CDC Dioxin Biomonitoring Data: Questions
July 21, 2005
1. What is the purpose of the CDC's biomonitoring reports
and how are data collected?
The purpose of the U.S. Centers for Disease Control and
Prevention (CDC) National Reports on Human Exposure to
Environmental Chemicals is to gain a better understanding
of the health and nutritional status of the U.S. population.
Under the National Health and Nutrition Examination Survey
(NHANES), CDC's teams visit 15 different locations around
the U.S. every two years to conduct health surveys and collect
statistically relevant information using questionnaires, physiological
measurements and analytical measurements from blood and urine,
including measurements of levels of environmental chemicals.
2. How long have dioxins been measured in human tissue
as part of the NHANES?
Although specialized data on dioxins1
have been collected by researchers for many years, the NHANES
represents the first national picture of levels of dioxins
in the U.S. population. Dioxin biomonitoring data were reported
first in the January, 2003 report and then again in the 2005
report, released July 21, 2005.
3. Has there been an increase in dioxin levels between
the 2003 and 2005 reports?
No. Dioxin levels have been declining since the early 1970s
and models project continued declines. There are several reasons
why there is an apparent increase in levels between the 2003
and 2005 reports:
Small blood sample volume available to the CDC for the 1999-2000
NHANES hampered CDC from measuring low levels of dioxins in
blood. With the larger blood sample volumes in the 2001-2002
NHANES, CDC was able to measure lower levels. With lower detection
levels, a more complete profile of dioxins is possible. It
is likely that the 2001-2002 NHANES data more accurately reflect
a dioxin level that is representative of the U.S. population.
4. What are the differences between the analytical measurements
for dioxins between the data reported in the 2003 and 2005
National Exposure Reports?
CDC identified the fact that the small volume of individuals'
blood samples available for the 2003 report were insufficient
to measure the very low levels of dioxins in blood. CDC took
steps to address these shortcomings in the 2005 report, increasing
the volume of blood sampled and pooling samples from each
of the age groups studied. Table 1 compares dioxin and furan
data from the two reports.
Table 1: Comparing Two Reports on Dioxins
in the U.S. Population
Age Groups Sampled
Four age groups:
12 - 19
20 - 39
40 - 59
Three age groups:
20 - 39
40 - 59
Blood Volume Sampled from Individuals
Approximately 5 ml
(contributed to lack of detection)
Approximately 8 ml
Dioxin Compounds Reported
15 of 17 compounds
17 of 17 compounds
Findings Regarding Dioxins
Levels in 50 percent of the population
were below the detectable limit.
Levels are low, more than 80% lower
than in the 1980s.
5. Why is the "limit of detection" important?
Accurately measuring low amounts of chemicals in blood depends
on the sample volume. The "limit of detection" (LOD) is the
level at which the measurement has a 95% probability of being
greater than zero. LOD depends, in part, upon the volume of
a sample. The larger the sample volume, the lower the level
chemists can measure.
For the 1999-2000 sampling years, the CDC measured dioxins
in individual blood samples having a size of about 5 milliliters
in volume. This resulted in CDC's reporting mostly non-detects
for many of the individual dioxin-like compounds in the 2003
report. (CDC only reported values for 5 congeners in the 2003
6. How did the 2003 CDC report the "limit of detection"
Each individual blood sample that CDC analyzed for dioxins
and furans had a unique limit of detection. That is because
the volume of blood available for analysis was slightly different
for each individual in the NHANES. For each chemical, CDC
reported the average amount, the standard deviation, and the
maximum value of the detection limit in a series of tables.
As a matter of policy, for chemicals that were detected
at the limits of detection, CDC decided not to report information
in its selected percentile tables if any individual limit
of detection exceeded the percentile estimate. Thus, in the
Second National Exposure Report CDC reported 'below
the limit of detection' for most of the dioxin-like chemicals.
7. Why did CDC pool (combine) individuals' blood samples
for measuring dioxins for the 2005 report?
For the 2001-2002 sampling years, the CDC strove to report
levels above the limit of detection-that is, to report actual
levels rather than reporting almost all "non-detects." To
do so, CDC needed larger volumes of blood than were collected
in the 1999-2000 survey. This was done by: 1) increasing each
individual blood sample volume from 5 to 8 milliliters and
2) making a very large sample-"pooled sample"-by combining
a small amount of blood from many people. Larger blood samples
permitted CDC to measure small amounts of dioxins, PCBs and
brominated flame retardants.4 Pooled samples allow CDC to report average (mean) levels of chemicals
in the U.S. population by age, race and gender.
1"Dioxins" refers to 7 dioxins and 10 furan chemical
compounds. There are 210 total known chlorinated dioxins and
furans, but only 17 of interest.
2Of the 17 dioxin compounds of interest, 15 were
measured for the 2003 report, but all 17 were measured for
the 2005 report. The two compounds added for the first time
were 1,2,3,4,7,8-HxCDD and 1,2,3,4,7,8,9-HpCDF.
3This refers to calculations performed by the
Chlorine Chemistry Division of the American Chemistry Council.
4Based on preliminary data presented by L. Needham,
CDC, to the National Academy of Sciences (NAS) Expert Panel
on Review, EPA's Exposure and Human Health Reassessment of
TCDD and Related Compounds, February 2, 2005. Slides accessed
by contacting NAS staff. http://www4.nas.edu/cp.nsf/Projects+_by+_PIN/BEST-K-03-08-A?OpenDocument.