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 
  
    
      
        
          
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    {\displaystyle {P_{a_{CO_{2}}}}}
   greater than 10 kPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.
Inadequate ventilatory response to exertion was indicated by the fact that, despite resting values in the normal range, 
  
    
      
        
          
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    {\displaystyle {P_{et_{CO_{2}}}}}
   rose markedly with exertion even when the divers breathed air at a depth of only a few feet.A variety of reasons exists for carbon dioxide not being expelled completely when the diver exhales:

The diver is exhaling into a vessel that does not allow all the CO2 to escape to the environment, such as a long snorkel, full-face diving mask, or diving helmet, and the diver then reinhales from that vessel, causing increased dead space.