An equation used to determine the alveolar partial pressure of oxygen (PAO₂): PAO₂ ≈ [FiO₂ × (atmospheric pressure - 47)] - [(PaCO₂)/R], where FiO₂ is the concentration of inhaled oxygen, PaCO₂ is the partial pressure of carbon dioxide in the artery, atmospheric pressure is 760 mm Hg at sea level, and R is the respiratory quotient.