Critical cartridge performance factors which must be considered prior to selecting an organic vapor air purifying respirator cartridge are reviewed. Organic respirator cartridges bind gas and vapor molecules by sorption mechanisms. The amount of gas and vapor sorbed depends on the physical and chemical nature of the sorbent and sorbate. The properties of an ideal sorbent are described and the factors which can affect the potency and efficiency of the sorbent are presented. Environmental factors which need careful examination are concentration and composition of the contaminants, amount of oxygen, explosiveness of the contaminant, temperature, and humidity. These elements control the flow pattern and are responsible for the service life of the cartridges. Procedures to test the breakthrough time and service life of different types of cartridges are presented with respect to tests performed with 121 organic vapors and gases. Besides environmental factors, work rate and other work related factors must be considered to assure safety of workers exposed to hazardous vapors and gases. The limitations placed on use of organic vapor cartridges by regulations to assure worker safety are emphasized, discussed and exemplified. Among the several environmental factors that affect the breakthrough time of cartridges, desorption of vapors, gases and impregnating agents have received the least attention. Studies on this subject are reviewed. The author concludes that if accurate assessment of the contaminants, their concentration, and their warning properties are not carefully examined, worker safety cannot be assured.