CDC Dioxin Biomonitoring Data: Questions and Answers

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:

  • The 2005 report analyzed for dioxin levels of two additional compounds.

    Two additional compounds2 were included for the first time in the 2003 report. These added a small amount to the overall dioxin measurement.

  • There were "missing data" in the 2003 report.
    The 1999-2000 NHANES data set had a large amount of "missing data" (samples that were "lost" due to spillage, mislabeling, handling and many other factors) that were treated as a null value ('0') in calculations.3 In the 2001-2002 NHANES data there are significantly fewer "missing data."
  • The volume of sampled blood in the 2003 report was too small.

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

2003 Report
2005 Report
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
Missing Data
23 %
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 report).

6. How did the 2003 CDC report the "limit of detection" for dioxins?

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.

End Notes

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.

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