An industrial hygiene approach was taken to rationalize monitoring needs on Space Station Freedom. The standard practice for monitoring in analogous ground-based situations, supplied air, confined spaces, and submarines was reviewed. The recommended minimum monitoring effort should include continuous monitoring for total pressure, humidity, O2, CO2, CO, H2, hydrocarbons, refrigerants, and halons. Additional specific monitoring requirements would depend on the substances likely to be present, the amount, and their hazard. This approach requires a thorough knowledge of the onboard processes, their products, and by-products. Other factors influencing the monitoring requirements for Space Station Freedom include: the enclosed space, the recirculation of supply air, the experiences of past missions, the unique experimental and manufacturing processes, and the interfacing of other modules with the U.S. core modules. Monitor development and selection will be complicated due to the many technologies competing for the life support systems, and the number of experimental payloads under development, each having their own unique monitoring requirements.