Apparent Half-Lives of Hepta- to Decabrominated Diphenyl Ethers in Human Serum as Determined in Occupationally Exposed Workers

1Polybrominated diphenyl ethers (PBDEs) have for decades been applied as additive flame retardants in polymers in electronics and electric goods, and in rubber, plastics, and textiles. The technical mixture of decabromodiphenyl ether, in which BDE-209 is the major congener, now dominates the market among the PBDE products. The concentrations of PBDEs, including BDE-209, have accumulated in humans and the environment worldwide. Hitherto, mainly PBDE congeners with four to six bromines have been assessed in human blood and milk. A few studies also included exposure to higher brominated PBDE congeners, both in humans occupationally exposed to PBDEs and in humans exposed to background concentrations.

2It is not yet fully understood how humans are exposed to the PBDEs, but ingestion (food and dust) and inhalation seem to be important routes of exposure. Many PBDEs have been reported to be present in food, in ambient and occupational air, and in household dust. Hence, dominant PBDE exposure routes differ from those of major historical persistent organic pollutants [e.g., polychlorinated biphenyls (PCBs) and 2,2-bis(4-chlorophenyl)-1,1,1-trichloroethane (DDT)] because there are numerous indoor sources for PBDEs but many fewer for the PCBs and DDT.

3All PBDEs are bioavailable, but the chemical and biologic properties of the congeners vary, as reviewed recently by,, and. Based on knowledge of the kinetics of halogenated aromatics, we should expect great variations in their degradation and excretion rates. recently presented estimated half-lives based on experimental data for a number of brominated flame retardants, including tetra- to hexa-BDEs, but not for any higher BDEs. These low- to medium-brominated PBDE congeners were indicated to have very slow elimination rates. A similarly long half-life for 2,2′,4,4′,5,5′-hexabromobiphenyl has also been reported. In contrast, observations in five electronics dismantlers sampled before and after 30 days of vacation suggested a rather rapid elimination of BDE-209, with a median decrease of 66% during this period of time. This was the first observation that BDE-209 is degraded or eliminated at a much higher rate than lower brominated congeners of PBDEs. Similarly, observed a short half-life (t1/2 = 2.4 days) for BDE-209 in rats dosed with this congener. Also, gray seals experimentally dosed with BDE-209 indicated a short half-life (8–13 days) of this compound.

4The objective of the present study was to calculate the apparent half-lives for BDE-209 and other higher BDEs in human serum, using data from occupationally exposed workers sampled before, during, and after a vacation period. If we assume that those who are known to be occupationally exposed to PBDE do not experience any other PBDE exposure beyond normal background outside their work environment, we can determine apparent half-lives of compounds with reasonably short half-lives compared with the duration of the vacation. Because two types of exposure have to be considered, work-related and general background exposure, we also used data from reference populations without any known occupational exposure for estimation of background exposure levels.

5We included five groups of workers in our study; these groups have all been previously described in detail. The serum concentrations of some PBDEs in these groups (cross-sectional investigations) are summarized in Table 1. For this study, we analyzed an additional number of samples from workers who donated blood samples during their vacations from the electronics dismantling plant and from the rubber manufacturer.

6Workers in a recycling plant performing manual dismantling of electronic goods were shown by to have elevated serum concentrations of BDE-47, BDE-153, BDE-183, and BDE-209 compared with referents. In the present study, three female and one male dismantler, 43–49 years of age, were sampled immediately before the summer vacation and at the end of their vacation, 28 or 29 days later. In addition, three or four samples were obtained from each worker during their vacation period. The sampling took place in 1998, 1 year after the first study.

