Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance
Results: 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.
There are confounding factors here. They used a poorly-ventilated office environment. It isn't only carbon dioxide levels that are increased, but other gases that are emitted from the various furnishings, paints, cleaning chemicals, and what not.
If they really want to do an accurate study, they could just use Carbogenetic's carbogen machine to continually breath in a gaseous mixture that involves mixing air with carbon dioxide. However, the lowest setting is at 5% CO2, which equates to 50,000 ppm. While this would be a high CO2 mixture, it at the very least ensures no toxic gases are included, and that no stale air is used.
It must be either a poorly designed experiment or one intended to make CO2 look really really bad.