Workers with a 20 year latency period did not exhibit a significantly elevated SMR for leukemia whether they had less than one year of exposure (SMR = 1.3, CI 0.2-4.6) or more than one year of exposure (SMR = 0.8, CI 0.1-2.8).
Studies have been undertaken at several facilities that produced 2,4-D and 2,4,5-T. Zober and colleagues (1990) examined German workers exposed to TCDD in a chemical accident in 1953. A single death due to leukemia was reported among the three subcohorts studied, with 0.2 expected. A Dow study (Bond et al., 1988) of 878 workers who manufactured 2,4-D between 1945 and 1983, identified two cases of leukemia/aleukemia with at least 15 years latency (SMR = 3.6, CI 0.4-13.0).
Lynge (1985) examined 3,390 male and 1,069 female Danish workers who were employed in the manufacture of phenoxy herbicides. The main herbicide produced was not 2,4,5-T, although some 2,4,5-T production did exist. Nonsignificantly elevated relative risks of 2.1 were found for all female employees with no latency (two cases) and 4.0 for females involved in the manufacturing or packaging of phenoxy herbicides. Results for 10 year latency in this latter comparison were the same, based on one observed case. For male workers, the RR values were 1.1 with no latency (5 cases) and 1.4 for men involved in the manufacturing or packaging of phenoxy herbicides. Results for 10 year latency in this latter comparison were the same, based on one case. Bueno de Mesquita and colleagues (1993) examined a cohort of workers exposed to phenoxy herbicides and observed a nonsignificant elevation of leukemia in two factories, with an SMR = 2.2 (CI 0.3-7.9) in the exposed group.
The IARC study, which included the Lynge and Bueno de Mesquita studies, found no significant elevation of the SMR for leukemia in any of four exposure categories assigned based on questionnaires and job classifications (Saracci et al., 1991). The period studied was 10-19 years after the first exposure in the cohort. The four groups and their associated SMRs for leukemia were exposed, 1.2 (CI 0.7-1.9, N = 18); probably exposed, no cases; unknown exposure, no cases; and among those not exposed, the SMR = 0.9 (CI 0.2-2.6, N = 3).
Another of the cohorts contributing to the larger cohort of Saracci consisted of 5,784 chemical workers in the United Kingdom involved in the production of MCPA (Coggon et al., 1986). Fourteen deaths from leukemia were observed (SMR = 1.8, CI 1.0-3.0). A suggestion of a dose-response relationship to phenoxy herbicides was reported for three exposed groups: background exposure (SMR = 1.8, N = 5), low potential exposure (SMR = 1.3, N = 3), and high exposure (SMR = 2.1, N = 6). No relationship was identified with duration of exposure for three categories: < 1 month (SMR = 5.9, N = 3), 1-6 months (SMR = 1.1, N = 2), and > 6 months (SMR = 1.4, N = 4). Risk was not elevated with increasing latency period to 10 years from