i'm aware of this one. clinical evidence - do moderately high CO2 levels CAUSE a disease? afaik no. the test in this study: they measure something real, but it is not used to diagnose someone routinely (despite what they say). they had a surprising result where performance on one of the subtests increased, and some were not affected (see https://pubmed.ncbi.nlm.nih.gov/23008272/#&gid=article-figur...). i am basically saying that, CO2 levels - do they cause headaches like the user is saying? I don't know, nobody has investigated that. 2500 ppm is very high concentration. it merits more study but not a panic.