I have no expertise in this at all. How do they determine if cognitive effects only show up well above 1000? Is it possible that the testing methodologies aren't capable of detecting changes at a lower level?

Edit(

>In fact, at 1400 ppm, CO2 concentrations may cut our basic decision-making ability by 25 percent, and complex strategic thinking by around 50 percent, the authors found.

If this is a continuous scale, it could be affecting us at lower levels already )

Further, isn't it reasonable to assume that regardless of the above, there will be 'hotspots' of c02 where the concentration may be hundreds of ppm higher than the average/background, at least temporarily (e.g. heavy industry centres, or cities in valleys)?

Edit: The article mentions indoor levels are ~500 ppm higher than outdoor, so if a home is in one of my presumed hotspots, some people might be hitting these levels on occasion.