Reactions — redness, swelling, and burning sensations — look the same whether caused by chemical irritants or allergy. Irritants, however, provoke an immediate reaction, while allergies can require weeks of constant exposure to develop. This foaming and cleansing detergent found in shampoo, body wash, and toothpaste may cause instant irritation to the skin, eyes, and mucous membranes and can eventually become an allergen. And because your skin and your airways aren't entirely separate, SLS may trigger respiratory symptoms as well. Similar chemicals found in foaming bath products include diethanolamine, monoethanolamine, and triethanolamine.

How to avoid it: “If it bothers you, reduce your exposure,” says Richard Honsinger, MD, an allergy specialist in Los Alamos, New Mexico. Choose products that contain natural foaming agents, such as coconut-derived lauric acid.