An abnormally large amount of thick, viscous or stringy mucous in therespiratory system (lungs and throat) and/or digestive passages. Additional Info: Phlegm is a general symptom of respiratory infections such as the common cold, or bronchitis. As described below, it can also be a symptom of respiratory irritation. MSDS Relevance: This term usually appears on an MSDS as a symptom of inhalation. Respiratory irritation by chemical dusts or vapors in the workplace can cause or increase the formation of phlegm. The best way to limit exposure to such materials is substitution of a safer or less irritating substance. If that's not possible,engineering controls such as ventilation are the next step to consider as they will protect all workers. Finally, personal protective equipment such as a dust mask or respirator can protect individual workers. See also: Bronchitis, Emphysema, Irritant