Appendix D Food Codes for NHANES - OEHHA

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Scientific Review PanelSRP Draft Version 2 February,June 2012 Each reference half-life was derived from data on occupational exposures (Flesch-Janys et al., 1996; van der Molen et al., 1996) or dietary intake of the general population (Ogura, 2004). Note that mean half-lives vary by more than 2fold among dioxin, 5-fold among furans and more than 100-fold among PCB congeners. In an initial review of the literature, Milbrath et al (2009) reviewed evidence about factors that can affect elimination rates. Personal factors such as body fat, smoking status and past lactation practices can affect body burden and elimination rates. For example, smoking has been associated with a 30% to 100% increase in elimination rates of some dioxin congeners (Flesch-Janys et al., 1996; Milbrath et al., 2009). As well, the onset of lactation sets a new elimination pathway into effect and can substantially reduce the maternal body burden of PCBs during 6 months of lactation (Niessen et al., 1984; Landrigan et al., 2002). Half-lives derived from children would be less than that from older adults due, in part, to the effects of the growing body on estimates of blood concentrations. Models based on rat data demonstrate a linear relationship between increasing fat mass and half-life length at low body burdens, with the impact of adipose tissue on half-life becoming less important at high body burdens (Emond et al 2006). At high body burdens, dioxins are known to up-regulate the enzymes responsible for their own elimination. Human data suggest that the serum concentration of TCDD where this transition occurs is 700 pg/g and 1,000 – 3,000 pg/g for PCDFs (Kerger et al 2006, Leung et al 2005). Therefore, investigators selected a subset of data based on the following criteria: • blood serum concentrations of PCDD/Fs were less than 700 pg /g blood lipid total toxic equivalents (TEQs) at the time of sampling • subjects were adults • measurements were not reported as inaccurate in later studies Milbrath et al selected the reference values to represent a 40- to 50-year-old adult with blood dioxin concentrations in the range where fat drives the rate of elimination (i.e. at lower body burdens). In addition, Milbrath rejected half-lives longer than 25 years if the original study calculated half-lives assuming steadystate conditions. For the retained subset, the investigators calculated the mean and range of halflives to establish a representative set of half-lives for each congener in a moderately exposed adult (Milbrath et al., 2009). They also adjusted reference half-lives for age, body fat, smoking habits and breast-feeding status as these factors were all strong determinants of half-life in humans (Milbrath et al., 2009). A generally accepted approach to estimating the concentration of a lipophilic chemical in milk is outlined by Smith (1987). This approach is based on average J-14
Scientific Review PanelSRP Draft Version 2 February,June 2012 maternal daily intake, an estimate of the half-life (t 1/2) of PCDDs/PCDFs and PCBs and body weight-normalized (BW) proportionality factors. The chemical concentration in breast milk can be calculated by equation J-4: Cm = (Emi)(t1/2)(f1)(f3)/(f2)(0.693) Eq. J-4 Cm = chemical concentration in milk (mg/kg milk) Emi = average daily maternal intake of contaminant (mg/kg-BW/day) t½ = biological half-life (days) f1 = proportion of chemical in mother that partitions into fat (e.g. 0.8) f2 = proportion of mother’s body weight that is fat (e.g. 0.33 = kg-fat/kg- BW) f3 = proportion of breast milk that is fat (e.g., 0.04 = kg-fat/kg-milk) Smith’s approach requires an estimate of the biological half-life of PCBs and PCDDs/PCDFs in the adult human and is restricted to poorly metabolized, lipophilic chemicals that act predominantly by partitioning into the fat component and quickly reaching equilibrium in each body tissue (including breast milk). Because of Milbrath’s approach, Tco-estimates for dioxins, furans and dioxin-like PCBs apply the following conservative assumptions regarding factors that affect elimination rates: • lower enzyme induction based on nonsmokers with a body burden below 700 ppt in the blood • adult age • no recent history of breast-feeding • body fat estimates based on older adults Transfer coefficients (Ng, 1982) are ideally calculated from the concentration of contaminant in milk following relatively constant long-term exposure that approximates steady state conditions. Because Smith’s equation is linear, it can be rearranged to solve ratio of the chemical concentration in milk to the chemical taken into the body per day, which is the transfer coefficient (Equation J-5). Tco = Cm/(Cf)(I) Eq J-5 Tco is the transfer coefficient (day/kg or day/liter) Cm = measured chemical concentration in milk (µg/kg or mg/liter milk) Cf = measured chemical concentration in exposure media (e.g. food) (µg/kg food) I = reported daily intake of exposure media (kg/day of food) The following equation (Eq-J-6) is equation Eq J-5 substituted into equation Eq J- 4 and rearranged to solve for Tco. J-15
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Appendix D Food Codes for NHANES - OEHHA

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