A normal carbon dioxide level for humans is 23 to 29 milliequivalents per liter, states MedlinePlus. CO2 is primarily found in the form of bicarbonate. A blood test determines how much bicarbonate is in the serum or liquid part of the blood, so the CO2 level is also found.
An abnormal level of carbon dioxide has many possible causes, according to MedlinePlus. Low levels are sometimes due to diarrhea, Addison or kidney disease, complications of diabetes, too much lactic acid, and methanol or ethylene glycol poisoning. High levels are sometimes the result of vomiting, breathing disorders, an adrenal gland disorder or Cushing's syndrome, which is caused by elevated amounts of the hormone cortisol.