[azernik]

Generally caves have elevated CO2 levels not because of human respiration, but because of outgassing - they're generally eroded out of carbonate minerals, and so the water flowing in carries CO2 inside at higher concentrations than the outside world. The smaller the subsystem the human is isolated in, the faster they perturb that equilibrium toward an even higher CO2 level.

The ESA, of all groups, has a pretty good write-up on this: http://blogs.esa.int/caves/2014/11/19/the-science-of-caves-e... They use cave exploration for astronaut team-building, process/workflow training, and acculturation.