":
Higher inspired oxygen (
  
    
      
        
          
            P
            
              
                i
                
                  
                    O
                    
                      2
                    
                  
                
              
            
          
        
      
    
    {\displaystyle {P_{i_{O_{2}}}}}
  ) at 4 atm (400 kPa) accounted for not more than 25% of the elevation in end tidal CO2 (ETCO2) above values found at the same work rate when breathing air just below the surface.
Hypercapnia also occurs when the breathing gas is contaminated with carbon dioxide, or respiratory gas exchange cannot keep up with the metabolic production of carbon dioxide, which can occur when gas density limits ventilation at high ambient pressures.In severe hypercapnia (generally 
  
    
      
        
          
            P
            
              
                a
                
                  C
                  
                    O
                    
                      2
                    
                  
                
              
            
          
        
      
    
    {\displaystyle {P_{a_{CO_{2}}}}}
   greater than 10 kPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.