Stapleton et al. and 0.34 (p = 0.003) respectively. In addition, the correlation of ∑BDE3 between serum and dust, r = 0.34 (p = 0.003) was less than that for serum and handwipes, but greater than the association of BDE-153 in serum and dust: r = 0.19 (p = 0.09). We therefore report results separately for ∑BDE3 and BDE-153 given that patterns of exposure appear to differ. Predictors of ∑BDE3 in children’s serum. Table 3 shows the results of regression of log- transformed serum ∑BDE3 with character- istics of the child and their parents, as well as by ∑BDE3 levels measured in handwipes and dust (by tertiles). Univariate estimates indicate that serum levels tended to increase with age (34%/year, p = 0.31) and were 45% higher in males than females (p = 0.12), although neither result was significant. Neither months of breast-feeding nor time away from home (hours/week) were signifi- cant predictors. Serum concentrations were strongly and inversely associated with father’s education, with levels about half as high in children whose fathers had college educations as those without (p = 0.003). Mother’s educa- tion and race (both related to father’s educa- tion) also predicted children’s serum levels. Because father’s education level was a slightly stronger predictor, we used this variable in the multivariate analysis. Serum ∑BDE3 levels increased with higher tertiles of ∑BDE3 levels in handwipes, with serum concentrations in the high handwipe group 3.66 times higher than those in the low group (p < 0.0001). Dust ∑BDE3 concentrations were not as strong a predictor of serum levels as handwipe levels, but still showed an increasing trend (serum concentrations 1.78 times higher in the high dust group compared to the low dust group; p = 0.05). A multivariate model including age, sex, father’s education, and handwipes indicated a stronger association between serum ∑BDE3 and age (61%/year, p = 0.05), little change in associations with sex or father’s education, and slightly weaker associations with ∑BDE3 lev- els in handwipes, although the latter remain strongly associated with serum levels. The explained variation in this multivariate model (r 2) was 0.39. Models that also included dust ∑BDE3 concentrations showed a weaker asso- ciation with dust concentrations compared with the univariate model estimates, but little change in associations with handwipe concen- trations from univariate estimates (data not shown), suggesting that handwipe concentra- tion is in part an intermediate variable. Predictors of BDE‑153 in children’s serum. In a univariate analysis of BDE-153 in children’s serum (Table 3), age was strongly associated with increased concentrations (141%/year, p = 0.002). In contrast with ∑BDE3, duration of breast-feeding was also a very strong predictor (7%/month, p < 0.0001) of serum BDE-153 levels. In this cohort, approximately 34% of the children were breast-fed for > 12 months, up to a maximum of 27 months. Also in contrast with ∑BDE3, BDE-153 in serum was positively (rather than negatively) associated with socioeconomic status variables, with mother’s education being the strongest predictor (80% increase in children whose mothers had a college degree, p = 0.03). Handwipe BDE-153 levels were also associated with serum BDE-153, although associations were not as strong as those between handwipe and serum ∑BDE3 levels. Consistent with the earlier correlation results, dust (categorized) was not a strong univariate predictor. Duration of breast-feeding and mother’s education were highly associated (data not shown), with the former being the most signif- icant predictor of BDE-153, so it was retained 10,000 ) 1,000 d i p i l um 100 g / g n ( r e s n i 3 E D B Σ 10 1 1 r = 0.59 p < 0.0001 10 100 1,000 10,000 ΣBDE3 in handwipes (ng) for multivariate analysis. The multivariate results show strong associations between serum BDE-153 concentrations and age (73%/year, BDE-153, 0.03), duration of breast-feeding (7%/month, p < 0.0001), and handwipe BDE-153 levels. Addition to this model of maternal education largely eliminated the asso- ciation with breast-feeding, but had little effect on the coefficients for the other variables (data not shown); these results suggest that breast- feeding may mediate the effect of maternal education on children’s serum BDE-153, but other explanations cannot be ruled out. The effect of handwipe levels showed an increasing trend across categories, with serum concen- trations 2.44 times higher in the high versus low handwipe categories (p = 0.0004). The explained variation in this model (r2) was 0.39. Discussion PBDE serum levels measured here in 1- to 3-year-olds are comparable to levels reported among the general U.S. population ≥ 12 years of age in the 2003–2004 NHANES (Sjödin et al. 2008). However, pentaBDE was vol- untarily phased out from use in the United States at the end of 2004, so we would expect to observe declining PBDE levels in the U.S. population in years following. Watkins et al. (2011) measured the same PBDE congeners (using the same labora- tory) in serum collected from a cohort of 31 adult Boston, Massachusetts, office workers in 2009. The geometric mean level of BDE-47 in their cohort was 14.2 ng/g lipid, signifi- cantly (p < 0.05) lower than measured in our toddler cohort (23.3 ng/g lipid). Although our cohort’s levels are higher than those of adults, they are lower than levels reported in a California cohort of 2- to 5-year-olds sampled between 2003 and 2005, 4–7 years before our cohort (Rose et al. 2010). The current results in toddlers—associations between ∑BDE3 concentrations in handwipes, dust, and serum—support an important role for exposure via the indoor environment, which is likely occurring through hand-to-mouth activity. The correlation between ∑BDE3 in serum and handwipes (r = 0.59) was stronger than the association between serum and house dust (r = 0.34). Our multivariate models sug- gest that handwipes may at least partly serve as an intermediate variable between dust and serum levels. Watkins et al. (2011) observed a similar general pattern in their recent analy- sis of adults working in offices, although the associations presented here are much stron- ger, likely because toddlers have more hand- to-mouth activity than adults and spend more time in the home environment than office workers spend in their offices. Handwipes may also reflect exposure in more than one micro- environment and may be modified by hand- washing (Watkins et al. 2011, 2012). Figure 1. Correlation between serum ∑BDE3 and handwipe ∑BDE3. 1052 ovlume 120 | number 7 | July 2012 • Environmental Health Perspectives