Background and design of a Danish cohort study of workers in phenoxy herbicide manufacture.

In 1982, a Danish cohort study was initiated of workers in the manufacture of 2,4-dichlorophenol- and 4-chloro-ortho-cresol-based phenoxy herbicides, after a governmental working group had indicated the need for further data on the long-term health consequences of exposure to these phenoxy herbicides. A cohort study of workers in the manufacture of these substances was considered to be a valuable supplement to the Swedish case-control studies of patients exposed mainly in spraying. Manufacture of phenoxy herbicides was commenced in Denmark by Kemisk Vaerk Koege (KVK) in 1947, and this company has produced 2,4-D and MCPA, and later 2,4-DP and MCPP. Very limited amounts of 2,4,5-T have been processed in this plant, mainly in the formation of esters based on a purchased acid. Manufacture of MCPA was commenced by Esbjerg Kemikaliefabrik (EK) in 1951, and this production was later supplemented with 2,4-DP. Furthermore, MCPA was produced by Cheminova and Danske Gasvaerkers Tjaerekompagni for short periods in the late 1950s. All persons employed at KVK and EK from the time when these plants began operation, in 1933 and 1951 respectively, until 1982 were intended to be included in this cohort study. The registration was based on company records and, from 1964 onward, supplemented with data from a public pension scheme (ATP). Linkage of company records from KVK with ATP records for the overlapping period 1964-1980 showed 2,163 persons to be known in both datasets, whereas 614 persons were known only from the ATP records. The data collection has consequently shown that ATP provides a valuable data source for control of company records in Denmark. For the study period before 1964, the number of registered employees could be controlled by comparison with the number of employees reported by the companies on questionnaires for the national industrial statistics 1945-1969. In the analysis of this cohort study, special attention was given to soft tissue sarcomas and malignant lymphomas, which are the diagnostic groups associated with exposure to phenoxy herbicides in the Swedish case-control studies. Soft tissue sarcomas, in both organs and connective tissue, can be identified using the classifications found in the Danish Cancer Registry.