2. Allergic reactions to chemicals in hair products and food: A lot of us don’t take the time to go through the ingredients in their hair products. Some chemicals in our products cause allergic reactions in the scalp known as contact dermatitis. Some common chemicals in our shampoos especially like the following listed below are major culprits in causing dermatitis. See below: Sulphates (Sodium lauryl sulphate, Sodium salt, Sodium Laureth sulphate, Sulfuric Acid, Sodium Dodecyl Sulphate, Sodium salt sulfuric acid and Monododecyl ester sodium salt sulfuric acid).