"Relative to 600 ppm, at 1,000 ppm CO2, moderate and statistically significant decrements occurred in six of nine scales of decision-making performance. At 2,500 ppm, large and statistically significant reductions occurred in seven scales of decision-making performance (raw score ratios, 0.06–0.56), but performance on the focused activity scale increased.

Conclusions: Direct adverse effects of CO2 on human performance may be economically important and may limit energy-saving reductions in outdoor air ventilation per person in buildings."

Since all the researchers seem to have been behavioral scientists, they concentrated on decision-making tests. But it would be interesting to know whether spending a large part of your day breathing a relatively high concentration of CO2 also has long-term health consequences. (That's not an experiment that's easy to perform on humans, however.)

Also, it would be interesting to know how many plants you'd need to put in an office space to absorb the CO2 that one human generates. Although the cost of maintaining the plants could be higher than the cost of providing adequate ventilation.