Silicon dioxide
Finely ground mineral sand used to stop powdered foods clumping together. One of the most chemically inert substances used in food.
EFSA could not confirm the acceptable daily intake of 'not specified' because of database limitations, judged the current EU specifications insufficient to characterise the additive (they do not exclude nanoparticles), and asked for the specifications to be revised including particle-size distribution.
What is it?
Silicon dioxide is the same mineral that makes up sand and quartz, processed into an ultra-fine amorphous (non-crystalline) powder. In food use it is called synthetic amorphous silica (SAS) and exists in several physical forms: fumed silica, precipitated silica, silica gel and hydrous silica. All are chemically identical mineral particles but differ in how they are manufactured and their surface texture.
What does it do?
Absorbs moisture on the surface of dry particles, preventing them from sticking together. The fine silica particles sit between food powder granules and act as tiny ball-bearings, keeping the product free-flowing. Because it is chemically inert and does not dissolve in water or fat, it passes through the gut essentially unchanged and is not absorbed into the body in any meaningful quantity.
Where you will see it
Powdered spices, salt, sugar, dried milk, coffee creamers, dried egg, garlic powder, onion powder, icing sugar, protein powders, powdered flavourings, supplement capsule fillers, and the outer coating of chewing gum. On a UK label it appears as silicon dioxide or E551.
What the science says
Nanoparticles and the ongoing specification question
E551 as used commercially contains a proportion of particles smaller than 100 nanometres. The EU's current legal specifications do not define or limit this nano fraction. EFSA's 2018 re-evaluation flagged this as a data gap and called for particle-size characterisation to be built into the specifications. A 2024 follow-up assessment reviewed additional data and concluded that the silica particles, even in the nano range, show very low absorption from the gut and do not appear to accumulate in the body. EFSA recommended that the EU revise specifications to include particle-size criteria and lower permitted limits for trace toxic elements (such as arsenic and lead) in the additive.
EFSA found current EU specifications for E551 were insufficient to characterise particle size distribution and did not exclude the presence of nanoparticles, identifying this as a data gap requiring follow-up.
In a 2024 follow-up opinion, EFSA concluded that E551 does not raise a safety concern for any population group at reported use levels, and that nano-range particles have limited bioavailability and do not accumulate in the body, though it recommended revised specifications to include particle-size criteria.
Absorption and what the body does with it
Silicon dioxide is not a nutrient and the body has no mechanism to take it up in significant amounts. Studies consistently show that amorphous silica passes through the gastrointestinal tract with minimal absorption. Any silica that does reach the bloodstream is cleared by the kidneys in urine. There is no evidence of tissue accumulation at food-additive exposure levels.
Amorphous silica has very low oral bioavailability in both animal and human studies, with absorbed fractions rapidly excreted in urine; tissue accumulation has not been observed at food-use exposure levels.
Distinction from crystalline silica (the industrial lung hazard)
Crystalline silica dust, produced when cutting stone, concrete or quartz, causes silicosis and is an IARC Group 1 carcinogen when inhaled occupationally. The silica used as a food additive is amorphous (non-crystalline), not crystalline, and is ingested rather than inhaled. Regulatory and toxicological assessments treat these as distinct hazards. IARC's Group 1 classification does not apply to the amorphous form used in food.
IARC first classified inhaled crystalline silica dust (quartz and cristobalite) as Group 1 (causes lung cancer in humans) in 1997 in the context of occupational exposure; this classification was reaffirmed in the comprehensive 2012 review of human carcinogens. It does not extend to amorphous silica consumed orally as a food additive.
Where it stands with the regulators
Who should be careful
No population group is currently advised to avoid it. People with very rare silica sensitivity or occupational lung disease from crystalline silica exposure sometimes ask about it, but the food additive form (amorphous, ingested) is chemically distinct from the occupational hazard. Look for silicon dioxide or E551 on the label.
The honest read
Silicon dioxide is about as chemically inert as food additives get. The body does not absorb it in meaningful amounts and it has been in food use for decades without any documented harm at additive exposure levels. The live question is a technical regulatory one: commercial E551 contains a proportion of nano-scale particles, and EU specifications do not yet define or cap that fraction. EFSA reviewed this specifically in 2018 and again in 2024 and concluded no safety concern, but is pushing for better characterisation in the specifications. The outstanding work is specification refinement, not a finding of harm. For the overwhelming majority of foods where E551 appears, the amounts involved are very small, often a fraction of a gram per serving.
Related additives
Common questions
Is E551 banned in the UK?
No. Silicon dioxide (E551) is approved for use in the UK under the assimilated EU Regulation 1333/2008 and appears on the UK FSA's approved-additives list. It has not been banned or restricted.
Is the nano-particle form of E551 a health risk?
Commercial E551 contains nano-scale particles (below 100nm) but current EU and UK specifications do not define that fraction. EFSA reviewed the evidence in both 2018 and 2024 and found no safety concern at the levels used in food, noting that these particles show very low absorption from the gut and do not appear to accumulate in the body. EFSA has recommended that specifications be updated to include particle-size characterisation. The issue is an open regulatory one, not a finding of harm.
What foods contain E551?
Powdered spices, table salt, icing sugar, dried milk, protein powders, coffee creamers, garlic and onion powders, dried egg, food supplement capsules, and chewing gum. It is one of the most widely used anticaking agents in dry foods.
Is E551 vegan?
Yes. Silicon dioxide is a mineral with no animal-derived components and is suitable for vegans and vegetarians.
Sources
- EFSA ANS Panel: Re-evaluation of silicon dioxide (E 551) as a food additive, EFSA Journal 2018;16(1):5088
- EFSA ANS Panel: Re-evaluation of silicon dioxide (E 551) as a food additive in foods for infants below 16 weeks of age and follow-up, EFSA Journal 2024;22(10):8880
- IARC Monographs Volume 68: Silica, Some Silicates, Coal Dust and para-Aramid Fibrils (1997)
- IARC Monographs Volume 100C: Arsenic, Metals, Fibres, and Dusts - A Review of Human Carcinogens (2012)
- UK FSA Approved Additives and E Numbers
- PubMed: Re-evaluation of silicon dioxide (E 551) 2018 (PMC7009582)
- PubMed: Re-evaluation of silicon dioxide (E 551) 2024 (PMC11483555)
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