NHANES is the only national survey that collects extensive health and nutrition information from face-to-face interviews and medical examinations.  The earliest environmental bio-monitoring program began in NHANES II (1976-80), with blood lead tests of all examinees three years of age and older. The bio-monitoring program for environmental chemicals has greatly expanded since then with the current assessment of more than 200 chemicals. The primary goal of the environmental bio-monitoring program is to provide baseline reference data for environmental chemicals.  Because it is population-based cross-sectional data, it is best used for descriptive purposes rather than hypothesis testing. 

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Whether novice or experienced, all users face similar challenges when using NHANES environmental chemical data due to the complexity of the survey design and data structure.  This tutorial was designed so that all users learn to:

• Efficiently locate pertinent information on the NCHS website, which is constantly evolving and being updated;
• Quickly retrieve NHANES environmental chemical data files, a frequent challenge given the complex structure and large amount of data available;
• Correctly prepare an analytic dataset taking into consideration the special issues which may impact the analyses of environmental chemical variables; and
• Correctly conduct statistical analyses that achieve the analyst’s research objectives with appropriate attention paid to the nuances of the data given the complex sample design, weighting requirements, and data structure.

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The tutorial is intended to be:

• a step-by-step course on environmental chemical data retrieval and analysis for new users;
• a self-learning tool for novice and infrequent users to supplement their knowledge of the NHANES database and the analysis techniques required to properly analyze environmental chemical data in NHANES; and
• a reference for experienced researchers highlighting NHANES' analytic nuances and sample program code.

To best serve the needs of a wide range of users, this tutorial includes a large amount of information.  The information is organized in such a way that those who would like to review all information may do so, while others can efficiently bypass sections that are not of interest to them.  Overall, this tutorial provides a complete on-line learning tool. Sections of the environmental chemical tutorial may repeat some concepts discussed in other NHANES tutorials, in order to provide a more useful presentation of the environmental chemical analytic topics.

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If you are a beginning NHANES environmental chemical data user, you may want to work through the tutorial in a linear fashion, starting with the modules that orient you to the data, then continuing with the modules about how to prepare an analytic dataset and how to conduct various types of analyses.

If you are an experienced NHANES data user and you are searching for specific information or types of assistance, you can pick and choose other topics of interest among the modules or even among tasks within modules.

You are encouraged to use the tutorial's features and content to suit your own needs, these include:

• A linear path for users who need step-by-step self-guided training;
• Iindex pages and easy navigation throughout modules that allow you to limit your training to information on an “as needed” basis;
• Checklists, summary tables, and animated demonstrations with audio explanation to enhance learning;
• Real-life examples to assist you through the various processes of data preparation and data analysis; 
• Sample code and data files for you to practice skills and run analyses using your own software;
• Annotated results with interpretation to help you troubleshoot your own analyses; and
• Links to related references and outside resources.

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Users will likely fall into one of these groups:

• Public health and other government staff involved in NHANES environmental chemical data analyses;
•   Researchers who use NHANES environmenta chemical data for scientific studies or educational purposes;
• Public health and medical professionals who use NHANES environmental chemical data for planning, policy making, or public health programs; and
• Other users of NHANES environmental chemical data or groups interested in NHANES.

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1. Issues specific to NHANES environmental chemical data

• NHANES does not test for every environmental chemical that might be of research interest due to factors such as cost, logistic or technical limitations.
• All environmental chemicals measured in NHANES are not measured in all NHANES participants, generally because of cost, participant burden, or other logistic reasons.
• Many of the chemicals are measured only on a subset (for example, of the examined NHANES study sample) thus understanding the sample design and sample weights is crucial (See Module 4 and Module 11).
• Currently, NHANES data can only be used to obtain national estimates and, for confidentality reasons, cannot be used to examine exposure levels by locality, state, or region.
• Currently, NHANES seasonal changes of the year cannot be considered with this NHANES data.

• Proximity to sources of exposure cannot be considered with the data.
• Use of particular products cannot be considered with this data.
• Trends over time can only be evaluated when a sufficient number of years of data are available to determine statistically reliable estimates.
• The presence of an environmental chemical does not imply disease or other health effects.
• Levels of environmental substances in blood and urine are not the same as those in air, water, food, soil or dust.
• Having only blood and urine levels measures do not determine which exposure source or which route of exposure has occurred.
• Assessment of persistent substances (with long half lives) might be more meaningful than assessments of non-persistent substances (with short half lives).

Users are encouraged to read further details on NHANES environmental chemical analytic issues in Module 11 of this tutorial.

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