Why do these people retain CO2?
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. This can occur due to multiple reasons. This includes hypoventilation (such as sleep apnea, COPD or rib cage issues like flail chest), diminished consciousness induced by drugs or metabolic causes like hypothyroidism, lung diseases. Hypercapnia can also occur due to environmental causes such as geothermal activity or rebreathing exhaled CO2.