Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and dioxin-like polychlorinated biphenyls (PCBs) [coplanar and several mono-ortho-substituted PCBs] are similar classes of chlorinated aromatic chemicals that are produced as contaminants or byproducts. They have no known commercial or natural use. Dioxins are produced primarily during the incineration or burning of waste; the bleaching processes used in pulp and paper mills; and the chemical syntheses of trichlorophenoxyacetic acid, hexachlorophene, vinyl chloride, trichlorophenol, and pentachlorophenol. Both the synthesis and heat-related degradation of PCBs will produce PCDF by-products. PCBs are chlorinated aromatic hydrocarbon chemicals that were once synthesized for use as heat-exchanger, transformer, and hydraulic fluids, and also used as additives to paints, oils, window caulking, and floor tiles.
Production of PCBs peaked in the early 1970s and was banned in the United States after 1979. Together with the PCDDs and PCDFs, cPCBs and mPCBs are often referred to as “dioxin-like” chemicals because they act in the body through a similar mechanism.
In the environment, these dioxin-like chemicals are persistent and usually occur as a mixture of congeners (i.e., compounds that differ by the numbers and positions of chlorine atoms attached to the dibenzo-p-dioxin, dibenzofuran, or biphenyl structures). The general population is exposed to low levels of these dioxin-like chemicals primarily through ingestion of high-fat foods such as dairy products, eggs, and animal fats, and some fish and wildlife.
Examined participants aged 12 years and older from a one-third sample were eligible.
Description of Laboratory Methodology
The 17 PCDDs/PCDFs, and four cPCBs were measured in serum by gas chromatography/isotope-dilution high-resolution mass spectrometry (GC/ID-HRMS). The analytical method for PCDDs/PCDFs/cPCB is described in Patterson et al., 1990 and Turner et al., 1994. The method for PCBs is described in Sjodin et al., 2004 and Barr et al., 2003.
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.
There were no changes to the lab method, lab equipment, or lab site for this component in the NHANES 2005-2006 cycle. However, the samples were measured in a pooled fashion for 2005-2006 rather than individual measurements.
Laboratory Quality Assurance and Monitoring
Serum samples were processed, stored and shipped to the Division of Laboratory Sciences, National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, GA for analysis.
Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedure Manual (LPM). Vials are stored under appropriate frozen (-30oC) conditions until they were shipped to the National Center for Environmental Health for testing.
The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Act mandates. Detailed QA/QC instructions are discussed in the NHANES LPM.
Mobile Examination Centers (MECs)
Laboratory team performance is monitored using several techniques. NCHS and contract consultants use a structured competency assessment evaluation during visits to evaluate both the quality of the laboratory work and the quality-control procedures. Each laboratory staff member is observed for equipment operation, specimen collection and preparation; testing procedures and constructive feedback are given to each staff member. Formal retraining sessions are conducted annually to ensure that required skill levels were maintained.
NHANES uses several methods to monitor the quality of the analyses performed by the contract laboratories. In the MEC, these methods include performing blind split samples collected during “dry run” sessions. In addition, contract laboratories randomly perform repeat testing on 2% of all specimens.
NCHS developed and distributed a quality control protocol for all CDC and contract laboratories, which outlined the use of Westgard rules (Westgard, et al. 1981) when running NHANES specimens. Progress reports containing any problems encountered during shipping or receipt of specimens, summary statistics for each control pool, QC graphs, instrument calibration, reagents, and any special considerations are submitted to NCHS quarterly. The reports are reviewed for trends or shifts in the data. The laboratories are required to explain any identified areas of concern.
All QC procedures recommended by the manufacturers were followed. Reported results for all assays meet the Division of Laboratory Sciences’ quality control and quality assurance performance criteria for accuracy and precision, similar to the Westgard rules (Caudill, et al. 2008).
Data Processing and Editing
The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.
Refer to the 2005-2006 Laboratory Data Overview for general information on NHANES laboratory data.
Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for details on the use of sample weights and analytic issues.
The original pooled-sample weight created for this file released in July 2013 was not correctly stratified to the U.S population total. This new file contains the corrected sample weight (WTSMSMPA). No changes or corrections were made to the lab analyte data in this new release. Any analyses of the data using the old public use data file should be repeated using the corrected sample weight on this file.
Dioxins and furans were measured in a one third subsample of persons 12 years and over, and samples were pooled in groups of 8 samples per pool within 32 demographic groups. The analysis of NHANES 2005-2006 pooled-sample data must be conducted with the basic demographic variables provided in this data file. This pooled-sample data file cannot be linked to other NHANES 2005-2006 data. Because each sample person does not have an equal probability of selection, sample weighting is needed to produce correct population estimates of means, percentiles, and other descriptive statistics.
Samples were pooled in groups of 8 samples per pool within 40 demographic groups. The analysis of NHANES 2005-2006 pooled-sample data must be conducted with the basic demographic variables provided in this data file. This pooled-sample data file cannot be linked to other NHANES 2005-2006 data. Because each sample person does not have an equal probability of selection, sample weighting is needed to produce correct population estimates of means, percentiles, and other descriptive statistics.
The pooled-sample weights required to produce estimates from these data are included in this data file. The analysis of pooled-samples from survey data is a relatively new field of study, and there is currently no established method to produce variance estimates for these pooled results. These data cannot be analyzed using software designed for complex surveys because design features are not available.
