For patients at sea level and breathing room air, FIO2 = 0.21, and the (A-a) DO2 can be simplified as follows: where (A-a) DO2 is typically < 20 but increases with age (because of age-related decline in pulmonary function) and with increasing FIO2 (because, although hemoglobin becomes 100% saturated at a PaO2 of about 150 mm Hg, oxygen is soluble in blood, and the oxygen content of plasma continues to increase at increasing FIO2).