In a letter to Mr. Edward J. Baier, Acting Director, NIOSH, dated December 16, 1974, Dr. John A. Zapp, Director, Haskell Laboratory, E.I. du Pont de Nemours and Company (Du Pont), Wilmington, Delaware, expressed concern over the potential carcinogenicity of chloroprene (2-chlorobutadiene). Du Pont had begun looking closely at this substance recently because of the similarity in chemical structure with vinyl chloride. Du Pont has utilized chloroprene in the production of neoprene (polychloroprene) since 1931. In the course of a literature search on chloroprene toxicity, Du Pont uncovered two recent Russian articles that suggest an increased incidence of skin and lung cancer in workers exposed to chloroprene. Also, two other articles in the Russian literature were located that described animal experiments in which chloroprene adversely affected embryo development in rats and mice. Du Pont has informed its employees of the Russian reports and has alerted its customers to the possibility of "escaping chloroprene" during the processing of neoprene. The company is conducting epidemiological studies in humans and animals to ascertain the carcinogenic potential of chloroprene. Chloroprene is a colorless liquid that is slightly soluble in water. It is soluble in alcohol and diethyl ether, and has a vapor density of 3.0, three times that of air, with a boiling point of 59.4 degrees-C. Chloroprene is used as a chemical intermediate largely as a monomer for the manufacture of a synthetic rubber. It is a chlorine-substituted derivative of 1,3-butadiene. Chloroprene can polymerize spontaneously at room temperature, the process being catalyzed by light, peroxides and other free radical initiators. It can also react with oxygen to form polymeric peroxides. Because of its instability, flammability, and toxicity, chloroprene has no end product uses. It is produced in large quantities mainly for polymerization and marketing under the trade name of Neoprene. Neoprene was developed in the U.S. by Carothers and was originally introduced by Du Pont in 1931 under the brand name Duprene. Although during recent years other suppliers have come on the market with their own brand name, neoprene is generally used as a generic name for polychloroprene rubbers. Neoprene is obtained by emulsion polymerization of chloroprene (2-chlorobutadiene) and consists mainly of 1,4-transpolychloroprene. There are two main classes, the sulfur modified type and the nonsulfur modified type, indicating the differences in polymerization techniques. Several subtypes of both are available, differing in viscosity and crystallization rate. Neoprene's most valuable properties are its resistance to weathering and oil. It is also resistant to abrasion, heat, flame, oxygen, ozone, and solvents. The main applications of neoprene are in high performance articles such as cable sheaths, hoses, fabrics, adhesives, and a large number of technical rubber articles. The automotive industry is the largest consumer of neoprene.