Sodium lauryl sulfate (SLS) is a common ingredient found in soaps and shampoos. SLS is a detergent, which means it does a good job of breaking up oil and grease. It's also the substance that makes soap get frothy when you rub it on your body.
So how does SLS contribute to contact dermatitis? One common skin-care myth is that the oil on our bodies is dirty, but the truth is that we need a reasonable amount of it for protection. While SLS is useful for breaking up greasy foreign substances, it also breaks up the layer of oil that keeps our skin from drying out. And while it's not technically an allergen because it doesn't provoke a reaction from the immune system, SLS can cause contact dermatitis and aggravate eczema by weakening that oily barrier on our skin. This means that SLS can usher other allergic elements into your body. After repeated exposure to these elements, you may develop reactions to things you weren't allergic to before.
If you're having a problem with dry, itchy skin, check your soap for sodium lauryl sulfate. It also appears in toothpaste and bubble bath -- pretty much anything that foams up to get you clean.