Procedures used by NIOSH in developing recommended permissible exposure concentrations for toxic substances in the working environment are reviewed. Proposed standards are intended to define atmospheric concentrations that will not injure a worker exposed 40 hours a week for the entire working lifetime. The majority of the NIOSH recommended standards are based on limited data from studies of human subjects and on the results of animal experiments. Epidemiologic studies often are not available, especially for new chemicals. Animal studies are used to provide information on the dose response effect of a chemical and may serve as the basis for extrapolation to the maximal tolerable concentration for humans. NIOSH standards may specify time weighted average concentrations throughout a workday of 8 to 10 hours, ceiling concentrations during a brief sampling period, or both. NIOSH also may define a threshold action concentration. The practicality of lower exposure limits is constrained by the sampling and analytic methods available. Biologic sampling, commonly of blood or urine, may be used in conjunction with environmental limits for determination and control of occupational exposure to toxic substances but involves the issues of specificity, sensitivity, and feasibility of the monitoring technique. Information on industrial exposure is used to understand the characteristics of the hazardous agent as it is used, manufactured, or stored in the workplace, including the physical and chemical properties of the agent, equipment used, production processes, exposure concentrations, and control measures. Summaries are presented of criteria documents developed by NIOSH that outline procedures used for developing exposure limits for nitric-acid (7697372), nitrogen-dioxide (10102440), nitric-oxide (10102439), lead (7439921), and chromium (7440473). The authors conclude that NIOSH recordkeeping requirements are crucial for setting and updating permissible exposure concentrations.