Level 1: Reasons for phlegm in the baby\’s throat There may be many reasons for the phlegm in the baby\’s throat, the most common of which are respiratory diseases such as colds and flu. In addition, air pollution, allergic reactions, inhalation of foreign bodies, etc. can also cause phlegm in the baby\’s throat. If left untreated for a long time, it may cause other complications. Level 2: How to relieve phlegm in the baby\’s throat. First, ensure that the indoor air is fresh and avoid air pollution.