Some flours are treated with potassium bromate, which is an oxidizing agent that strengthens dough so that it doesn't collapse between the time that it proofs and is to be baked. Consequently, the dough can have an improved oven spring. The problem with the bromate is that it has been linked to cancer in laboratory animals, including tumors of the kidney, thyroid, and other organs. Several countries, including Canada and many European countries (and the EU) have banned bromates. California requires that products including bromates be labeled to that effect, and the product is on the state's list of carcinogens. Instead of banning bromate, since 1991 the FDA has tried to convince bakers to voluntarily stop using it. It has achieved only modest success. Flours that are bromated are labeled as such on the packaging.