Aspiration of gastric acid causes a chemical pneumonia, or aspiration pneumonitis (Mendelson’s syndrome). This occurs due when a large volume of gastric acid is aspirated (>/= 4ml/kg of gastric contents with a pH of < 2.5 are considered to be critical factors), which creates a chemical reaction in the lung tissue. Other types of aspiration pneumonia are bacterial (aspiration of material from oral and pharyngeal areas), and exogenous lipoid pneumonia (aspiration of some type of oil, ie, mineral oil or vegetable oil).