• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Data Processing and Editing
• Laboratory Quality Assurance and Monitoring
• Analytic Notes
• Codebook
  • SEQN - Respondent sequence number
  • WTSA2YR - Subsample A weights
  • URXUHG - Mercury, urine (ug/L)
  • URDUHGLC - Mercury, urine comment code
  • URXUCR - Creatinine, urine (mg/dL)

National Health and Nutrition Examination Survey

2011-2012 Data Documentation, Codebook, and Frequencies

Mercury: Inorganic - Urine (UHG_G)

Data File: UHG_G.xpt

First Published: October 2013

Last Revised: October 2014

Component Description

Mercury is widespread in the environment and originates from natural and anthropogenic sources. The general population may be exposed to three forms of mercury: elemental, inorganic, or organic (primarily methylmercury). The concentration of total mercury, in urine, is a biomeasure of exposure primarily to elemental and inorganic mercury. Elemental and inorganic mercury exposure can result from mercury spills, dental amalgams, and occupational exposures. Both elemental and inorganic mercury are nephrotoxic and neurotoxic. Health effects related to low exposure in the general population are not well defined. In the 1999-2002 NHANES, urine mercury levels were measured in all women aged 16-49 years. From 2003-2004 forward, urine mercury levels were measured in a one-third subsample of participants aged 6 years and older.

Eligible Sample

Examined participants aged 6 years and older from a one-third sample.

Description of Laboratory Methodology

Urine specimens were processed, stored and shipped to Division of Laboratory Sciences, National Center for Environmental Health, National Centers for Disease Control and Prevention, Atlanta, Georgia.

Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM).

Urinary mercury concentrations are determined by ICP-DRC-MS (Inductively Coupled Plasma Dynamic Reaction Cell Mass Spectroscopy). This multielement analytical technique is based on quadrupole ICP-MS technology and includes DRC™ technology. Coupling radio frequency power into a flowing argon stream(seeded with electrons) creates the plasma, the heat source, which is ionized gas suspended in a magnetic field. Predominant species in the plasma are positive argon ions and electrons. Diluted urine samples are converted into an aerosol using a nebulizer inserted within the spray chamber. A portion of the aerosol is transported through the spray chamber and then through the central channel of the plasma, where it is exposed to temperatures of 6000-8000 ºK. This thermal energy atomizes and ionizes the sample. The ions and the argon enter the mass spectrometer through an interface that separates the ICP, which is operating at atmospheric pressure (approximately 760 torr), from the mass spectrometer, which is operating at approximately 10^-5 torr. The mass spectrometer permits detection of ions at each mass-to-charge ratio in rapid sequence, which allows the determination of individual isotopes of an element. Once inside the mass spectrometer, the ions pass through the ion optics, then through DRC™, and finally through the mass-analyzing quadrupole before being detected as they strike the surface of the detector. The ion optics uses an electrical field to focus the ion beam into the DRC™. The DRC™ component is pressurized with an appropriate reaction gas and contains a quadrupole. Electrical signals resulting from the detection of the ions are processed into digital information that is used to indicate the intensity of the ions and subsequently the concentration of the element. Traditionally ICP-MS has been a trace analysis technique and the typical measurement ranges from 0.1µg/L to around 100 µg/L. DRC technology provides additional control of ICP-MS sensitivity; therefore appropriate adjustments of the reaction cell parameters can significantly extend the useful concentration measurement range. In this method, iodine (isotope mass 127), tellurium (isotope mass 130), mercury (isotope mass 202) and bismuth (isotope mass 209) are measured in urine by ICP-DRC-MS using 100% argon as the Dynamic Reaction Cell™ (DRC) gas utilizing collisional focusing. Urine samples are diluted 1+1+ 8 (sample+ water + diluent) with water and diluent containing tellurium and bismuth for internal standardization.

There were no changes to the site or laboratory from the previous two year cycle.

Refer to NHANES 2011-2012 Lab Methods for Iodine & Mercury for detailed description of the laboratory method used.

Data Processing and Editing

The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.

Laboratory Quality Assurance and Monitoring

The NHANES quality control and quality assurance protocols (QA/QC) meet the 1988 Clinical Laboratory Improvement Amendments mandates. Detailed QA/QC instructions are discussed in the NHANES Laboratory Procedures Manual (LPM).

Analytic Notes

Refer to the 2011-2012 Laboratory Data Overview for general information on NHANES laboratory data.

Subsample Weights
Urinary mercury was measured in a one third subsample of persons 6 years and older. Special sample weights are required to analyze these data properly. Specific sample weights for this subsample are included in this data file and should be used when analyzing these data.

Variance Estimation
The analysis of NHANES laboratory data must be conducted with the key survey design and basic demographic variables.  The recommended procedure for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.

Links to NHANES Data Files
This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier SEQN.

Detection Limits
The lower detection limit for urinary mercury was constant during this two-year cycle: Urinary Mercury= 0.05 ng/mL. The variable named URDUHGLC indicates whether the result was below the limit of detection. There are two values: “0” and “1”. “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 other variable named URXUHG provides the analytic result for urinary mercury. For cases with analytic results below the lower limit of detection (i.e., URDUHGLC=1), an imputed fill value was placed in the analyte results field. This value is the lower limit of detection divided by square root of 2 (LLOD/sqrt(2)). 

Other NHANES Data Files

Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial  for further details on the use of sample weights and other analytic issues.

Codebook and Frequencies