A 58-year-old woman comes to the pulmonology clinic because she can no longer climb a flight of stairs without stopping to catch her breath — a problem that has worsened steadily over four years. She also has a persistent dry cough. She has never smoked and spent 22 years working in a textile factory before retiring. On examination, bibasilar fine inspiratory crackles are present. Pulmonary function tests show an FVC of 64% predicted, an FEV1/FVC ratio of 0.85, and a TLC of 70% predicted. DLCO is 42% predicted. HRCT shows bilateral lower lobe ground-glass opacities and fine reticular markings; there are no bullae and no hyperinflation. PaO₂ is 66 mmHg on room air. Hemoglobin is 13.8 g/dL.
Which of the following best explains this patient's reduced DLCO?