Body plethysmography is more accurate than the nitrogen washout method for patients with obstructive lung diseases like emphysema. Nitrogen washout often underestimates lung volumes because it only measures air in communicating pathways. Studies published in the Journal of Applied Physiology show that plethysmography captures trapped air that nitrogen techniques miss. This makes it the gold standard for measuring Functional Residual Capacity in 100% of the lung space, regardless of airway blockages.