AHR and squamous cell lung carcinoma: For lung squamous cell carcinoma (SCC) development in the central airways induced by higher exposure levels of combustion PM from smoking, occupational exposure, or indoor coal combustion, the initiation step appears to be a key mechanism driven by mutagenic PAH-metabolites through the classical AhR:Arnt-CYP pathway, acting in combination with other combustion-derived mutagens.