In packaging, any of a variety of thermoplastic materials used in the packaging of foods, beverages, and other materials that require some degree of protection from environmental forces, such as water, oxygen, etc. Of particular importance is the ability to impede the penetration of oxygen. A polymer is thus described as a "barrier" when its oxygen permeability is less than 10 cm3 • mil/100 in.2 • d • atm at 23ºC. Such a barrier will also impede the penetration of other gases (such as carbon dioxide) into the package, as well as other substances (such as odors) out of the package. Another consideration in the use of barrier polymers is the extent to which the packaging material will leech the flavor or odor of the material being packaged, as well as the extent to which the packaged material will absorb the flavor and odor of the packaging. Some commonly used barrier polymers are polyvinyl alcohol, polyacrylonitrile, ethylene:vinyl alcohol, PVCD homopolymer, cellophane, as well as various polyvinyl chloride and polyethylene formulations. Where once glass and metal were used as barriers in packaging, advances in polymeric formulations have replaced these materials in many applications. Even more recent advances in polymer chemistry have resulted in packages which can be specifically formulated for certain materials.