Carbon dioxide is the by-product of respiration produced in body and eliminated from blood by lungs in expiration. When this oxygen is not removed enough, it stays in blood and the condition is called hypercapnia. People who retain excess carbon dioxide in blood are called carbon dioxide retainers. High carbon dioxide levels produce headaches, drowsiness, lethargy, confusion and may lead to coma or death. Causes of hypercapnia include conditions like: brainstem disease, hypothermia, metabolic disorders like thyroid conditions, obesity, sleep apnea, sedative overdose, stroke, ankylosing spondylitis, spinal cord disorders like myasthenia gravis, G.B syndrome or muscular dystrophy, Toxins such as tetanus or botulism, upper airway disorders and most of all COPD.