Squamous epithelial metaplasia is generally a reversible process and can be recovered if treated in time; if it persists, it may turn into squamous cell carcinoma, that is, carcinoma has occurred. Squamous epithelial metaplasia refers to the tissue that is not normally squamous epithelium, but is transformed into squamous epithelial tissue when it is damaged by some factors and regenerated and repaired for a long time, for example, the normal tracheal and bronchial mucosal epithelium is columnar epithelium, which is transformed into squamous epithelium when it is repeatedly damaged by chemical stimulation or chronic inflammation from smoking for a long time, which is the normal response of the body to external damage and is usually reversible. The mucosal epithelium can return to a columnar epithelium if the damage is eliminated. If the damage factor persists, it may turn into squamous carcinoma of trachea and bronchus, which is often referred to as squamous carcinoma of lung, but the chance of turning into squamous carcinoma is very low.