, causing irritation.
"Though sulfate-based shampoos do effectively clean the hair, they can also strip the hair of essential oils as well as irritate the skin,"
-cosmetic chemist and BeautyStat founder of Ron Robinson told Glamour.