Rationale and Methods Used to Create Pooled Results
The Centers for Disease Control and Prevention (CDC) provide an ongoing assessment of the US population's exposure to environmental chemicals by using Biomonitoring in conjunction with CDC's National Health and Nutrition Examination Survey (NHANES). Characterizing the distributions of concentrations of environmental compounds or their metabolites in the US population is a primary objective of CDC's Biomonitoring program. Historically, this characterization has been based on individual measurements of these compounds in body fluid or tissue from representative samples of the population. Pooling samples allows for larger sample volumes, which can result in lower limits of detection and reduces the number of measurements and costs. For the first time in NHANES 2005-2006, a weighted pooled-sample design was implemented to facilitate pooling samples before making analytical measurements (pooled results were not weighted in NHANES 2001-2002). Table 1 lists the IUPAC (International Union of Pure and Applied Chemistry (IUPAC) names, common abbreviation, and NHANES variable name for the analytes included in this data file.
Pools were prepared from serum collected from a random one-third subset of the NHANES 2005–2006 participants aged 12 years and older. Samples were pooled based on gender, race and Hispanic origin, and age. To implement the pooled-sample design, each participant sample was identified as belonging to one of 32 demographic groups based on race and Hispanic origin (non-Hispanic white: NHW, non-Hispanic black: NHB, Mexican American: MA,: OTHER, not non-Hispanic black, non-Hispanic white or Mexican-American), gender (Male, Female), and age group (12-19, 20-39, 40-59, and 60+ years of age and older). Eight (8) samples were included in each pool. The number of pools created for each of the 32 demographic groups varied depending on the total number of individual samples available in a demographic group. The one-third subset of NHANES 2005-2006 represents 2345 individual samples, but because the pooled-sample design requires that all samples be of sufficient volume and that there be the same number of samples in each pool, only 1973 samples were available to create 247 pools with 8 samples per pool. See more details on pool sample formation and exceptions in Table 2. The variable SAMPLEID denotes the identification number for each pool and ranges from 1 through 247. Please refer to the Pooled-Sample Technical Support file (POOLTF_D) for detailed information on individual participants included in each pool.
In order to incorporate sample weighting into the pooled-sample design it was necessary to use a different volume of material from each sample contributing to a pool. The volume chosen for each sample in a pool was based on the ratio of its sampling weight to the sum of the sampling weights of all samples in the pool. To physically accomplish the pooling in the laboratory required that the ratio of the largest to the smallest sampling weight of samples in the same pool be no larger than about 4 or 5. The individual samples were sorted/stratified by sampling weight within each of the 32 demographic groups and pools were formed with sampling weights adjacent to one another in the sorted list. The number of samples in the one-third subset, the number of samples available, the number of these samples that were usable, and the number of pools formed in each demographic group are presented in Table 2. Once the pools were created, summed sampling weights were further adjusted to account for the unused samples. These adjusted summed sampling weights are represented by the variable named WTSMSMPA.
Demographic and Other Related Variables
The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2005-2006 Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample design variables. The recommended procedure for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.
The Fasting Questionnaire File includes auxiliary information, such as fasting status, length of fast and the time of venipuncture.
This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier (i.e., SEQN).
In the dataset, the whole weight detection limit (fg/g serum) is a variable and dependent on the available sample size, however, the variation in sample size was low. Three variables are provided for each of these analytes. The variable name ending in “LC” (ex., LBDD01LC) indicates whether the result was below the limit of detection: the value “0” means that the result was at or above the limit of detection, “1” indicates that the result was below the limit of detection. The varibale ending in “LA” (ex., LBCD01LA) is the lipid adjusted detection limits in ng/g lipid, because the lipid levels are measured in individual pools/specimens. The third variable prefixed LBC (ex., LBCD01) provides the analytic result for that analyte. For analytes with analytic results below the lower limit of detection (ex., LBDD01LC= 1), an imputed fill value was placed in the analyte results field. This value is the lower limit of detection divided by the square root of 2 (LLOD/sqrt). The lower limit of detection (LLOD in pg/g serum and ng/g lipid) for DOX:
Table 1. NHANES Variable name, common abbreviation and International Union of Pure and Applied Chemistry (IUPAC) name for each analyte reported. Maximum limit of detection (MLOD) expressed as pg/g serum and ng/g lipid.
Table 2. Number of subjects per demographic group in the NHANES 2005–2006 one-third subsample, number of individual serum samples available, number of usable samples, and number of pools formed from usable samples.
1 Value of this categorical variable in the data set.
2 With only 23 usable samples, two 8 sample pools and one 7 sample pool were created.
3 With only 6 usable samples, one 6 sample pool was created.
Methodological Considerations for analysis
The analysis of pooled-samples from survey data is a relatively new field of study, and there is currently no established method to produce variance estimates for these pooled results. These data cannot be analyzed using software designed for complex surveys because design features are not available.
There are a few pooled-sample methodological publications, which address issues related to the following topics (See References).
• Variance estimation and bias correction when pooling samples from log-normally distributed populations (Caudill, 2010a; Caudill, 2010b; Caudill et al., 2007; Li et al., 2014);
• Models that incorporate measurement error when analyzing pooled-samples from normally or log-normally distributed populations (Caudill, 2010a);
• Incorporation of sample weighting into a pooled-sample design (Caudill, 2012; Caudill, 2010a; Li et al., 2014);
• Estimation of standard errors and confidence limits for point estimates from pooled-samples (Caudill, 2012; Caudill, 2010a; Li et al., 2014; Caudill, 2015).