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A study reported by Johnson et a), in the Journal in November 1992 is fundamentally flawed (/). In that study, recent serum samples from 37 workers who had sprayed chlorinated phenoxy acid herbicides in the past were analyzed for 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) in order to estimate the workers' TCDD levels at the time of the cessation of spraying and the rate of exposure while spraying. The past levels were estimated from current serum levels using a mathematical model that incorporated the half-life of TCDD in serum, the duration of exposure, and the time since the cessation of spraying. The current serum levels of TCDD ranged from 2 to 18 parts per trillion (ppt, adjusted for lipid weight) for all but one of the sprayers who had a concentration of 34 ppt. The authors noted that the maximum level of TCDD reported for the general population is 26 ppt. The workers were divided into three groups according to the period when they were spraying: pre-1965, 19651974, and post-1974. The current serum TCDD levels were similar in all three groups of workers. When the mathematical model was used to extrapolate back to the time of ceasing employment, the estimated exposure levels were higher for the earlier years. For example, there was at least one worker in each period with a current serum level of 3 ppt TCDD (i.e., at the lower end of the normal range for unexposed people). This level was extrapolated back to 78 ppt at the time of ceasing employment for a pre-1965 sprayer, to 14 ppt for a 1965-1974 sprayer, and remained at 3 ppt for a post-1974 sprayer. The authors concluded that, "Estimated rates prior to 1965 were more i than an order of magnitude higher than those after 1974" and that "...some sprayers may have been exposed to levels comparable with those that produce cancer in laboratory animals." The fundamental flaw in the paper is that Johnson et al. did not take the normal background levels of serum TCDD into account when they extrapolated back to estimate serum TCDD levels at the time of exposure. They stated, "These calculations implicitly assume that all levels at the time that blood was drawn represent decayed levels residual from past occupational exposures." In the Appendix describing their mathematical methods, they stated: "Note that this calculation assumes that exposure from nonoccupational sources is negligible." This assumption is obviously untenable when the current serum levels detected are indistinguishable from those of the unexposed population. In order to estimate past TCDD levels, one should first subtract the background level to be expected in an unexposed person. Only the residual amount could be regarded as arising from past occupational exposures and used for extrapolating back to the time of employment. Fingerhut et al. (2) reported a mean concentration of 7 ppt TCDD in serum in 79 unexposed people, with all values less than 20 ppt. The mean concentration of serum TCDD in the workers in the paper by Johnson et al. was 7.8 ppt (calculated using the detection limit value for the nine workers in whom TCDD was not detected). If the upper limit for the general population, namely 26 ppt, was subtracted from each of the current serum levels reported by Johnson et al. to obtain a conservative estimate of the residual TCDD levels, only one worker, with 34 ppt, would have had a positive contribution from past occupational TCDD exposure. This worker's level would be estimated as 77 ppt at the time of ceasing employment if the residual TCDD level of 8 ppt was used for extrapolation (compared with 329 ppt at the time of ceasing employment calculated by Johnson et al.). All the other workers' residual TCDD levels would be zero and hence not suitable for extrapolation. The fact that all but one of the workers had current TCDD levels within the normal range completely destroys the justification for the elaborate modeling of estimated past exposure levels and hence the conclusions drawn by Johnson et al. The only valid conclusion that can be drawn from their data is that the level of residual TCDD in the serum of all but one of the herbicide sprayers was too low to be detected above background levels in the general population.

Serum Levels of 2,3,7,8- Tetrachlorodibenzo-p-dioxin in Phenoxy Herbicide Sprayers