What it does
The EU's CosIng database describes sodium benzoate as the sodium salt of benzoic acid and lists three functions: preservative, anticorrosive and fragrance.1 As a preservative it inhibits the growth of microorganisms in the product. Anticorrosive, in CosIng's terms, means protecting the packaging from corrosion.
Why acidity matters
A World Health Organization assessment explains the chemistry. It is undissociated benzoic acid that does the antimicrobial work. Benzoic acid itself dissolves poorly in water, so sodium benzoate, about 200 times more soluble, is often used instead: under acid conditions it converts to benzoic acid.2
The same assessment notes that, in food, about 0.1% is usually enough to preserve a product adjusted to pH 4.5 or below.2 So sodium benzoate does more in an acidic formula than in a neutral one. An ingredient list does not state a product's pH, so the list alone cannot tell you how well it is working.
The EU limits
| Product type | Maximum, counted as benzoic acid |
|---|---|
| Rinse-off products, except oral products | 2.5% |
| Oral products | 1.7% |
| Leave-on products | 0.5% |
Benzoic acid's other salts and its esters share a separate entry, 1a, with a single maximum of 0.5% counted as the acid.3 A cosmetic may only be preserved with substances on this list, within their conditions.4
Also a food additive: E 211
The EU food additive regulation lists sodium benzoate as E 211, next to benzoic acid as E 210.5 Food and cosmetics have separate rules, so the E number has no bearing on the cosmetic limits above.
What sources say
The US Cosmetic Ingredient Review assessed benzoic acid and its salts with related ingredients in 2017. It described them as working mostly as fragrance ingredients or preservatives, and concluded they are "safe in the present practices of use and concentration" described in its assessment.6
The WHO assessment notes that benzoic acid and sodium benzoate can cause non-immunological contact reactions, such as hives, in sensitive people, particularly those with frequent urticaria or asthma. In one study of 2,045 dermatology patients, about 0.2% reacted in patch tests, and the assessment concluded that skin reactions to them are rare in the healthy general population.2
How it appears on a label
As Sodium Benzoate. Benzoic Acid is listed under its own name and shares the same Annex V entry.3 In the example list in how to read a shampoo ingredient list it sits among the small print near the end. The list gives no concentration, so it cannot show which of the limits above a product is near.