If there is only one cough, there is no need to be nervous. Because coughing is a very important physiological reflex of the body in order to keep the lungs and the respiratory tract inside, like the trachea, lung tissue clean. If there is no cough, a drop of water choked in inadvertently, or eating into the rice particles can not come out. Because of the cough reflex, when a foreign body enters the lungs or trachea, it can be expelled by coughing. For example, if you inhale a pollutant, it can be wrapped up in secretions inside the lungs and trachea, forming a grayish-black phlegm that may come out. When inflammation occurs, you can also cough to expel the inflamed secretions from the lungs, which is the yellow phlegm you normally see. Therefore, in the absence of underlying lung disease and other discomforts, an occasional cough may be irritated by dust in the air while breathing, so there is no need to be nervous.