Continued epidemic of bladder cancer in workers exposed to ortho-toluidine in a chemical factory.

Ortho-toluidine (o-toluidine), an aromatic amine, is classified by the International Agency for Research on Cancer as a probable human carcinogen. A cohort study published in 1991 reported a 6.5-fold excess incidence of bladder cancer in a chemical plant that used o-toluidine. We report 19 additional cases of bladder cancer among workers in this cohort, yielding a total of 34 cases of bladder cancer in the cohort to date. The number of bladder cancers diagnosed in the recent period has increased. The timing of onset of exposure to o-toluidine of numerous cases of bladder cancer after 1968, and especially 1975, suggests that potentially confounding occupational exposures other than o-toluidine were not responsible for the observed excess bladder cancer. A formal cohort update is strongly indicated. This study further supports the human bladder carcinogenicity of o-toluidine.