How did the military reach these CO2 levels in their tests? Were these levels created artificially or through human aspiration? It might be that CO2 in the air

How did the military reach these CO2 levels in their tests? Were these levels created artificially or through human aspiration? It might be that CO2 in the air is simply a proxy. My personal observation is that usually good air quality is reached when CO2 levels are around 600-1000 ppm. Lower it is fresher the air feels.