Pneumonia can lower the oxygen in your blood because the air spaces in your lungs fill with fluid. The oxygen you breathe does not get into your bloodstream. It is important that the amount of oxygen in your blood be measured within 24 hours of arriving at the hospital to see if you need oxygen therapy. The assessment may include an ABG (arterial blood gas) or pulse oximetry (electrodes attached to a part of your body like a finger, earlobe, or skin fold).