| I | Ciliated respiratory epithelium with underlying squamous metaplasia, at least 12 cell layers thick, associated with early inversion of squamous metaplasia. |
| II | Partially ciliated respiratory epithelium with luminal squamous metaplasia and increased prominence of inversion including the involvement of seromucinous glands. |
| III | Near complete absence of respiratory epithelium, metaplasia replaced by stratified squamous epithelium, and varying degrees of dysplasia. |
| IV | Invasive squamous cell carcinoma in the presence of inverted squamous metaplastic and dysplastic change; features of grades II and III will be present when grade IV is diagnosed. |