Sodium Benzoate
In plain terms
A common preservative that stops mold, yeast, and bacteria, used in acidic foods, drinks, and some personal care. Safe by itself; the issue is one specific pairing.
What the evidence shows
Sodium benzoate is safe on its own and the body clears it within a day. The real issue is a reaction. Work published in the Journal of Agricultural and Food Chemistry showed that when benzoate sits in a drink alongside vitamin C, it can break down into benzene, a known human carcinogen, with heat, light, and trace metals speeding the reaction. The FDA confirmed the effect, found a number of beverages above the drinking-water limit for benzene in its testing, and worked with makers to reformulate. The amounts are small and most products test low, so this is not a blanket reason to avoid the preservative. It is a reason to watch one combination, which is why the verdict turns on the product rather than the ingredient. In personal care the same preservative also appears as benzoic acid (E210), its uncombined free-acid form, which is a recognized mild irritant and can cause brief non-allergic stinging or flushing on sensitive skin, though it is well tolerated at the low levels used.
On the label
Listed as sodium benzoate or E211. Benzene is never named on a label; the thing to spot is sodium or potassium benzoate sharing an ingredient list with ascorbic acid (vitamin C) or erythorbic acid, especially in fruit or diet drinks stored warm.