7At a plant manufacturing technical rubber, batches of deca-BDE flame-retarded rubber compound were regularly produced, usually during campaigns of 1–2 days, two to four times a month. The commercial deca-BDE used as an additive consisted of mainly BDE-209 with traces of nona-BDEs and octa-BDEs. The mixers had elevated serum levels of octa-BDEs to BDE-209, whereas the millers had serum levels comparable with male referents. In the present study, we include blood samples from four male rubber mixers, 25–60 years of age, and three male millers, 31–45 years of age, drawn immediately after a period of production of flame-retarded rubber in June 2000, on the day before the start of the summer vacation, and again at the end of vacation (31, 34, or 43 days later). Additionally, we obtained between one and seven samples from each of the workers during their vacation.

8In 2002, the production of flame-retarded rubber had decreased substantially at the plant, and no batches were produced between the end of September and the end of November that year. Two mixers were resampled on the morning before a 1-day production period and at days 2 and 7 after the handling of deca-BDE. Data from these samplings were not included in the modeling of half-lives but are reported separately.

9In June 1997 blood samples were obtained from 20 female clerks, 25–61 years of age, regularly using computers 8 hr/day. Samples from their vacation period were not obtained.

10In September 1997, blood samples were obtained from 20 female hospital cleaners 30–60 years of age. Their work was performed without any computer support, and their electrical and flame-retarded environment was considered to be at a minimum. They all had none or very limited computer experience. Samples from their vacation periods were not obtained.

11Blood samples were obtained from 18 male abattoir workers 24–60 years of age in March 2000. Their work was performed without any computer support, and the electrical and flame-retarded environment was considered to be at a minimum at the plant. They all had none or very limited computer experience. No samples from any vacation periods were obtained.

12Blood was drawn from the cubital vein into evacuated plain tubes (Vacutainer; Becton Dickinson Vacutainer Systems, Rutherford, NJ, USA). The serum was centrifuged, transferred to dark-colored acetone-washed glass bottles, frozen, and kept at –20°C until the start of the chemical analysis. Informed consent was obtained from all subjects, and the study was approved by the Ethics Committee at Lund University, Sweden (protocol LU 227-97).

13Solvents, reference standards, and other chemicals used in analysis of serum samples, as well as the instrumental support of the work, have been reported previously.

14Extraction of serum, lipid determination, and partitioning between an organic solvent and aqueous sodium hydroxide have been described in detail elsewhere. We performed the cleanup procedure with sulfuric acid lipid removal, and quantitative analyses of PBDEs by gas chromatography–mass spectrometry [electron capture negative ion (ECNI) monitoring, m/z = 79, 81] as previously described in detail for the individual studies. We used the response factor for BDE-203 for quantification of structurally unidentified octa- and nona-BDEs, which probably underestimate the nona-BDE levels. For all other PBDE congeners, authentic reference standards were available. Selected ion monitoring (SIM) chromatograms of the PBDE peak patterns in a referent (abattoir worker), an electronics dismantler, and a rubber worker are presented in Figure 1 and show the differences in their PBDE patterns.

15The blood samples were analyzed at different time points between 1998 and 2004. All samples from each study group were analyzed together. Because the original limit of quantification (LOQ) varied between data sets, we had to recalculate uniform LOQs for the present study. For all serum samples analyzed, we used a signal-to-noise (S:N) ratio > 5 to define the LOQ, when no interference was present in the blank samples. If interferences were present in the blank samples, the amount of the analyte in the sample had to be at least five times the average blank level to be accepted. The amounts of PBDEs in blank samples, if any, were subtracted from sample concentrations before being reported. When LOQs differed between sets of data recorded and analyzed at different time points, the LOQ was set as the highest measured LOQ (based on S:N ratio or blank sample levels) or the lowest quantified value in all data sets.

16For modeling of the half-life of each PBDE congener, we assumed that each subject had a certain, constant non-work-related exposure independent of profession. Each of the occupationally exposed workers also had a work-related exposure considered to be at a steady state before vacation but different between groups and subjects. Considering the start of the vacation as day 0, Equation 1 for each subject can be expressed as follows:

17Based on the results from the cross-sectional studies, we selected groups for inclusion in the models. Clerks, cleaners, and abattoir workers were considered occupationally unexposed in all half-life calculations. We considered the electronics dismantlers to be occupationally exposed to all PBDE congeners studied. Four rubber mixers were considered to be occupationally exposed to deca-, nona-, and octa-BDEs but not to lower brominated congeners. The remaining three rubber millers had PBDE concentrations that did not differ from the referents and were considered occupationally unexposed.

