Normal CO2 or carbon dioxide levels are between 23 to 30 mEq/L in humans, according to WebMD. However, this is just a reference point as normal ranges vary from lab to lab. The lab report given should contain the normal range for that particular lab.
High carbon dioxide levels can be caused by vomiting, blood transfusions, dehydration, anorexia, chronic obstructive pulmonary disease, heart disease, Conn's syndrome, Cushing's disease, fluid in the lungs and overusing medications that contain bicarbonate such as antacids. Low-carbon dioxide levels can be caused by hyperventilation, diarrhea, dehydration, severe malnutrition, hyperthyroidism, uncontrolled diabetes, a massive heart attack, liver or kidney disease and alcohol or aspirin overdose.