Triethanolamine
In plain terms
An ammonia-derived liquid used to balance a product's pH and help oil and water blend. On its own it is a mild ingredient; the concern is about what it can react with.
What the evidence shows
Triethanolamine (TEA) is a workhorse pH adjuster, and the concern is not the molecule itself but a reaction it can drive. The CIR safety assessment lays it out: TEA carries small amounts of related amines, and amines can combine with nitrosating agents in a formula to form nitrosamines, some of which are carcinogenic. Whether that happens depends on the rest of the recipe. The EU set a limit of 50 micrograms per kilogram for nitrosamines in both raw materials and finished cosmetics and directs that TEA not be paired with nitrosating agents. The most common nitrosamine found in cosmetics, NDELA, traces back to TEA or its cousin DEA meeting such an agent. Reviewers also flag skin irritation from leave-on products above roughly 2.5 percent. So TEA is fine in a clean formula and a problem in the wrong company.
On the label
Listed as triethanolamine, TEA, or trolamine. The nitrosamine it can form is never on the label, so the signal is TEA appearing alongside a nitrosating preservative such as bronopol (2-bromo-2-nitropropane-1,3-diol) or certain nitrite-containing ingredients.