18The rubber millers and abattoir workers (all male) were sampled in 2000, whereas the female referents were sampled in 1997. Thus, we added a separate addition factor for millers and abattoir workers to the model (whether it represents a calendar year effect or a sex difference cannot be distinguished). For BDE-209 the inclusion of this addition factor yielded a better fit of the model but did not change the half-life estimate. For all other congeners, there were no differences in the model fits, and this additional factor was set to zero.

19In total, 107 observations from 68 subjects for BDE-209 and BDE-183 were available for the calculations. Because octa- and nona-BDEs had not been determined for cleaners and clerks, 67 observations from 28 individuals were available. For serum concentrations below the uniform LOQ, the level was set to LOQ/2, and the values could thus be used in the kinetic calculations.

20We used NONMEM (version 5, level 1.1; ) for the calculations. We determined the variance in the populations, expressed as a coefficient of variation for both the non-work-related and work-related exposures. Approximate estimations of the 95% confidence interval (CI) were calculated as the estimate ± 2SE of the estimate.

21Additionally, we performed a nonlinear regression analysis of concentration versus time data between days 10 (to ensure that the uptake phase had ended) and 34 in two rubber workers with a rich data set. We assumed a monoexponential decline on top of the baseline (background) level. We used NLIN Procedure in SAS (version 8.2; SAS Institute, Cary, NC, USA).

22PBDE serum concentrations from four electronics dismantlers and four rubber mixers sampled two to seven times from the beginning to the end of their vacation are given in Table 2. The half-lives of the higher BDEs increased with decreasing number of bromine substituents (Table 3). For BDE-209, the calculated t1/2 was 15 days (95% CI, 11–18 days). For the three nona-BDEs (BDE-206, BDE-207, and BDE-208), the calculated t1/2 values were 18, 39, and 28 days, respectively, with the shortest half-life for the PBDE con-gener with three ortho-bromines (BDE-206). The four octa-BDE isomers (octa-1, octa-2, BDE-203, and octa-3) had calculated half-lives of 72, 85, 37, and 91 days, respectively, with the shortest half-life for BDE-203. BDE-183 had a calculated half-life of 94 days (95% CI, 68–120 days). The apparent half-lives for BDE-153 and several other medium- and low-brominated congeners could not be determined but were indicated to be much longer than our 1-month observation period (BDE-153 data in Table 2).

23In two rubber mixers with a rich data set, a traditional elimination model yielded similar results for BDE-209, with half-lives of 14 and 16 days, respectively, calculated between days 10 and 34, to ensure that the uptake phase was over (Figure 2).

24We observed a rapid uptake of BDE-209 after an isolated 1-day period of deca-BDE exposure during production of a batch of flame-retarded rubber (Figure 3). The levels of BDE-209 increased between days 0 and 2 and had clearly declined at day 7 in both workers. In contrast, the octa-BDE levels were at their highest at day 7. For nona-BDE, only a slight decrease between days 2 and 7 was observed. Moreover, the relative increase of BDE-209 between day 0 and day 2 was higher than the relative increase of the octa-and nona-BDEs in both workers.

