Iron oxides
Mineral pigments giving food yellow, red, brown, or black colours. Used on confectionery coatings, processed meats, cheese rinds, and many black olives.
Red and black iron oxides tested positive in laboratory genotoxicity assays on mammalian cells. Regulators have not been able to rule this out because no gut site-of-contact data exist. Carcinogenicity and reproductive toxicity studies were unavailable when EFSA last reviewed this additive; a new assessment is ongoing.
What is it?
Iron oxides and hydroxides are inorganic mineral pigments derived from iron. Three forms are used in food: yellow (goethite, FeO(OH)), red (haematite, Fe2O3), and black (magnetite, FeO.Fe2O3). Mixtures produce brown and intermediate shades. Commercially they are synthesised from iron powder and purified to food-grade specifications rather than being taken directly from mined ore.
What does it do?
Provides colour by absorbing specific wavelengths of visible light. The pigments are added to surfaces or coatings of foods and to the bodies of confectionery, capsules, and processed products. They do not contribute flavour. Because the particles are poorly soluble, they largely pass through the gut without dissolving, which also means they do not deliver meaningful amounts of dietary iron.
Where you will see it
Confectionery coatings and sugar shells; cake decorations and icings; the rinds and surfaces of processed cheeses; meat pies, sausage casings, and processed meat products; commercially prepared black olives (typically picked green then recoloured); smoked salmon and prawns (surface reddening); pharmaceutical and supplement capsules. On UK labels it appears as 'E172', 'iron oxides', or 'iron hydroxides'.
What the science says
In vitro genotoxicity: a signal regulators could not dismiss
In laboratory tests on mammalian cells, both red iron oxide (Fe2O3) and black iron oxide (FeO.Fe2O3) produced signs of DNA damage, and this was seen for both nano-sized and micro-sized particles. The in vitro signal alone does not prove harm in people, but EFSA noted it could not be dismissed because no data from the site where exposure actually happens, the gut wall, were available. In vivo oral dosing studies in rats did not find genotoxic effects in blood cells, but EFSA said those tests did not cover the gastrointestinal lining, which is the relevant target.
Red (Fe2O3) and black (FeO.Fe2O3) iron oxide, both in nano and micro form, were positive in in vitro genotoxicity assays in mammalian cells; in vivo oral studies did not elicit genotoxic effects in rat blood cells, but no data for the gastrointestinal site-of-contact were available.
EFSA concluded that 'an adequate assessment of the safety of E 172 could not be carried out because a sufficient biological and toxicological database was not available', and recommended that additional toxicological data be provided.
Carcinogenicity and reproductive toxicity: data were not available
When EFSA conducted its 2015 re-evaluation, studies covering long-term cancer risk and effects on reproduction or fetal development either did not exist or were submitted as unpublished company data that the Panel could not independently evaluate. This left two major safety questions formally unanswered. A follow-up EU data call was issued in 2016 and a new risk assessment (EFSA-Q-2021-00178) is ongoing as of 2025.
Carcinogenicity and reproductive and developmental toxicity data were not available and could not be evaluated by the EFSA Panel during the 2015 re-evaluation.
The European Commission issued a call for scientific and technical data on E172 in December 2016 to address the identified data gaps; a follow-up risk assessment (EFSA-Q-2021-00178) is listed as an ongoing risk assessment.
Nanoparticles in commercial E172 samples
Studies measuring real commercial E172 pigments found that nano-sized particles, those below 100 nanometres, were present in most samples tested. Particle size was not included in food-grade specifications at the time of the 2015 opinion, so manufacturers were not required to control or declare this fraction. Iron oxide nanoparticles have been shown to reach the intestinal wall in particle form after passing through artificial digestion. Cell studies showed minimal uptake across the intestinal barrier and no cytotoxicity at low concentrations, but liver cell studies at higher concentrations showed signs of stress. EFSA flagged the absence of particle-size specifications as a gap.
Nanoparticles were detected in all five commercial E172 pigments examined by transmission electron microscopy or small-angle X-ray scattering; four of five had a size median below 100 nm.
Iron oxide E172 particles passed through artificial gastrointestinal digestion without dissolving and reached intestinal Caco-2 cells in particle form; minor uptake into cells was detected but almost no transport across the barrier.
EFSA recommended that particle size and particle size distribution should be included in the specifications for E172 and that grouping of different E172 forms for risk assessment purposes is not advisable due to variation in physicochemical properties.
Gut microbiota: high-dose animal findings
Rat studies using food-grade iron(III) oxide nanoparticles at high doses found changes in the gut microbiota, with increases in bacteria associated with inflammation and obesity and reductions in beneficial Bifidobacterium. These effects appeared at doses substantially higher than typical dietary exposure from food colouring use. Extrapolating animal high-dose findings directly to human consumers at normal intake levels is not straightforward, and no human studies have been conducted.
