are identifying numbers that are assigned to chemical substances by the Chemical Abstract Service of the American Chemical Society and used by the Environmental Protection Agency (EPA) to aid in registering chemicals under the federal Toxic Substances Control Act (TSCA) of 1976. CAS numbers are assigned to generic refinery process streams, such as kerosene and lube basestocks, that contain no additives. Petroleum products containing additives are termed "mixtures" by the TSCA and, as such, do not have CAS numbers. All chemical substances used in such mixtures are assigned CAS numbers and must be listed with the EPA by the refiner or the additive supplier.