What it does
The EU's CosIng database identifies disodium EDTA as disodium dihydrogen ethylenediaminetetraacetate, gives its international non-proprietary name as edetate disodium, and lists two functions: chelating and viscosity controlling.1 A chelating agent forms complexes with metal ions that could affect a product's stability or appearance.2 The US Cosmetic Ingredient Review (CIR) describes EDTA and its salts in the same way, as chelating agents, with EDTA typically used below 2% and its salts at even lower concentrations.3
In shampoo, chelating agents such as EDTA bind the magnesium and calcium ions of hard water. Without them, those ions form insoluble soaps, a "scum" on scalp and hair that can make hair look dull.4 That makes a chelating agent part of an ordinary shampoo recipe, which is why its presence does not make a shampoo clarifying: see clarifying vs gentle shampoo.
A helper for preservatives, not a preservative
A review of cosmetic preservation describes EDTA as known for its synergy with several preservatives: it disrupts the outer lipid layer of bacteria and lets other antimicrobial compounds into the cell. Chelating agents in general also withhold the iron microbes need to grow.5
That does not make it a preservative in EU law. A cosmetic may only be preserved with the substances listed in Annex V6, and EDTA and its salts appear in none of the regulation's annexes, so no EU maximum or condition applies to them.6 The cosmetic preservatives guide covers the listed preservatives and their limits.
What the US expert panel concluded
The CIR panel's 2002 report covered EDTA and ten related ingredients, disodium EDTA among them. It found that clinical tests reported no absorption of an EDTA salt through the skin, but that because these ingredients bind calcium, they are likely to affect how other chemicals pass into the skin. It therefore asked formulators to keep this in mind when combining them with ingredients judged safe mainly because they were poorly absorbed.3
The panel also looked at products that may be inhaled as a spray. A conservative exposure estimate put the maximum EDTA dose from inhaling an aerosolised cosmetic below the doses shown to cause reproductive or developmental effects in animals. On the available data, it concluded that the ingredients are safe as used in cosmetic formulations.3
How it appears on a label
As Disodium EDTA.1 Related salts are listed under their own names, such as Tetrasodium EDTA, which CosIng lists as chelating7, and Trisodium EDTA; both were part of the same CIR review.3 At the low levels the panel describes, it may fall in the part of the list below 1%, where EU rules let ingredients appear in any order6, so its position says little. To see which names on your own list are chelating agents, paste it into the ingredient checker.