Mortality study of workers employed at a plant manufacturing chemicals for the rubber industry: 1955-86.

A chronic gavage study on rats and mice carried out by the US National Toxicology Program found that 2-mercaptobenzothiazole (MBT) produced some evidence of carcinogenic activity in male and female rats.' A chronic feeding study with mice provided no evidence for involvement of MBT in the neoplastic process.2 The relevance of these findings to workers exposed at work to MBT (or its derivatives) is uncertain, and information on the safety (or otherwise) of MBT has been sought by means of an epidemiological investigation. MBT is manufactured and used at a chemicals production plant in north Wales, along with vul-canisation inhibitors and accelerators, antioxidants, and many other proprietary products for the rubber industry. It was first manufactured on the site in 1932. Operations also involved sodium MBT and zinc MBT. Production of MBT derivatives such as dibenzothiazyl disulphide (MBTS), n-oxydiethyl-ene-2-benzothiazolesulphenamide (MBS), and n-cyclohexyl-2-benzothiazolesulphenamide (CBS) began in 1939. After discussions with company medical advisers and occupational hygienists, the occupational exposures to the following compounds were also selected for investigation: (1) polymerised 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), (2) n-cyclohexylthiophthalimide (CTP), (3) phenyl-,f-napthylamine (PBN), and (4) aniline and orthotoluidine. The results of an analysis of cancer mortality experienced by production workers employed at this plant are reported here. A mortality study of chemical production workers exposed to MBT is also being carried out at a United States plant that has processes in common with the United Kingdom plant. Materials The company supplied a computer tape comprising identifying particulars for all employees known to have worked at the plant in the period 1955-84 and detailed coded job histories (for hourly paid personnel only). Two subcohorts were identified; a survivor population in employment on 1 January 1955 and an entry cohort of employees who commenced work in the period 1955-84. Employees working for less than six months were excluded. The classification of salaried and hourly staff referred to the most recent job carried out at the plant and it was necessary to categorise personnel on the basis of first job. Job histories for all salaried personnel were sought, therefore, both from paper and microfilmed records stored in the company's personnel department. A number of such employees were found to have commenced employment at the plant as hourly paid staff, and the final data file contained information on 2410 hourly paid personnel and 312 salaried personnel (based on first job). The analysis proceeded on the basis of the hourly paid …