The word "azo" is a prefix commonly used in chemistry to refer to a molecule that contains nitrogen. It usually indicates that the molecule has a double bond connecting two nitrogen atoms. Full definition
Like artificial sweeteners, consumption of azo dyes might not be as harmful as some claim, but there is evidence that these chemicals can cause various negative effects in the body. (legionathletics.com)
These are said to be processed only with Skal * approved bleaches (no dioxin), use azo free and heavy metal free dyes, and be finished («sized») with an Indian soap nut, instead of the usual formaldehyde. (treehugger.com)
Jinhui Feng, Carl E. Cerniglia, and Huizhong Chen, Toxicological Significance of Azo Dye Metabolism by Human Intestinal Microbiota,» Frontiers in Bioscience (Elite Edition) 1, no. 4 (2012): 568 — 86 ↑ (legionathletics.com)