Aspiration Pneumonitis and Pneumonia By Sanjay Sethi, MD, University at Buffalo, Jacobs School of Medicine and Biomedical Sciences Reviewed / Revised Sep 2022 View Patient Education Pathophysiology Symptoms and Signs Diagnosis Treatment Prevention Key Points Aspiration pneumonitis and pneumonia are caused by inhaling toxic and / or irritant substances, most commonly large volumes of upper airway secretions or gastric contents, into the lungs.