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.