The human body can tolerate a short sauna session. It cannot tolerate living in a sauna 24/7. That said, the authors note that many people regularly do indeed

The human body can tolerate a short sauna session. It cannot tolerate living in a sauna 24/7. That said, the authors note that many people regularly do indeed experience long periods of higher CO2 levels, and that this may in part explain the HCO3− blood level trend they're seeing and has to be taken into consideration. They also note "more recent studies of humans and animals have shown harmful effects of CO2 exposure at [1000 to 3000 ppm CO2], such as changes in heart rate, kidney calcification, oxidative stress, neural damage and inflammation".