Toxic substances are chemicals that may present an unreasonable risk of damage to employee health. They include chemical substances and mixtures like formaldehyde, lead, beryllium, and more. Hazardous substances are chemicals that pose a health hazard (toxic, carcinogenic, corrosive, etc.) or physical hazard (flammable, explosive, oxidizing, etc.). Hazardous substances also include simple asphyxiants and combustible dusts.

Subpart Z, Toxic and Hazardous Substances, is the largest subpart under both 29 CFR 1910 and 1926, for general industry and construction, respectively. It contains the substance-specific requirements for chemicals such as asbestos and respirable crystalline silica. For many, but not all, substances (acrylonitrile, benzene, ethylene oxide, formaldehyde, methylene chloride, etc.) the construction standards simply note that the requirements are identical to the general industry standards.