During inhalation naphthalene (NA) exposure, the airway epithelial cells are exposed simultaneously to NA aerosols in the airway, and NA, naphthalene oxide (NAO), and other NA metabolites (M) arriving from systemic circulation. Blue double arrow represents diffusion of NA into cells. Both “air-borne” NA and “blood-borne” NA can undergo target tissue bioactivation and cause cytotoxicity, whereas circulating NA metabolites, such as NAO, can also cause airway toxicity. However, the relative contributions of the various sources of reactive NA metabolites to airway toxicity are currently unknown.