Hypoxia. Any cause leading to low oxygen in blood - low oxygen in the air ( altitude ) , poor breathing ( poor ventilation) , obstruction in airflow tract ( asthma, copd, mechanical, sleep apnea), problems in lungs loosing area of air and blood contact ( collapse, pneumonia, bloodclots) , bad lungs ( emphysema, fibrosis), lung blood vessels( pulmonary hypertension, vasculitis) and chemicals (cyanide, met.
Lack of oxygen. Lack of oxygen can happen from 1.) high altitude 2.) inadequate ventilation into the lung 3.) airflow obstruction 4.) ventilation and perfusion mismatch and 5.) impaired or loss of surface area in the lung. All causes of low oxygen (hypoxia) can be fit into one of these 5 categories.