Final answer:
Hypercapnia is caused by impaired respiratory functions or reduced breathing, while hypocapnia can occur with hyperventilation due to various causes.
Step-by-step explanation:
Hypercapnia, or abnormally elevated blood levels of CO2, can be caused by impaired respiratory functions, reduced breathing (hypoventilation) due to drugs, or even just holding one's breath. On the other hand, hypocapnia, or abnormally low blood levels of CO2, can occur with hyperventilation due to causes such as salicylate toxicity, elevated room temperatures, fever, or hysteria.