Cosmetic Organic Standard (COSMOS)1 is an international and internationally recognized standard for organic and natural cosmetics. COSMOS accepts five preservatives of Annex VI of the European Directive as molecules present in nature: benzoic acid and its salts, benzyl alcohol, dehydroacetic acid and its salts, salicylic acid and its salts, and sorbic acid and its salts.
The main activity of benzoic acid is antifungal but it shows some efficacy against bacteria. It is ineffective against pseudomonas. Furthermore, its optimal efficacy is at pH levels below three. It is safe in concentrations of up to 0.5%.2
Benzyl alcohol is most effective against Gram positive bacteria. It has some efficacy against Gram negative bacteria and yeast, but is ineffective against mold. It shows poor efficacy above pH 7. It is safe in concentrations of up to 1%, but insufficient data supports its safety when inhalation is the primary route of exposure. Furthermore, benzyl alcohol is one of twenty-six allergen substances whose presence must be indicated on the packaging per the European Directive of Cosmetics.
The activity of dehydroacetic acid is strongest against fungi. It has some anti-bacterial properties but is ineffective against Pseudomonas. It is safe as used in current practices and concentration.3
The major activity of salicylic acid is antifungal but it is more effective against bacteria than benzoic acid. It is not as strong an antimicrobial agent. Salicylic acid should be used only in products where the pH is 4 or lower. It is incompatible with iron salts, poorly soluble in water (but shows solubility in fats and oils), and sensitive to UV light. Salicylic acid is safe when formulated to avoid skin irritation and exposure to the sun.4
Sorbic acid is very effective against mold, somewhat effective against yeast, and ineffective against most bacteria. It shows efficacy at a pH of 4.5 or lower. Being an unsaturated fatty acid, sorbic acid is subject to oxidation. It is sensitive to UV light and will turn yellow in solution. Sorbic acid is safe as a cosmetic ingredient as used in current practices and concentration.5