The formation of foam inside sputum is actually caused by poor airflow during the process of sputum discharge, and coughing may be one of the main causes. If if you do not cough, due to the strength of breathing is not whole, phlegm can not be directly discharged, inside the lungs, in the airway up and down, in fact, it is a bit like the action of beating eggs. Phlegm is actually originally secreted out to be sticky, or it may be a heap, or it may be mucus. Under normal circumstances there are no bubbles when it is just generated, but in the process of coming out upwards will not be so smooth, in the process of coming out because breathing is to be done constantly, it is impossible to hold the breath for a mouthful of phlegm to inhibit coming out, unless the breathing exercise to that extent, have the ability to also be similar to coughing a little out. But most cases cannot go out at once, and excessive coughing or excessive wheezing will lead to the generation of a lot of foam inside the phlegm. The foam inside the sputum is particularly harmful because it is elastic and expands when inhaling in the trachea and then squashes when exhaling. In fact, there may not be much air in the lungs, but all the foam cancels it out. So once there is sputum discharge, try to use more phlegmolytic drugs, cough less, and then reduce the formation of foam, which also reduces the consumption of lung gas.