E-numbers / Other additives / E538
also: Calcium hexacyanoferrate(II) · Tetracalcium ferrocyanide
An anti-caking agent added to table salt and salt substitutes to keep them free-flowing. One of three ferrocyanide salts used in food.
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The kidneys are the target organ identified in animal toxicity studies.
Why it’s worth knowing. The kidneys are the target organ identified in animal toxicity studies. No reproductive toxicity data have been assessed by regulators, leaving a gap in the safety picture for people planning pregnancy.
Calcium ferrocyanide is an inorganic salt made up of calcium, iron and cyanide ions bound tightly together in a stable complex (hexacyanoferrate). Despite containing cyanide, the ferrocyanide complex is chemically very different from free cyanide: roughly 95% passes through the gut without being broken down.
Acts as an anti-caking agent. The fine crystals of calcium ferrocyanide coat the surfaces of salt granules and stop them sticking together in humid conditions, keeping the salt granular and easy to pour.
Used almost exclusively in table salt, sea salt, and salt substitutes. Occasionally used in dried spice blends where salt is the carrier. On a UK label it appears as 'calcium ferrocyanide' or 'E538'. It is typically listed in the ingredients of table salt in small print.
In a two-year rat study, the kidneys were the organ most affected at higher doses of ferrocyanide. The critical finding was increased excretion of cells in urine. EFSA used this result to set the acceptable daily intake, applying a safety margin to it. The effect was described as occasional and transient at the doses studied.
A two-year rat study identified the kidney as the target organ for ferrocyanide toxicity, with a NOAEL of 4.4 mg/kg body weight per day in males based on increased renal cell excretion in urine.
The ferrocyanide complex is chemically stable and around 95% of an oral dose passes through the gut and is excreted unchanged in faeces. A small fraction is absorbed and some free cyanide can in principle be released, but at the quantities used in food, EFSA concluded the amount of free cyanide released would not be a safety concern at the permitted level.
Approximately 95% of ingested ferrocyanide is excreted unchanged in faeces. The Panel noted that at the established ADI the potential amount of free cyanide released would not be of safety concern.
When EFSA reviewed ferrocyanides in 2018, it flagged that no reproductive toxicity studies were available, and only limited prenatal developmental data existed. There were also no hypersensitivity or allergenicity studies. These gaps mean the safety picture for people trying to conceive or during pregnancy relies on inference rather than direct evidence.
EFSA identified the absence of reproductive toxicity studies, limited prenatal developmental data, and no hypersensitivity or allergenicity data as data gaps in the safety dossier for ferrocyanides.
No specific group is advised to avoid E538 under current guidance. However, people who are pregnant or planning pregnancy may want to note that reproductive toxicity has never been formally studied for ferrocyanides. Look for 'calcium ferrocyanide' or 'E538' in the ingredients of table salt and salt substitutes.
E538 is one of the more obscure food additives, used only in salt at very low levels. The chemistry sounds alarming because of the word cyanide, but the ferrocyanide complex is structurally very different from poisonous free cyanide. EFSA reviewed the ferrocyanides thoroughly in 2018 and estimated that even people who use a lot of salt consume far less than the permitted daily limit. The honest gap in the picture is reproductive toxicity: no studies have been done, so regulators cannot confirm or rule out an effect on fertility or pregnancy. That gap is not a signal of harm, but it is a known unknown.
No. E538 is permitted in the UK and EU as an anti-caking agent in salt and salt substitutes at a maximum of 20 mg/kg.
The word cyanide is alarming, but ferrocyanide is a chemically stable complex where cyanide ions are tightly bound to iron. Around 95% passes through the gut without being absorbed. At the levels permitted in food the amount of free cyanide that could be released is very small. EFSA reviewed this specifically in 2018.
Almost exclusively table salt, sea salt and salt substitutes. It is occasionally present in spice blends where salt is the main carrier. Check the ingredients list of any table or cooking salt.
Yes. Calcium ferrocyanide is a mineral salt with no animal-derived ingredients.
Last reviewed: 20 June 2026
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