Increased work of breathing accounted for most of the elevation of 
  
    
      
        
          
            P
            
              
                a
                
                  C
                  
                    O
                    
                      2
                    
                  
                
              
            
          
        
      
    
    {\displaystyle {P_{a_{CO_{2}}}}}
   (alveolar gas equation) in exposures above 1 atm (100 kPa), as indicated by the results when helium was substituted for nitrogen at 4 atm (400 kPa).
Vasodilation is another effect, notably in the skin, where feelings of unpleasant heat are reported, and in the brain, where blood flow can increase by 50% at a 
  
    
      
        
          
            P
            
              E
              
                T
                
                  C
                  
                    O
                    
                      2
                    
                  
                
              
            
          
        
      
    
    {\displaystyle {P_{ET_{CO_{2}}}}}
   of 50 torrs (0.066 atm), Intracranial pressure may rise, with a throbbing headache.