Iodopropynyl Butylcarbamate (IPBC)
In plain terms
An iodine-based preservative borrowed from the paint and wood industry. It works well but is a known skin sensitizer, and because it releases iodine the EU caps how much can be used and keeps it out of products for young children and out of lip and mouth products. Fine within limits for most, worth avoiding if your skin reacts.
What the evidence shows
Iodopropynyl butylcarbamate, or IPBC, is a broad-spectrum preservative that started as a wood and paint preservative and moved into cosmetics, where it guards against bacteria and fungi. Two things shape how to think about it. First, it is a recognized skin sensitizer: as its cosmetic use grew, cases of allergic contact dermatitis rose, and it now sits on European patch-test panels with a sensitization rate around or above 1 percent. Second, it contains and releases iodine, which led the EU to limit it tightly, capping it at 0.02 percent in rinse-off products, 0.01 percent in leave-on, and less in deodorants, while banning it from lip and oral products and from products for children under three, out of concern about iodine load. Leave-on products above a set level must carry a Contains Iodine warning. It also penetrates skin readily and is toxic to breathe in, so it is kept out of sprays. Within those limits it is treated as acceptable for most adults, which is why this lands at depends rather than avoid: the question is whether you are in a group it is restricted for, whether you react to it, and whether the product respects the limits. If you have sensitive skin, react to iodine antiseptics, or are choosing products for a young child, it is one to skip.
On the label
Listed as iodopropynyl butylcarbamate or IPBC, sometimes flagged by a Contains Iodine note. Reasonable to avoid if you have sensitive skin or an iodine sensitivity, and worth keeping out of young children's products. For most adults, in a rinse-off product within the legal limits, it is lower-risk.