Inadequate exhalation of CO₂ causes  

Any condition that decreases the movement of ${\mathrm{CO}}_2$ from the blood to the alveoli of the lungs to the atmosphere causes a decrease in blood pH due to a build-up of  ${\mathrm{H}}^{+}$. 

${\mathrm{CO}}_2 + {\mathrm{H}}_2\mathrm{O} \stackrel{ }{\to } {\mathrm{H}}_2{\mathrm{CO}}_3 \stackrel{ }{\to } {\mathrm{H}}^{+} + {{\mathrm{HCO}}_3}^{-}$