Key Concepts About Concentations of Chemicals in Blood and Urine Compared to Concentrations of Chemicals in the Environment

Many factors can influence the net exposure to an environmental chemical. For example variations in the absorption, metabolism, elimination, and storage of chemicals in urine or blood may affect the net exposure of an individual. Also, the structure of the chemical itself, different routes of exposure, and the exposure location may also make a difference. Therefore, the assessment of exposure requires an examination of the following factors when conducting analyses:

• Chemical half-life (i.e., persistence of chemical in blood or urine);
• Route of exposure;
• Genetic suseptibility;
• Demographic characteristics (e.g., age or gender);
• Health status and nutrition(e.g., reduced kidney function, iron deficiency);
• Lifestyle or behavioral factors (e.g. , smoker versus non-smoker, or occupation); and
• Geography (e.g., proximity to environmental chemical sources, or climate).

The role of geography in environmental chemical exposure is one factor that is of interest, and NHANES data have been used to examine the relationship between environmental exposure and place of residence. While this topic cannot be fully addressed in this tutorial, it deserves a brief discussion.  Because NHANES is currently designed as an overall nationally representative sample, the analysis of a particular environmental exposure according to geographic area may be limited. Within a two-year data collection cycle for NHANES, the design is such that approximately 30 locations in the United States are selected for the identification of the entire sample of persons.  It is usually necessary to combine the data from several consecutive two-year NHANES data collection cycles to achieve adequate sample size for geographic analyses. While earlier NHANES (i.e., NHANES I, II, III) provided the region of the country and level of urbanization of place of residence for all sample persons, NHANES 1999+ data sets no longer provide this detail due to potential risks to participant confidentiality.  Researchers may have access to geography-related information through the National Center for Health Statistics’ Research Data Center (RDC). The RDC also provides a venue for NHANES data to be matched with other data from geographic information systems (GIS) or from federal data sets with relevant geo-coded variables.  An example of an analysis that combined and additional data set linked to NHANES participants on the basis of geography is provided in a recent paper by Pickett et al. which examined the relationship between state or county-based smoke-free air and secondhand smoke exposure in NHANES 1999–2002.

Most of these factors are not unique to NHANES data. Further discussion of these and other factors can be found at:

• The International Programme on Chemical Safety's Guidelines on Studies in Environmental Epidemilogy, and
• Environmental Health Criteria No. 27 (1983).

References

Pickett MS, Schober SE, Brody DJ et al. Tob Control 2006;15:302-307 doi:10.1136/tc.2005.015073.

Lauwerys RR and Hoet P. Industrial Chemical Exposure: Guidelines for Biological Monitoring. 2001. Lewis Publishers, Boca Raton FL.