Bromated flour contains potassium bromate, a chemical additive that the International Agency for Research on Cancer classifies as a Group 2B substance, meaning it is possibly carcinogenic to humans. Most of the world has already banned it from food. The European Union, the United Kingdom, Canada, China, Brazil, India, Australia, and New Zealand all prohibit potassium bromate as a flour additive. The United States still allows it, though that is starting to change.
What Potassium Bromate Does in Flour
Potassium bromate is a slow-acting oxidizer added to flour to strengthen dough. It helps gluten networks form more tightly, which gives bread a better rise and a chewier texture. Commercial bakers have relied on it for decades, particularly for products like hamburger buns and dinner rolls where consistent volume and structure matter. Under federal regulations, bromated flour can contain up to 50 parts per million of potassium bromate.
The idea behind its use is that bromate does its work during mixing and fermentation, then converts into potassium bromide (a harmless salt) when exposed to baking heat. In theory, no bromate should remain in the finished loaf. In practice, that conversion depends on baking temperatures being high enough and baking times being long enough. When bread is underbaked, baked at lower temperatures, or when too much bromate is added, residual amounts can persist in the final product. Testing of commercial baked goods in multiple countries has confirmed that measurable bromate residues do show up in bread sold to consumers.
Cancer Risk From Animal Studies
The cancer concern comes from extensive animal research. When rats were given potassium bromate in their drinking water over roughly two years, they developed kidney tumors (both benign and malignant), thyroid tumors, and tumors in the abdominal lining. These results appeared across multiple independent studies using different doses and durations.
The dose matters. In one key study, kidney tumors appeared at concentrations of 125 parts per million and above in drinking water, while precancerous changes in kidney tissue showed up at doses as low as 30 parts per million. Thyroid tumors and abdominal lining tumors appeared at the highest tested dose of 500 parts per million. Mice and hamsters also developed kidney tumors, though at lower rates than rats.
No controlled studies exist in humans for obvious ethical reasons, which is why the IARC classification is “possibly” rather than “probably” carcinogenic. But the consistency of tumor development across species and across multiple organ systems is what prompted dozens of countries to ban it outright rather than wait for human data.
Kidney and Thyroid Damage Beyond Cancer
Cancer is not the only concern. Potassium bromate is directly toxic to the kidneys. It works by triggering a flood of reactive oxygen species, unstable molecules that damage cell membranes, proteins, and DNA. The kidneys are the primary target organ because they filter bromate from the blood, concentrating it in kidney tissue. In animal studies, bromate exposure caused shrunken filtration units in the kidneys, damaged kidney tubules, and elevated blood markers of kidney stress like creatinine and uric acid. In cases of acute human poisoning (typically from accidental ingestion of large amounts), kidney failure is the most immediate life-threatening effect.
The thyroid is also vulnerable. Animal studies consistently show thyroid follicular tumors developing alongside kidney damage, suggesting bromate disrupts thyroid tissue through a similar oxidative mechanism. The oxidative stress bromate generates overwhelms the body’s natural antioxidant defenses, leaving cells unable to repair the damage.
Why the U.S. Still Allows It
The FDA does not regulate potassium bromate as a standard food additive. Instead, it holds a special status as a “prior sanctioned substance,” meaning it was approved for use before modern food safety laws took effect. This legal category has made it harder to remove through the normal regulatory process. The FDA has encouraged bakers to voluntarily stop using it but has not issued an outright ban at the federal level.
That is changing at the state level. California passed the California Food Safety Act, which bans the manufacture, sale, and distribution of food products containing potassium bromate starting January 1, 2027. The same law also bans brominated vegetable oil and propylparaben. Because California is such a massive consumer market, this law is expected to push many national brands to reformulate their products, effectively reducing bromate exposure well beyond the state’s borders.
How to Avoid Bromated Flour
Checking ingredient labels is straightforward. If a product contains potassium bromate, it will appear in the ingredients list. Some flour brands are labeled “bromated flour” directly. Others list “potassium bromate” as an individual ingredient. Either phrasing means the same thing.
Many flour brands now prominently label their products as “unbromated” to attract health-conscious buyers. These flours use safer dough conditioners to achieve similar baking results. The most common alternative is ascorbic acid (vitamin C), which strengthens gluten in a comparable way without the toxicity concerns. Some commercial flours also use enzymes derived from fungi to improve dough structure. Both alternatives are widely available, and many large bakeries have already switched to them without any noticeable difference in their finished products.
If you bake at home, look for “unbromated” or “unbleached, unbromated” on the bag. If you mostly buy bread and baked goods, checking ingredient lists at the store takes a few extra seconds but is the most reliable way to avoid it. Artisan bakeries and organic brands almost never use bromated flour, while some conventional brands still do, particularly for inexpensive white bread and rolls.
Putting the Risk in Perspective
The amount of bromate in a single slice of bread is small. The real concern is cumulative, low-level exposure over years and decades of eating baked goods made with bromated flour. Bromate’s mechanism of harm, generating oxidative damage to DNA, is the type that accumulates over time rather than causing immediate symptoms. You would not feel sick from eating one hamburger bun made with bromated flour. But the pattern of consistent organ damage across animal studies, combined with the fact that residual bromate does survive baking, is why the vast majority of developed nations decided the risk was not worth taking when safe alternatives exist.
The simplest way to think about it: bromated flour is not an acute poison at the levels found in food, but it contains an ingredient with no nutritional value that has well-documented toxic effects and readily available replacements. Over 30 countries reached the same conclusion and banned it.