25Initial findings in a small number of electronics dismantlers had indicated that the half-life of BDE-209 might be shorter than the half-lives of PBDEs with a lower degree of bromination. In the present study, we used a larger number of individual measurements from studies of occupationally exposed subjects and referents without known occupational exposure to PBDEs to further investigate PBDE congener half-lives. We found that the apparent half-lives of deca- to hepta-BDEs in serum increased with decreasing number of bromine substituents. For the fully brominated BDE-209, the apparent half-life was as short as 15 days. Data from two different types of modeling agreed well. The results obtained from the present study thus confirm our first observation in electronics dismantlers and are corroborated by observations in two rubber workers after a single day of exposure to deca-BDE 16 months later (Figure 2). Thus, it is not likely that fluctuations in background exposure levels can invalidate our findings. Our data are too sparse to investigate whether the kinetics are dose dependent. However, the half-life estimations in workers with initial higher (i.e., rubber workers; Figure 2) versus lower (i.e., dismantlers) BDE-209 levels seem to agree. A previous NONMEM calculation, based on data from the four dismantlers only (Table 2, subjects A–D), having initial BDE-209 levels much lower than the rubber workers, yielded the estimate of 7 days (95% CI, 3–16 days).

26Experimental data are well in accordance with our observations. A rapid clearance of BDE-209 has been indicated in carp and rats. The half-life calculated in gray seals, 8.5–13 days, agrees well with our estimate of 15 days (95% CI, 11–18 days). From a theoretical point of view, it is also reasonable that BDE-209 should have a relatively shorter half-life than other PBDEs because of its susceptibility to undergo, for example, reductive dehalogenation and substitution reactions under experimental conditions. Also, hexabromobenzene, another perbrominated aromatic compound, has been shown to be readily metabolized in the rat.

27The general pattern of decreasing half-lives of PBDE congeners with increasing number of bromine substituents was thus expected. Within each group of isomers, such as the nona-BDEs and octa-BDEs, the estimated apparent half-lives varied nominally, albeit with wide and overlapping confidence intervals. It can be noted, however, that the PBDE congeners with the shortest half-lives within each group of isomers have a hydrogen substituent in one of the phenyl rings and the other ring fully brominated. No further conclusions can yet be drawn from these observations.

28The metabolism of BDE-209 is not yet fully elucidated. In the rat, reported that BDE-209 was metabolized, excreted, and only marginally distributed to adipose tissue, but it was found in plasma and blood-rich tissues. Moreover, traces of nona-BDEs were also observed. For octa- and nona-BDE, experimental data are entirely lacking. In rainbow trout, found a clear accumulation of nona- and octa-BDEs over time, after exposure to deca-BDE. On the other hand, studies of carp have shown a rather rapid degradation of BDE-209, but no accumulation of hepta-BDEs to nona-BDEs. It seems likely that nona-BDEs and octa-BDEs are formed in humans after exposure to BDE-209, as previously indicated in our cross-sectional study of rubber workers, using a technical deca-BDE product containing only trace levels of octa- and nona-BDEs (Figure 1). Furthermore, our findings from rubber mixers investigated after a 1-day deca-BDE exposure (Figure 3) support metabolic formation of octa- and nona-BDEs from BDE-209.

29There are yet no other half-life calculations based on observational human data. Estimates based on animal experimental data have been published previously for BDE-47, BDE-99, BDE-100, BDE-153, and BDE-154, indicating long half-lives for all these congeners. Their significant tissue burdens in both wildlife and humans further support the persistence of these PBDE congeners. Our observations for BDE-153 agree well with the estimated long half-life, but we refrained from calculation because our observation period was too short.

30BDE-209 has been shown to have developmental neurotoxicity in mice, and its potential carcinogenicity as observed in the must also be considered. It is obvious that BDE-209 differs from low- to medium-brominated PBDEs because, despite its short half-life in blood, it is present in male Swedes representing the general public at levels similar to those of BDE-47. Taking the rapid turnover of this compound into account, humans must be exposed more or less continuously to keep up the blood concentration. It seems likely that BDE-209 is metabolized to nona-BDEs and octa-BDEs, but the metabolism is probably more complex, as indicated in an experimental study in the rat, finding also hydroxylated metabolites that may be more toxic than the parent compound. BDE-209 and the other higher BDEs are of special concern because large amounts of deca-BDE are still used, while the penta- and octa-BDE products are phased out. Thus, better knowledge of the metabolism and kinetics of higher BDEs, along with a better understanding of their toxicology, is needed.