High-dose iron(III) oxide nanoparticle exposure in rats (200 mg/kg body weight over 28 days) changed the abundance and composition of gut bacteria, increasing inflammation-associated species and reducing Bifidobacterium; lower doses showed minimal effects on microbial diversity.
Heavy metal impurities in the pigment
Iron oxide pigments can carry trace impurities of lead, arsenic, cadmium, and mercury from their production process. EU specifications set maximum limits for these contaminants in food-grade E172, and regulators have called for revision of those limits. The concern is about the impurities in the pigment, not the iron oxide itself.
Purity concerns for E172 include possible contamination with heavy metals including lead and arsenic; regulatory reviews have called for stricter impurity limits and revision of current EU specification maximum levels for cadmium, arsenic, lead, and mercury.
Where it stands with the regulators
Who should be careful
People with hereditary haemochromatosis or other iron overload conditions should note that E172 contributes very little absorbable iron, but should discuss total iron intake from all sources with their doctor. Anyone wishing to reduce exposure to additives with outstanding safety questions can check ingredient lists for 'E172', 'iron oxides', or 'iron hydroxides', most commonly found on confectionery coatings, processed meat products, and commercially packaged black olives.
The honest read
E172 sits in an unusual position: it has been used for decades in food and pharmaceuticals, yet the 2015 EFSA re-evaluation formally concluded it could not confirm whether it was adequately safe, because the toxicological database was incomplete. Red and black iron oxide produced DNA-damage signals in mammalian cell tests, and neither carcinogenicity nor reproductive toxicity data were available for independent review. The additive is poorly absorbed in the gut, which regulators treat as a mitigating factor, but nano-sized particles present in commercial pigments reach the intestinal wall intact, and what happens there in long-term use at site-of-contact is unknown. A new EFSA assessment is in progress. The honest picture is that the science is unresolved, not settled.
Related additives
Common questions
Is E172 banned in the UK?
No. E172 is approved for use in the UK under the assimilated EU Regulation (EC) No. 1333/2008 and is listed on the UK FSA's approved additives register. It remains permitted despite EFSA's 2015 conclusion that a full safety assessment could not be completed, because no specific harm was demonstrated that would trigger a ban.
Why did EFSA say it could not confirm E172 was safe?
In its 2015 re-evaluation EFSA found that red and black iron oxides produced positive results in laboratory genotoxicity tests on mammalian cells, but no data were available from the gut wall, the relevant point of contact. Carcinogenicity studies and reproductive toxicity studies were also missing. This left three key safety questions unanswered. A new assessment is ongoing.
What foods contain E172?
E172 is most commonly found in confectionery with coloured sugar coatings or chocolate shells, cake decorations, meat pies, sausage casings, processed cheese rinds, and commercially packaged black olives (many black olives are picked green and recoloured with iron oxides). It also colours pharmaceutical and supplement capsules. On the label it appears as 'E172', 'iron oxides', or 'iron hydroxides'.
Is E172 vegan?
Yes. Iron oxides are mineral compounds with no animal-derived ingredients. E172 is suitable for vegans, vegetarians, and is considered acceptable under halal and kosher dietary rules.
Sources
- EFSA Scientific Opinion on the re-evaluation of iron oxides and hydroxides (E 172) as food additives, EFSA Journal 2015;13(12):4317
- UK FSA Approved Additives and E Numbers
- UK FSA Regulated Products Register: E-172
- European Commission call for scientific and technical data on E172 iron oxides, December 2016
- Particulate iron oxide food colorants (E 172) during artificial digestion and their uptake and impact on intestinal cells, Food and Chemical Toxicology, 2024
- Effects of food-grade iron(III) oxide nanoparticles on cecal digesta- and mucosa-associated microbiota and short-chain fatty acids in rats, MDPI Nutrients, 2024
- Intestinal and hepatic effects of iron oxide nanoparticles, Archives of Toxicology, 2021
- The presence of iron oxide nanoparticles in the food pigment E172, Food Chemistry, 2020
- Safety and efficacy of iron compounds (E1) as feed additives for all species: ferric oxide, EFSA Journal 2025
- EU Commission Regulation (EU) No 231/2012 laying down specifications for food additives
- Physicochemical characterisation of iron oxides and hydroxides applied as food additive E172, ScienceDirect 2025
Aaron Keen is the founder of NutraSafe. He researches and writes every additive entry himself, from the primary sources. About the research →
This is a guide, not medical advice. If an additive affects you, speak to your GP or a dietitian.
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