Some properties of wool have been replicated by cheaper synthetic materials, but no single synthetic fiber can achieve all the things wool can, nor is any synthetic fiber used in clothing as durable as wool. Cotton fibers work in almost the opposite fashion as wool, absorbing vast quantities of water, holding onto it, then exposing the wearer's skin to that moisture. Cotton, for instance, has been blamed for deaths caused by hypothermia in cold weather and feels clammy and gross in hot weather.

That’s why it’s positively insulting to call any material made from cotton or synthetic fibers “flannel.” Flannel should only ever be made from wool. To suggest that other materials can work as well, by giving them that name, is dangerously misleading.