Hedley et al. concentrations of the individual dioxin or dioxin-like compounds multiplied by their toxic equivalent factors (TEFs) (van den Berg et al. 1998). We expressed body burdens in terms of picograms of dioxin TEQs per gram of fat in breast milk. The TEQs contributed by PCDDs, PCDFs, and PCBs were described as PCDD-TEQs, PCDF-TEQs, and PCB- TEQs, respectively, whereas the PCDD/F- TEQ was used for the sum of PCDD-TEQ and PCDF-TEQ. Treating undetected values as zero resulted in a lower-bound TEQ con- centration, and an upper-bound concentration was given by assuming the undetected value as the limit of detection. Mass concentrations and TEQ concentrations of dioxins were reported to three signiﬁcant ﬁgures. Data analysis. We use the Statistical Package for Social Sciences (SPSS for Windows, version 10.1; SPSS Inc., Chicago, IL, USA). The differences among geographic groups/pools were accessed by chi-square tests (for categorical variables), analysis of variance, t-test, or Mann- Whitney U-test (for continuous variables). Spearman correlation coefﬁcients were calcu- lated to assess the relationship among the con- centrations of different congeners as well as that between pooled TEQs and pool characteristics. All p-values were two tailed, and an association with a p-value < 0.05 was considered statistically signiﬁcant. The overall mean and median levels of WHO-TEQs were weighted by the number of subjects in each pool. Data are presented as means ± SE. Results The most toxic congener, TCDD, was present in all 13 pooled milk samples with a mass con- centration ranging from 0.88 to 1.49 pg/g fat. In eight of the 13 pools, 1,2,3,7,8,9-hexa- chlorodibenzofuran (HxCDF) was not detected, and PCB-123 was undetected in all 13 pools (Table 3). The analytical difference between upper-bound and lower-bound calculations of TEQ values is negligible (< 0.1%), so only upper-bound concentrations are reported here. The total TEQs of the 29 target congeners across the 13 pools ranged from 8.97 to 16.7 pg/g fat (weighted mean, 12.9 pg; weighted median, 13.4 pg). On average, 76% of the total TEQs was contributed by five congeners: TCDD (9.5%), 1,2,3,7,8-pentachlorodibenzodioxin Table 1. Summary of selection criteria and thus characteristics of the 13 pools. No. 25 17 17 32 29 21 34 22 23 18 12 24 42 316 0.9 0.8 1.0 0.6 0.7 0.5 0.9 0.6 0.7 0.7 0.7 0.4 0.7 0.7 Have been living in Hong Kong for a period of 2–6 years since immigration Have been living in Hong Kong for 7 years or more since immigration Hong Kong born and have lived abroad for 1 to 10 years Overseas born, and recently migrated to Hong Kong Other criteria — — — — — — — — — 3.4 1.5 6.1 1.2 2.8 1.0 3.1 3.2 3.4 2.3 3.6 0.8 3.3 2.7 Geographic classiﬁcation Hong Kong Hong Kong Hong Kong Hong Kong Hong Kong Mainland China Mainland China Mainland immigrant Mainland immigrant Overseas Overseas 26.5 31.0 30.5 31.4 32.6 26.6 28.1 27.6 30.1 32.1 25.7 30.2 30.8 29.7 20.6 20.5 20.1 19.9 20.5 19.1 18.9 19.4 20.1 20.2 21.6 20.9 21.2 20.2 4.2 4.2 4.1 4.4 4.5 4.2 4.2 4.1 4.2 4.4 4.1 4.6 4.5 4.3 Smoking habits Ever smoker Never smoker Never smoker Never smoker Never smoker — — — — — — — — 68.0 94.1 82.4 87.5 86.2 71.4 73.5 81.8 87.0 88.9 100.0 79.2 90.5 83.2 Dietary criteria — 17 highest dairy product intake and dairy product intake > 2.5 × seafood intake 17 highest seafood intake and seafood intake > 2.5 × dairy product intake < 2,000 g/month dairy product intake and < 2,000 g/month seafood intake ≥ 2,000 g/month dairy product intake or ≥ 2,000 g/month seafood intake < 2,000 g/month dairy product intake and < 2,000 g/month seafood intake ≥ 2,000 g/month dairy product intake or ≥ 2,000 g/month seafood intake — < 2,000 g/month dairy product intake and < 2,000 g/month seafood intake ≥ 2,000 g/month dairy product intake or ≥ 2,000 g/month seafood intake 52.0 52.9 41.2 62.5 41.4 81.0 70.6 77.3 56.5 44.4 75.0 66.7 53.7 59.4 — — — Percent 100.0 0.0 0.0 0.0 0.0 4.8 5.9 9.1 17.4 11.1 0.0 4.2 9.5 13.0 Pool identiﬁer 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 Pool identiﬁer 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 All 204 Abbreviations: BMI, body mass index; CHI, China immigrants; HK, Hong Kong; MIX, mixture of residence characteristics; ML, mainland China; OS, overseas. 12. MIX: low dairy and seafood Mixed residency 13. MIX: high dairy or seafood Mixed residency Total Table 2. Summary of the maternal characteristics in the 13 pools. Abbreviations: —, variables not used to deﬁne the pool; CHI, China immigrants; HK, Hong Kong; MIX, mixture of residence characteristics; ML, mainland China; OS, overseas. Estimated dietary intake (kg/month) Age (years) BMI Baby’s age (weeks) First Exclusive Ever Income pregnancy breast-feeding smoker < US$1,282 Education: Dairy ≥ senior high product Seafood Meat Poultry Egg 28.0 0.0 0.0 3.1 0.0 33.3 50.0 31.8 0.0 0.0 8.3 8.3 17.1 15.6 88.0 100.0 100.0 100.0 100.0 42.9 64.7 59.1 82.6 100.0 83.3 66.7 69.0 80.1 2.8 6.9 0.9 1.0 2.9 0.5 3.5 2.0 3.3 3.8 2.4 0.6 3.7 2.6 3.2 2.8 2.8 2.0 2.2 1.5 1.9 1.6 2.0 2.6 1.6 1.5 2.5 2.2 1.2 0.9 1.4 0.9 1.1 0.5 0.8 0.9 0.7 1.0 1.2 0.6 0.9 0.9 VOLUME 114 | NUMBER 2 | February 2006 • Environmental Health Perspectives