The purpose of this study was to examine the effects on humans of exposure to carbon dioxide (CO2 ) and bioeffluents. In three of the five exposures, the outdoor air supply rate was high enough to remove bioeffluents, resulting in a CO2 level of 500 ppm. Chemically pure CO2 was added to this reference condition to create exposure conditions with CO2 at 1000 or 3000 ppm. In two further conditions, the outdoor air supply rate was restricted so that the bioeffluent CO2 reached 1000 or 3000 ppm. The same 25 subjects were exposed for 255 min to each condition. Subjective ratings, physiological responses, and cognitive performance were measured. No statistically significant effects on perceived air quality, acute health symptoms, or cognitive performance were seen during exposures when CO2 was added. Exposures to bioeffluents with CO2 at 3000 ppm reduced perceived air quality; increased the intensity of reported headache, fatigue, sleepiness, and difficulty in thinking clearly; and reduced speed of addition, the response time in a redirection task, and the number of correct links made in the cue-utilization test. This suggests that moderate concentrations of bioeffluents, but not pure CO2 , will result in deleterious effects on occupants during typical indoor exposures.