Objectives At the 85th Annual Meeting of the Japan Society for Occupational Health (31 May, 2012, Nagoya, Japan), we reported a cluster of five patients with cholangiocarcinoma among workers in the offset colour proof-printing section of a small printing company in Osaka, Japan. This was the first report of occupational cholangiocarcinoma in the world. Further investigation led to the identification of a total of 11 patients (Occup Environ Med 2013, in print). Here, we present an epidemiological characteristic of the workers in the printing section.

Methods We identified 62 men who had worked in the proof-printing section for at least one year between 1991 and 2006. We interviewed them to learn about the chemicals they used, and estimated their chemical exposure levels. We also investigated medical records of the 11 patients and calculated the standardised mortality ratio (SMR) from 1991 to 2011.

Results Workers used 1,2-dichloropropane (1,2-DCP) from approximately 1985 to 2006 and dichloromethane (DCM) from approximately 1985 to 1997/98. Exposure concentrations were estimated to be 100–670 ppm for 1,2-DCP and 80–540 ppm for DCM among proof-printing workers. All patients were pathologically diagnosed with cholangiocarcinoma. Age at diagnosis ranged from 25–45 years. Primary cancer sites included the intrahepatic bile duct (five patients) and the extrahepatic bile ducts (six patients). All patients were exposed to 1,2-DCP for 7–17 years and diagnosed with cholangiocarcinoma 7–20 years after the first exposure. Ten patients were also exposed to DCM for 1–13 years. Six of the patients had died by 2011 and the SMR for cholangiocarcinoma was 2,900 (expected deaths: 0.00204, 95% confidence interval: 1.100–6.400).

Conclusion These findings suggest that 1,2-DCP and/or DCM may cause cholangiocarcinoma in humans. New patients appeared among the company workers in 2012 and 2013, and the current number of patients has reached 17.