you and I might not do too well trying to breath 1000 ppm CO2 air though. Too much higher than that and you reach the levels they use to exterminate pests from greenhouses. Its the methane from the bovine excretions that is a whole other can of worms in terms of atmospheric impacts more than the CO2
Data collected on nine nuclear-powered ballistic missile submarines indicate an average CO2 concentration of 3,500 ppm with a range of 0-10,600 ppm, and data collected on 10 nuclear-powered attack submarines indicate an average CO2 concentration of 4,100 ppm with a range of 300-11,300 ppm (Hagar 2003).
My bad, it was 10,000 ppm for extermination.