Hedley et al. the 100 pooled samples from the 26 countries/ regions ranged from 2.7 pg WHO-TEQ/g fat (Brazil) to 51.5 pg WHO-TEQ/g fat (Egypt) (Malisch and van Leeuwen 2003). Median PCDD/F-TEQs in different countries demon- strated that the median for Hong Kong was ranked 14th from the top, 40% higher than Australia and double the other Asia-Pacific countries. Fiji, the Philippines, and Australia were the lowest. Comparison of the levels of PCB-TEQs (1.3–28.5 pg WHO-TEQ/g fat) shows that the rank order of PCB-TEQs among the 26 countries/regions varied from that of the PCDD/F-TEQs. The median PCB-TEQs of Hong Kong ranked 17th from the top, and its value was lower than that of most of the European countries. The median PCB-TEQ level in Hong Kong was two orders lower than that found in Sweden, the Netherlands, Slovak Republic, Belgium, Russia, Germany, Luxembourg, Czech Republic, Italy, and Ukraine. The other Asia-Pacific countries (New Zealand, Australia, the Philippines, and Fiji) occupied the six lowest rankings for PCBs along with Hungary and Brazil. Discussion Factors affecting dioxin levels. The linear rela- tionship between the total WHO-TEQs and mean age of mothers indicates that duration of exposure in all environments is a major deter- minant of the total body burden of dioxins in a population. In this study, because primi- parous mothers in Hong Kong were older than those who had migrated from mainland China and overseas and their age increased as the number of years lived in Hong Kong increased, the age effect probably explains the higher dioxin body load among the Hong Kong pools. Mothers with higher seafood consumption had higher total TEQs. Food is the main source of dioxin exposure, and dietary habits may have strong inﬂuence on the dioxin body load. Food consumed in Hong Kong is mainly imported. According to the Hong Kong Department of Agriculture, Fisheries and 1. HK: smoking 2. HK: high dairy 3. HK: high seafood 4. HK: low dairy and seafood 5. HK: high dairy or seafood 6. ML: low dairy and seafood 7. ML: high dairy or seafood 8. CHI: 2–6 years in HK 9. CHI: ≥ 7 years in HK 10. OS: 1–10 years 11 OS: ≥ 11 years 12. MIX: low dairy and seafood 13 MIX: high dairy or seafood A B Conservation, only around 25% of the fish and seafood consumed in Hong Kong and around 5% of meat and poultry (live, chilled, and frozen) were produced in Hong Kong in 2004. PCB levels in mussels in coastal waters in Hong Kong were found to be higher than those obtained from many Asian countries (Liu and Kueh 2005); however, the most toxic con- geners were not detected in 6 years of monitor- ing, and positive data were recorded mostly for two compounds of low toxicity. Another local study reported very low PCB levels in ﬁsh pur- chased in Hong Kong markets (Chan et al. 1999), and a recent local survey carried out by the Hong Kong Food and Environmental Health Department (HKFEHD 2002) did not report dioxin concentrations in any food groups that would raise safety concerns. In that study, Hong Kong secondary school students’ intake was estimated to be 0.85 pg WHO- PCDD/F-TEQ/kg body weight/day (26 pg WHO-PCDD/F-TEQ/kg body weight/ month). In a four-region China study (Wu et al. 2002), the estimated intake was 36 pg WHO-PCDD/F-TEQ/kg body weight/ month. Although both estimates are within the tolerable daily intake of 70 pg WHO-TEQ/kg body weight/month, neither includes coplanar PCBs, so the true value is likely to be double these figures. Unless there are special point sources, the difference between the dioxin exposure in geographic areas may depend more on the amount of intake than on the absolute dioxin content in foods. Higher habitual ani- mal food consumption in Hong Kong com- pared with that in rural mainland China, as indicated in a previous study (Woo et al. 1999) and the dietary assessments carried out in this study, might partly explain the higher dioxin body load found in Hong Kong mothers. Other factors may inﬂuence dioxin body load. It has been suggested that cigarette smoking increases dioxin elimination rate in humans (Flesch-Janys et al. 1996). The lower dioxin level in Hong Kong mothers who had smoked at some time during their lives could be associated with both younger age and faster dioxin elimination rate induced by cigarette smoking. In a population, subgroup dioxin levels may also reﬂect long-standing historical exposure, rather than simply recent or con- temporary exposure. It is important to con- sider the fact that part of the body load of dioxins in our subjects is likely to be the result of transfers from their own mothers in utero and during breast-feeding. The age-dependent relationship in dioxin body load might also inﬂuence the compari- son of dioxin concentrations in breast milk in the WHO dioxin exposure study. The mean age of primiparous women in Hong Kong, about 30 years, is comparable with that in other postindustrial developed countries but higher than many developing countries. Without further information on the age of mothers in the WHO survey, comparison of dioxin levels between different countries would be incomplete. The heterogeneity of pools for age might also further limit the interpretation of the pooled TEQ concentra- tions. We suggest that age should be consid- ered in future international studies as an important factor in the design of the pooling strategy to allow valid comparisons between different countries. Geographic speciﬁcity in dioxin exposure. Because PCDD, PCDF, and PCB congeners are associated with different degrees of bioavail- ability, the concentration of congeners found in human samples is the result of concentrations in the environment, bioavailability in animals in the food chain, and relative absorption and elimination rates. The mix of residential back- grounds among the subjects we recruited locally in this study reﬂected the pattern of previous exposure to dioxins not only in Hong Kong but also in mainland China and neighboring overseas countries. Immigrant mothers with a shorter period of stay in Hong Kong appeared to have been exposed to relatively lower propor- tions of PCDDs and higher proportions of PCDFs and mono-ortho PCBs compared with mothers resident in Hong Kong for longer peri- ods. Another illustration of geographic variation in exposure to dioxin-like compounds was the higher contribution of PCBs in the total TEQ )40 s r a e y (35 s r e h t o30 m f o e g a25 n a e M 20 0 10 20 30 40 50 Percent PCDF-TEQ of PCDD/F-TEQ 0 10 20 30 40 50 60 Percent mono-ortho PCB-TEQ of PCB-TEQ Figure 1. (A) Percent PCDF-TEQ of PCDD/F-TEQ, and (B) percent mono-ortho PCB-TEQ of PCB-TEQ, by pool. 206 VOLUME 114 | NUMBER 2 | February 2006 • Environmental Health Perspectives 6 8 10 12 14 16 Total WHO-TEQ (pg/g fat) 18 20 Figure 2. Correlation between total WHO-TEQ in breast milk and the mean age of mothers in 13 pools. r = 0.85 (95% CI, 0.55–0.96), R 2 = 0.73, p < 0.0005.