Volatile Organic Compounds (Human Blood):
Exposure to volatile organic compounds (VOCs) is ubiquitous. Chronic exposure
to extremely high levels of some VOCs can lead to cancer and neurocognitive
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 levels in blood is essential to determine the need for
regulatory mechanisms to reduce the levels of hazardous air pollutants to which
the general population is exposed.
Volatile Organic Compounds (Home Tap Water):
In addition to assessing levels of VOCs in blood, VOC levels will be measured
in home tap water specimens provided by NHANES participants. Specifically,
trihalomethanes (THMs) and the fuel additive methyl tertiary-butyl ether (MTBE)
are measured in thesesamples.
In 1999, the eligible sample was a one-fourth subsample of persons 20–59
years; in 2000, it was a one-third subsample of persons 20–59 years. There were
no component-specific exclusions.
Description of Laboratory
Measurements of Trihalomethanes (THMs) and MTBE in Tap
The prevalence of water disinfection by-products in drinking water
supplies has raised concerns about possible health effects from chronic
exposure to these compounds. The objective is to support studies
the relationship between exposure to THMs and health
This automated analytical method uses headspace solid-phase microextraction
(SPME) coupled with capillary gas chromatography and mass spectrometry (3). This
method quantitates trace levels of THMs (chloroform, bromodichloromethane,
dibromochloromethane, and bromoform) and MTBE in tap water. Detection
limits of less than 100 pg/mL for all analytes and linear ranges of three orders
of magnitude are adequate for measuring the THMs in tap water samples tested
from across the United States. THMs are stable for extended periods in tap water
samples after quenching of residual chlorine and buffering to pH 6.5, thus
enabling larger epidemiologic field studies with simplified sample collection
Measurements of THMs and MTBE in Whole Blood
The prevalence of water disinfection by-products (e.g. THMs) in tap water has
raised concerns about possible health effects from chronic exposure to these
compounds. Exposure to the fuel oxygenate MTBE has also raised concerns. People
can be exposed to THMs and MTBE through a variety of sources, including use of
household tap water that contains these chemicals. The objective of this study
is to support studies exploring the relationship between health effects and
exposure to THMs and MTBE. THMs and MTBE were quantified in human blood using
capillary gas chromatography (GC) and high-resolution mass spectrometry (MS)
with selected ion mass detection and isotope-dilution techniques (4). This
method quantified trace levels of THMs (chloroform, bromodichloromethane,
dibromochloromethane, and bromoform) and MTBE in human blood. Analyte responses
were adequate for measuring background levels after extraction of these volatile
organic compounds with either purge-and-trap extraction or headspace SPME.
Detection limits ranged from 0.3–2.4 pg/mL, with linear ranges of three orders
of magnitude. This method provided adequate sensitivity for measuring the THMs
and MTBE in most blood samples tested from diverse U.S. reference
Measurements of other VOCs in Whole Blood
An additional 11 VOCs were measured in human blood using SPME in conjunction
with gas chromatography and bench top quadrupole mass spectrometer (5). A
combination of SPME and multiple single-ion monitoring minimizes the
interferences and chemical noise associated with whole-blood samples. Detection
limits are below 50 ppt (pg/mL) for a majority of the VOCs tested.
Data Processing and Editing
Automated data collection procedures for the survey were introduced in NHANES
1999–2000. In the MECs and analytical laboratories, data for the laboratory
component are recorded directly onto a computerized data collection form. The
system is centrally integrated, and it allows for ongoing monitoring of much of
the data. Although the complete blood count and pregnancy analyses are performed
in the MEC laboratory, most analyses are conducted elsewhere by approximately 21
laboratories across the United States.
Guidelines have been developed that provide standards for naming variables,
filling missing values, and handling missing records. NCHS staff, assisted by
contract staff, has developed data-editing specifications that check data sets
for valid codes, ranges, and skip- pattern consistencies and examine the
consistency of values between interrelated variables. Comments have been
reviewed and recoded. NCHS staff verifies extremely high and low values whenever
possible, and numerous consistency checks are performed. Nonetheless, users
should examine the range and frequency of values before analyzing
The data editing specifications are as follows:
- Age and gender checks
- Total number of observations complete for each field
- No field overlap, truncated values, or weird results
- Direct data entry (DDE) errors
- Abnormal results confirmed by lab
- Test algorithm performed • Checked comment codes to resolve missing
results and missing records
- All missing results and missing MEC-examined records are accounted
- Duplicate records were verified and deleted
Assurance and Monitoring
Mobile Examination Centers (MECs)
Laboratory team performance is monitored using several techniques.
NCHS and contract consultants use a structured quality assurance evaluation
during unscheduled visits to evaluate both the quality of the laboratory work
and the quality-control procedures. Each laboratory staff person is observed for
equipment operation, specimen collection and preparation; testing procedures and
constructive feedback are given to each staff. Formal retraining sessions are
conducted annually to ensure that required skill levels were maintained.
The NHANES QA/QC protocols meet the 1988 Clinical Laboratory Improvement Act
mandates. Detailed QA/QC instructions are discussed in the NHANES LPM.
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. In addition, contract laboratories randomly
perform repeat testing on 2.0% of all specimens.
NCHS developed and distributed a quality control protocol for all the
contract laboratories which outlined the Westgaard rules 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 and Westat quarterly. The reports are reviewed for trends or
shifts in the data. The laboratories are required to explain any identified
areas of concern. NCHS/Westat is currently reviewing these reports.
Measures of volatile organic compounds in blood and water were assessed in a
subsample of participants aged 20–59. Use the special weights included in this
data file when analyzing data. Please refer to the Analytic Guidelines for
further details on the use of sample weights and other analytic issues.
The analysis of NHANES 1999-2000 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), which are included in the demographic data file for each
data release. The NHANES 1999-2000 Household Questionnaire and Demographic Data
Files contain demographic data, health indicators, and other related information
collected during household interviews. The phlebotomy file includes auxiliary
information such as the conditions precluding venipuncture. The demographic,
household questionnaire and phlebotomy files may be linked to the laboratory
data file using the unique survey participant identifier SEQN.
The detection limit was variable for many of the analytes in the data set. In
addition two variables are provided for each of these analytes. The variable
named LBD___LC indicates whether the result was below the limit of detection.
There are three values: "0", "1", and "2". "0" means that the result was at or
above the limit of detection. "1" indicates that the result was below the limit
of detection. "2" means the result was above the limit of detection.
The other variable named LBX___ provides the analytic result for that
analyte. In cases, where the result was below the limit of detection, the value
for that variable is the detection limit divided by the square root of two.