Inhaling too much of any gas that does not contain oxygen is deadly. A condition known as hypercapnia(too much carbon dioxide) occurs. The human body would respond by hyperventilating, sensing it was low on oxygen (hypoxemia), and the person would become dizzy, panicky, with bluish skin, eventually passing out and after about 4 minutes or so, brain death from lack of oxygen will occur, in addition to massive organ failure if the CO2 exposure was prolonged.
Inhaling too much CO2 produces more H2CO3 in the blood.
CO2 + H2O <===> H2CO3
The H2CO3 ionizes to produce more H+ in the blood.
H2CO3 <===> H+ + HCO31-
More H+ in the blood makes the blood more acidic; the pH is lowered below the normal value of 7.35. If it is lowered too much, a person may get acidosis.