Naphthalene animal carcinogenicity and human relevancy: overview of industries with naphthalene-containing streams

Bioassays in mice and rats exposed via inhalation to naphthalene show incidences of lung and nasal cancer, respectively. To address the question of human relevancy, a literature search for cancer case reports in workers exposed to naphthalene was performed, along with an evaluation of major studies from industries with naphthalene-containing streams having the highest naphthalene exposures and/or most extensive epidemiology data. Although no epidemiologic studies of workers exposed solely to naphthalene were found, a population-based case-control study of oral and oropharynx cancer found no relation with naphthalene exposure. Limited case reports of laryngeal and colorectal cancer and naphthalene exposure exist, but these data are inadequate for evaluating human cancer risk. No case reports of nasal tumors were found, which is informative because case reports have historically identified several occupational carcinogens. Combined with anatomic and metabolic differences between rodent and human upper airways and data suggesting that cancer potency based on the rat bioassay is overestimated, relevancy of rat nasal tumors to humans is questionable. For lung cancer, existing human studies are insufficient to make firm conclusions about the presence or absence of a potential naphthalene-related risk, although no occupationally-related lung cancer risks were identified in the industries evaluated.