Although it's not toxic, CO2 can be dangerous in high concentrations because it is an asphyxiant (displaces oxygen). Concentrations exceeding 35,000 ppm can cause shortness in breath. Above that level, the central nervous system begins to shut down. Ambient CO2 levels are usually in the 350 to 375 ppm range. The recommended CO2 concentration level for an occupied space is about 0.1%, or 1,000 ppm (refer to ASHRAE Ventilation Standard 62-89).
Building ventilation systems must continuously introduce a sufficient amount of outside air to maintain the CO2 concentration below this level. The above ASHRAE standard provides the minimum amount of outdoor air required per person to accomplish this for various spaces and activity levels. These recommendations are based on the calculated CO2 production per person plus a small margin of safety (see Appendix D of the standard). As an example, the recommended minimum ventilation rate for a typical office space is 20 cfm/person.