How can you get oil and water ingredients to mix in dog shampoo? If for example a manufacturer wishes to add an essential oil to their product, they would use this stabilizing agent to allow the oil to dissolve with other ingredients, including water.
This is where polysorbates enter the picture. In its natural form, a polysorbate is a harmless sugar alcohol (a sorbitol). However, it becomes a dangerous agent when it is treated with ethylene oxide.
The name you see in a dog shampoo's ingredients list will have the word "polysorbate" followed by a number. This number is critical as it informs how much ethylene oxide was used to treat the natural sugar in its original form. The higher the number, the more parts of ethylene oxide it has been treated with and therefore higher toxicity.
The main concern arises as 1,4-dioxane is created as a by-product of this treatment with ethylene oxide. 1,4-dioxane is a known animal carcinogen that can be absorbed thru the skin and result in developmental and reproductive toxicity, not to mention irrevocable skin damage and allergies.
Avoid buying dog shampoos and conditioners that contain any reference to polysorbates or "ethoxylated" ingredients.