also: Iron oxide black · Magnetite-type pigment · CI Pigment Black 11
A mineral-based black pigment used to colour the surface of foods such as olives, confectionery, and edible coatings. Listed on labels as 'E172' or 'iron oxides and hydroxides'.
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Black and red iron oxide were positive in laboratory genotoxicity tests on mammalian cells.
UK supermarket foods that declare E172c: 1 of the 43,325 we checked (under 0.1%)
Counted by its code or its name in the ingredient list.
Counted by NutraSafe from UK supermarket ingredient lists, September 2026. How we counted
Why it’s worth knowing. Black and red iron oxide were positive in laboratory genotoxicity tests on mammalian cells. EFSA's 2015 re-evaluation could not complete a full safety assessment because the required toxicological studies, covering potential cancer risk, reproductive effects, and developmental effects, had not been provided by the industry. The particle size of the pigment is unregulated, meaning some batches contain nanoparticles whose behaviour in the body is not yet fully understood.
Black iron oxide is an inorganic mineral pigment with the chemical formula FeO.Fe2O3 (ferrosoferric oxide, also called magnetite). It is synthetically produced from iron powder through controlled oxidation reactions, giving a stable, heat-resistant black colour. It is one of three iron oxide variants authorised in the UK and EU under the shared code E172, alongside red (E172b) and yellow (E172a) iron oxides.
It provides an opaque black surface colour to food through light absorption by iron oxide particles. The pigment sits on or near the food surface rather than dispersing through the product. It is chemically stable at high processing temperatures and does not bleed into surrounding ingredients.
Most commonly used to recolour green olives that are picked unripe and then treated with iron salts and oxidants to turn them black, giving them the appearance of fully ripened olives. Also used in edible cheese rinds, sausage casings, chocolate and sugar confectionery coatings, cake decorations, and the shells of dietary supplement capsules. On a UK label it appears as 'E172', 'iron oxides and hydroxides', or occasionally 'iron oxide, black'.
In the 2015 EFSA re-evaluation, both red and black iron oxide (in nano and micro particle forms) showed positive results in laboratory genotoxicity tests on mammalian cells. However, the EFSA panel concluded it could not properly evaluate these signals because the toxicological data package submitted was insufficient. The panel stated outright that an adequate assessment of the safety of E172 could not be carried out due to the absence of required studies.
Red and black iron oxide, in both nano and micro particle forms, were positive in in vitro genotoxicity assays in mammalian cells.
The EFSA panel concluded that an adequate assessment of the safety of E172 could not be carried out because a sufficient biological and toxicological database was not available, and recommended additional toxicological data on carcinogenicity, reproductive toxicity, and developmental toxicity.
Commercial E172 pigment samples have been found to contain iron oxide nanoparticles (particles below 100nm) at significant proportions, but current EU/UK specifications do not set any limit on particle size or nanoparticle fraction. This matters because nanoparticle behaviour in the body can differ from larger particles. EFSA specifically called for particle size data to be included in the specification, and a 2023 industry update confirmed that EFSA issued a further request for data in November 2022.
Nanoparticles were detected in all E172 commercial samples tested by TEM or SAXS, with the majority of pigments having a size median below 100nm.
Current EU specifications for E172 do not limit particle size or the nanoparticle fraction, and EFSA recommended that particle size distribution should be included in specifications.
Iron oxide particles are poorly absorbed through the gut wall; the bulk of what is ingested passes through unchanged. However, laboratory studies on intestinal cells showed that nanoparticle-sized iron oxides can be taken up into cells to a minor extent, and one study in rats at high doses found shifts in gut bacteria including a reduction in beneficial Bifidobacterium. These doses were far above typical food exposure and the authors acknowledged this was a new area of research.
Iron oxide particles are poorly absorbed in the gastrointestinal tract, with low systemic availability from food-grade pigment.
At high doses (200mg/kg body weight), food-grade iron(III) oxide nanoparticles altered rat cecal microbiota, reducing Bifidobacterium and increasing bacteria associated with inflammation, though effects at lower doses were minimal.
Iron oxide pigments can carry traces of heavy metals including arsenic, lead, cadmium, and mercury as manufacturing impurities. The EFSA 2015 panel specifically recommended that the maximum permitted limits for these toxic elements in E172 specifications should be revised downward to ensure the pigment is not a significant source of heavy metal intake from food. Some commercial samples have also been found to contain unlabelled titanium and aluminium particles.
EFSA recommended revising the maximum limits for arsenic, cadmium, lead, and mercury in E172 specifications to ensure iron oxides are not a significant source of these toxic elements in food.
Analysis of commercial E172 pigment samples found significant proportions of titanium- and aluminium-containing particles that are not required to be declared on labels.
No declarable allergen status under UK food law. Mineral-based, so suitable for vegetarians and vegans. People with hereditary haemochromatosis (iron overload disorder) may wish to note any source of iron in their diet, though absorption from iron oxides is low. Look for 'E172', 'iron oxide, black', or 'iron oxides and hydroxides' on the ingredient list.
Black iron oxide is one of the more unusual food additives in everyday use. Most people encounter it without realising, in the black olives they eat from a jar (many are green olives that have been processed and recoloured) or in confectionery coatings. The chemistry is ordinary mineral pigment chemistry. The genuine uncertainty here is not dramatic but it is real: EFSA's own 2015 review panel found that it could not complete the safety assessment because the required long-term toxicology studies simply had not been done. On top of that, the pigment contains nanoparticles, and the regulation does not currently limit how many. The positive in vitro genotoxicity signals for black and red iron oxide have not been resolved. As of 2026, EFSA is still in the process of gathering additional data. This is not the same as a finding of harm, but it is an honest description of where the science stands.
No. Black iron oxide (E172c) is an approved food additive in the UK under the assimilated EU Regulation 1333/2008 and appears on the UK FSA's approved-additives list. It may only be used in specific permitted food categories and must meet purity specifications set in Commission Regulation 231/2012.
In its 2015 re-evaluation, the EFSA panel found that the toxicological data provided by industry was insufficient to draw firm conclusions on carcinogenicity, reproductive toxicity, and developmental toxicity. In vitro genotoxicity tests on mammalian cells showed positive results for black and red iron oxide, but without the required follow-up animal studies, the panel could not determine whether these signals represent a real risk at food-use levels. EFSA called for additional data; industry submitted data in 2020 and EFSA issued a further request in November 2022. A final updated opinion had not been publicly confirmed as of mid-2026.
Black iron oxide is most commonly found in commercially sold black olives (many of which are green olives that have been processed with iron salts and recoloured), edible cheese rinds, sausage casings, chocolate confectionery and sugar-coated dragees (for surface decoration), and the shells of some dietary supplement capsules. On the label it appears as 'E172', 'iron oxides and hydroxides', or 'iron oxide, black'.
Yes. Black iron oxide is a mineral-based pigment with no animal-derived ingredients. It is considered suitable for vegan, vegetarian, halal, and kosher diets. Cross-contamination during manufacturing is a separate question that individuals may wish to verify with specific producers.
Last reviewed: 20 June 2026
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