> 420/800/1200 ppm has no meaningful difference

That claim contradicts the results of the NIEHS study "Is CO2 an Indoor Pollutant?"[1], which has found that "Relative to 600 ppm, at 1,000 ppm CO2, moderate and statistically significant decrements occurred in six of nine scales of decision-making performance."

[1] https://web.archive.org/web/20160305212909/http://ehp.niehs....