because of the possibility that, for example, some health effects may be associated with exposure to 2,4-D in agriculture and forestry. The herbicide 2,4-D does not contain TCDD, although small quantities of other dioxins are present.

For TCDD-exposed populations, serum measures of TCDD concentrations can be collected from a representative sample of those exposed. Serum biomarkers of TCDD exposure are sometimes used to estimate the degree of prior exposure of individuals; however, there are limitations to their use.

The available information on occupational and environmental exposures to dioxin, the contaminant found in 2,4,5-T includes studies of residents living in and around Seveso, Italy, who were exposed during industrial accidents; chemical plant workers who were occupationally or accidentally exposed to TCDD during the production of 2,4,5-T or other phenoxy herbicides or chlorophenols such as hexachlorophene or trichlorophenol; sawmill workers exposed to higher chlorinated dioxins from contaminated wood preservatives; and pulp and paper workers exposed to dioxin through the pulp bleaching process.

Occupational Studies

Production Workers

One of the most extensive sets of data on workers engaged in the production of chemicals potentially contaminated with TCDD has been compiled by NIOSH. From 12 chemical companies, more than 5,000 workers were identified from personnel and payroll records indicating whether the worker had been involved in production or maintenance processes associated with TCDD contamination (Fingerhut et al., 1991). In an update, Steenland (1999) constructed an exposure matrix for a subcohort of the workers, and attempted to evaluate the relationship between estimated TCDD exposures and mortality.

A multisite study by the International Agency for Research on Cancer (IARC) involved 18,390 production workers and herbicide sprayers from 10 countries (Saracci et al., 1991). Exposure was estimated from a combination of factory records, work histories, spraying data, and questionnaires. Several other occupational studies of workers involved in chemical production plants have relied upon job titles as recorded on individual work histories and company personnel records to classify exposure (Ott et al., 1980; Zack and Gaffey, 1983; Coggon et al., 1986, 1991; Cook et al., 1986; Zober et al., 1990). Similarly, exposure in chemical plant workers has been characterized by worker involvement in various production processes such as synthesis, packaging, waste removal, shipping, and plant supervision (Manz et al., 1991; Bueno de Mesquita et al., 1993). Flesch-Janys et al. (1995) did an update of this cohort and added quantitative exposure assessment based on blood or adipose measurements of polychlorinated dibenzo-p-dioxin and furan (PCDD/F). Using a first-order kinetics model, half-lives from an elimination study in 48 workers from this cohort, and background levels for the Ger-

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