The NHANES program
suspended field operations in March 2020 due to the coronavirus disease 2019
(COVID-19) pandemic. As a result, data collection for the NHANES 2019-2020
cycle was not completed and the collected data are not nationally
representative. Therefore, data collected from 2019 to March 2020 were combined
with data from the NHANES 2017-2018 cycle to form a nationally representative
sample of NHANES 2017-March 2020 pre-pandemic data. These data are available to
the public. Please refer to the Analytic Notes section for more details on the
use of the data.
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 toxicants 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.
All examined participants aged 3 to 5 years and participants aged 6 years and older from a one-third subsample in the NHANES 2017-March 2020 pre-pandemic sample 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
Refer to the Laboratory Method Files section for
a detailed description of the laboratory methods used.
Laboratory Method Files
Volatile Organic Compounds (VOCs) Metabolites
Volatile Organic Compounds (VOCs) Metabolites
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 2017-2018 and 2019-2020 NHANES Laboratory Procedures Manuals (LPMs). Vials are stored under appropriate frozen (–30°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 Amendment mandates. Detailed QA/QC instructions are discussed in the NHANES LPMs.
Mobile Examination Centers
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
NHANES uses several methods to monitor the
quality of the analyses performed by the CDC and 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 QC protocol for
all CDC and contract laboratories, which outlined the use of Westgard rules
(Westgard, et al., 1981) 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 Division
of Laboratory Sciences’ 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.
The COVID-19 pandemic required suspension of NHANES 2019-2020 field operations in March 2020 after data were collected in 18 of the 30 survey locations in the 2019-2020 sample. Data collection was cancelled for the remaining 12 locations. Because the collected data from 18 locations were not nationally representative, these data were combined with data from the previous cycle (2017-2018) to create a 2017-March 2020 pre-pandemic data file. A special weighting process was applied to the 2017-March 2020 pre-pandemic data file. The resulting sample weights in the demographic data file should be used to calculate estimates from the combined cycles. These sample weights are not appropriate for independent analyses of the 2019-2020 data and will not yield nationally representative results for either the 2017-2018 data alone or the 2019-March 2020 data alone. Please refer to the NHANES website for additional information for the NHANES 2017-March 2020 pre-pandemic data, and for the previous 2017-2018 public use data file with specific weights for that 2-year cycle.
Refer to the 2017-2018 and 2019-2020 Laboratory Data Overview documents 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. 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 is necessary.
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.
The analytes included in this dataset were measured in all examined participants aged 3-5 years, and in a one-third subsample of participants 6 years and older. Special sample weights are required to analyze these data properly. Variable (WTSAPRP) encoding of the specific sample weights for this subsample is included in this data file and should be used when analyzing these data. These special sample weights were created to account for the subsample selection probability, as well as the additional nonresponse to these lab tests. Therefore, if participants were eligible for the subsample, but did not provide a urine specimen, they would have the sample weight value assigned as “0” in their records.
Demographic and Other Related Variables
The analysis of NHANES laboratory data must be conducted the appropriate survey design and demographic variables. The NHANES 2017- March 2020 Pre-Pandemic 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.
This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier (i.e., 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. All data are rounded to three significant figures or three decimal places, whichever is less precise.
The lower limit of detection (LLOD, in ng/mL) for urinary VOC metabolites:
|| ANALYTE NAME
||2-Aminothiazoline-4-carboxylic acid (ng/mL)
||Mandelic acid (ng/mL)
||2-Methylhippuric acid (ng/mL)
||3- and 4-Methylhippuric acid (ng/mL)
||Phenylglyoxylic acid (ng/mL)