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.
Compounds and Trihalomethanes/MTBE (Whole Blood)
compounds (VOCs) are a large group of chemicals that have been used as
solvents, degreasers, and cleaning agents in industry and consumer products.
Many of the VOCs were found to contaminate ground water and drinking water
sources. Because of human health concerns, these VOCs have been banned or
restricted from most uses.
solvents are VOCs consisting of a hydrocarbon chain or one hydrocarbon
substituted with one or more chlorine or bromine atoms. Most of these chemicals
are used as degreasers and solvents in various products, such as paint. In the
past, 1,1,1-trichloroethane was used as a dry-cleaning agent, insect fumigant,
and solvent in consumer products. Methylene chloride, tetrachloroethene, and
trichloroethene are other VOCs that were widely used in the past.
ethylbenzene, and xylene collectively referred to as BTEXS, are components of
tobacco smoke. Along with 2,5-dimethylfuran, these VOCs are usually detected in
the blood of cigarette smokers at higher levels than in non-smokers.
Chlorobenzene (monochlorobenzene) and the three dichlorobenzenes are halogenated
aromatic hydrocarbons primarily used in industrial and chemical synthetic
processes. Chlorobenzene has been used to produce DDT, phenol, and
nitrobenzene. The dichlorobenzenes are also chemical intermediates in the
synthesis of dyes, pesticides, and other industrial products.
1,4-Dichlorobenzene (para-dichlorobenzene) is used also as a moth repellent and
as a deodorizer. Disinfection by-products (DBP), including bromodichloromethane,
dibromochloromethane, bromoform, and chloroform are formed when chlorine
interacts with natural organic materials found in water. Primary sources of
DBPs are chlorinated drinking water and recreational water bodies, such as
The prevalence of
disinfection by-products in drinking water supplies has raised concerns about
possible adverse health effects from chronic exposure to these potentially
carcinogenic compounds. Methyl-tert-butyl ether (MTBE) was used as an additive
to replace lead in gasoline, but its use was banned after widespread ground
water contamination was discovered.
Inhalation is the
most common VOC route of exposure in the general population, including indoor
sources such as paints, adhesives, cleaning solutions, and aerosolized
insecticide sprays; industries producing these solvents; and contaminated waste
disposal sites. Drinking water may contribute to exposure when underground
drinking water supplies are contaminated. After they are absorbed in the body,
VOCs are rapidly eliminated in exhaled breath, or may be rapidly metabolized and
eliminated in the urine.
Examined participants aged 12 years and older from a one-half sample in the NHANES 2017-March 2020 pre-pandemic sample were eligible.
Description of Laboratory Methodology
An automated analytical method was developed using capillary gas chromatography (GC) and mass spectrometry (MS) with selected-ion monitoring (SIM) detection and isotope-dilution. This method quantifies levels of individual VOCs and trihalomethanes (THMs) and methyl tert-butyl ether (MTBE) in whole blood to low-parts-per-trillion range. Because non-occupationally exposed individuals have blood VOC concentrations within this range, this method is applicable for determining these quantities and investigating cases of sustained or recent low-level exposure.
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.
Laboratory Method Files
Volatile Organic Compounds (VOCs) & Trihalomethanes/MTBE Laboratory Procedure Manual
Volatile Organic Compounds (VOCs) & Trihalomethanes/MTBE Laboratory Procedure Manual
Laboratory Quality Assurance and Monitoring
Whole blood specimens were processed, stored, and shipped to Division of Laboratory Sciences, National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, GA for analysis.
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
The NHANES quality
assurance and quality control (QA/QC) protocols meet the 1988 Clinical
Laboratory Improvement Act mandates. Detailed QA/QC instructions are discussed
in the NHANES LPMs.
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.
several methods to monitor the quality of the analyses performed by the
contract laboratories. In the MEC, these methods include performing blind split
samples collected on “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) used when running 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. For example, in 2017-March 2020 approximately 76% of children aged 1-17 years who were examined in the MEC provided a blood specimen through phlebotomy, while 95% 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 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.
A volatile toxicant questionnaire (VTQ) was administered on the mobile
examination center (MEC), by trained interviewers, using the Computer-Assisted
Personal Interview (CAPI) system. The VTQ section includes data about the participant’s
home, activities, amount of time spent in various locations, and exposure to different
chemicals over the past 48 hours. This questionnaire data can be used in
conjunction with the VOC laboratory dataset and found in the Volatile Toxicant
Data File (P_VTQ) in
the NHANES 2017- March 2020 Pre-Pandemic Questionnaire Data section.
Whole blood VOCs
were measured in a one-half subsample of participants 12 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.
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.
The Fasting Questionnaire File includes auxiliary information, such as fasting status, length of fast and the time of venipuncture.
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 all of the analytes in the data set. Two variables are provided for each of these analytes. The variable name ending in “LC” (ex., LBD2DFLC) indicates whether the result was below the limit of detection: “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 prefixed LBX (ex., LBX2DF) provides the analytic result for that analyte. All data are rounded to three significant figures or three decimal places, whichever is less precise. For analytes with analytic results below the lower limit of detection (ex., LBD2DFLC=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 ).
Lower Limit of Detection (LLOD, in ng/mL) for Whole Blood VOCs:
||Blood 2,5-Dimethylfuran (ng/mL)
||Blood 1,1,1,2-Tetrachloroethane (ng/mL)
||Blood Hexane (ng/mL)
||Blood Heptane (ng/mL)
||Blood Octane (ng/mL)
||Blood 1,2-Dichlorobenzene (ng/mL)
||Blood 1,2-Dichloroethane (ng/mL)
||Blood 1,3-Dichlorobenzene (ng/mL)
||Blood Tetrachloroethene (ng/mL)
||Blood a-pinene (ng/mL)
||Blood Bromoform (ng/mL)
||Blood Bromodichloromethane (ng/mL)
||Blood Benzene (ng/mL)
||Blood Benzonitrile (ng/mL)
||Blood Cyclohexane (ng/mL)
||Blood Chlorobenzene (ng/mL)
||Blood Chloroform (ng/mL)
||Blood Dibromochloromethane (ng/mL)
||Blood Carbon Tetrachloride (ng/mL)
||Blood 1,4-Dichlorobenzene (ng/mL)
||Blood 1,2-Dibromoethane (ng/mL)
||Blood Diethyl Ether (ng/mL)
||Blood Ethyl Acetate (ng/mL)
||Blood Ethylbenzene (ng/mL)
||Blood Chloroethane (ng/mL)
||Blood Furan (ng/mL)
||Blood Isobutyronitrile (ng/mL)
||Blood Isopropylbenzene (ng/mL)
||Blood Methylene Chloride (ng/mL)
||Blood Methylcyclopentane (ng/mL)
||Blood MTBE (ng/mL)
||Blood Methyl Isobutyl Ketone (ng/mL)
||Blood Nitrobenzene (ng/mL)
||Blood o-Xylene (ng/mL)
||Blood Trichloroethene (ng/mL)
||Blood 1,1,1-Trichloroethane (ng/mL)
||Blood aaa-Trifluorotoluene (ng/mL)
||Blood Tetrahydrofuran (ng/mL)
||Blood 1,2,3-Trichloropropane (ng/mL)
||Blood Vinyl Bromide (ng/mL)
||Blood m-/p-Xylene (ng/mL)