What CO2 level is acceptable in normal breathing?

In normal breathing, the acceptable level of carbon dioxide (CO2) in the air is typically around 0.04% (400 parts per million). In human physiology, the concentration of CO2 in exhaled air is much higher, usually around 4-5%. While elevated CO2 levels in the environment can indicate poor air quality, concentrations below 1% are generally considered safe for normal breathing.