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.
If associated with a high 
  
    
      
        
          
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    {\displaystyle {P_{a_{CO_{2}}}}}
   the high delivery of oxygen to the brain may increase the risk of CNS oxygen toxicity at partial pressures usually considered acceptable.In many people a high 
  
    
      
        
          
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    {\displaystyle {P_{a_{CO_{2}}}}}
   causes a feeling of shortness of breath, but the lack of this symptom is no guarantee that the other effects are not occurring.