Exposure to volatile organic compounds (VOCs) is ubiquitous. Chronic exposure to extremely high levels of some VOCs can lead to cancer and neurocognitive dysfunction. Urinary metabolites of VOCs can be detectable in urine for a longer period of time than the parent VOCs can be detected in blood.
Nearly 200 air toxics have been associated with adverse health effects in occupational studies or laboratory studies, but have not been monitored in general population groups. Information on levels of exposure to these compounds, as measured by their metabolite levels in urine, is essential to determine the need for regulatory mechanisms to reduce the levels of hazardous air pollutants to which the general population is exposed.
Examined participants aged 6 years and older from a one-third subsample were eligible.
Description of Laboratory Methodology
This method is a quantitative procedure for the measurement of VOC metabolites in human urine using ultra performance liquid chromatography coupled with electrospray tandem mass spectrometry (UPLC-ESI/MSMS) as described by Alwis et. al (2012). Chromatographic separation is achieved using an Acquity UPLC® HSS T3 (Part no. 186003540, 1.8 µm x 2.1 mm x 150 mm, Waters Inc.) column with 15 mM ammonium acetate and acetonitrile as the mobile phases. The eluent from the column is ionized using an electrospray interface to generate and transmit negative ions into the mass spectrometer. Comparison of relative response factors (ratio of native analyte to stable isotope labeled internal standard) with known standard concentrations yields individual analyte concentrations.
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 2013-2014 cycle.
Laboratory Method Files
Volatile Organic Compound (VOCs) and Metabolites-Urine
Laboratory Quality Assurance and Monitoring
Urine specimens are 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 Procedures Manual (LPM). Vials are stored under appropriate frozen (–20°C) conditions until they are shipped to National Center for Environmental Health for testing.
The NHANES quality assurance
and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Amendments 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 QC 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 second examinations on previously examined participants and blind split samples collected on “dry run” sessions.
NCHS developed and distributed a QC protocol for
all CDC and contract laboratories, which outlined the Westgard rules (Westgard,
et al. 1981) used when testing 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 CDC/NCEH Tobacco and Volatile Branch QA/QC 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 2013-2014 Laboratory Data Overview for general information on NHANES laboratory data.
There are over
800 laboratory tests performed on NHANES participants. However, not all
participants provided biospecimens or enough volume for all the tests to be
performed. The specimen availability can also vary by age or other population
characteristics. For example, in 2013-2014, approximately 82%of children aged
1-17 years who were examined in the MEC provided a blood specimen through
phlebotomy, while 96% of examined adults age 18 and older provided a blood
specimen. Analysts should evaluate the extent of missing data in the dataset
related to the outcome of interest as well as any predictor variables used in
the analyses to determine whether additional re-weighting for item non-response
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.
VOC metabolites in urine were assessed in a subsample of participants aged 6 years and older. Use the special one-third weights included in this data file when analyzing data. 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.
Demographic and Other Related Variables
The analysis of NHANES laboratory data must be conducted with the key survey design and basic demographic variables. The NHANES 2013-2014 Demographics File contains demographic and 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.
This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier SEQN.
The detection limits were constant for the analytes in the data set. Two variables are provided for each of these analytes. The variable named ending in “LC” (ex., URDAAMLC) 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. For analytes with analytic results below the lower limit of detection (ex., URDAAMLC=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 other variable prefixed URX (ex., URXAAM) provides the analytic result for the analyte.
The lower limit of detection (LLOD, in µg/L) for urinary VOC metabolites:
||Urinary 2-Aminothiazoline-4-carboxylic acid
||Urinary Mandelic acid
||Urinary 2-Methylhippuric acid
||Urinary 3- and 4-Methylhippuric acid
||Urinary Phenylglyoxylic acid
||Urinary 2-Thioxothiazolidine-4-carboxylic acid
Bias adjustments Applied to N-Acetyl-S-(3-Hydroxypropyl-1-Methyl)-L-Cysteine (URXPMM)
A systematic bias was discovered for the measurement of urinary VOC metabolite N-Acetyl-S-(3-hydroxypropyl-1-methyl)-L-cysteine (URXPMM) in the NHANES 2005 – 2006, 2011 – 2012, 2013 – 2014, and 2015 – 2016 cycles. The systematic nature of these biases permitted retrospective adjustment through an algebraic correction factor of 0.565. The measurement values in variable URXPMM were adjusted as below and released in the present file:
URXPMM = URXPMMOriginal x 0.565
In the 2005-2006 and 2011-2016 NHANES cycles, calibrators used in these tests were externally prepared from a multi-analyte stock solution formulated from neat materials by a vendor. Recently in 2021, accuracy solutions from a new vendor indicated a systematic bias. Validations conducted by the CDC testing lab confirmed that the large mass fraction of salt in the neat materials was not accounted for during formulation of the externally prepared calibration materials from previous vendor. This discrepancy between the calculated concentration of the calibrators and the actual concentration led to a systematic bias in results for URXPMM.