A new study shows strong effects of what should be a negligible CO2 concentration.
The paper "Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance" exposed people to three levels of CO2: 600, 1000, and 2500 ppm while keeping constant other factors such as ventilation, humidity, temperature, and indoor pollutant concentration. Of course, the participants didn't know how much CO2 was in the air.
At the 2500 ppm level, most cognitive performance measures were temporally reduced (they used the SMS management test). Some changes were drastic.
Since normal air is 400 ppm, 2500 ppm sounds like a lot. However, it is dwarfed by internal production. Humans exhale 50,000 ppm CO2 in the lungs, meaning that the lung air averages 25,000 ppm (as a rough approximation). The extra 2500 ppm is only a 10% increase in the average "atmosphere" that the blood is exposed to. Furthermore, people partially compensate increased P(CO2) with deeper ventilation (the effect takes much larger concentrations to be noticeable). Why would this extra 10% have such a strong effect?
Personally I am skeptical. This study has such high impact (millions of people's performance) that it needs replication. Maybe the hissing sound of the injector got people worried (more hissing at a higher concentration!). Maybe an experimenter (who knew the concentration) was watching the people and that subconsciously affected them.