Dioxin Exposures and Body Levels Fall Dramatically
Tracking Dioxin Trends
Measured emissions and environmental levels
of dioxins have declined dramatically over the past three
decades as a result of government regulation and industry
initiative designed to curb releases of these compounds.
Given the encouraging progress to date, the
question arises: How have dioxin emissions reductions affected
human exposure? Two new scientific studies show that steadily
decreasing levels of dioxins in human tissue are evidence
of significantly diminished exposure to these persistent compounds.
Humans are exposed to dioxins predominantly by ingesting animal
fats. Air emissions1 of pollutants, including dioxins,
settle on vegetation that is fed to livestock. These dioxins
then accumulate in animal fats and are conveyed to humans
through meat and dairy products in the diet.
Researchers Aylward and Hays (2002) and
Lorber (2002) collected and plotted historical human tissue
dioxin concentration data and found that average levels of
these compounds have dropped steadily in humans over the past
several decades. In each of their studies, the scientists
employed mathematical and biological models to reconstruct
the exposure level trends that would be required to produce
observed trends in body levels of dioxins.
The Science: Low Exposure Low Body Levels
Absorbed Intake of the Most Toxic Form
of Dioxin is Down More Than 95% Since 1972
Aylward and Hays focused their research
on 2,3,7,8-TCDD, one of the most toxic and widely studied
of the dioxin family compounds. They found that average levels
of this compound in residents of the United States, Canada,
Germany and France have declined by a factor of 10 over the
past 30 years. Current levels are at approximately two parts
per trillion-lipid adjusted (ppt), down from an estimated
average level of approximately 20 ppt in the early 1970s (see
Figure 1, p. 2). Back-calculating with the aid of scientific
models, the researchers determined the average "absorbed intake"
(the amount of 2,3,7,8-TCDD actually absorbed by the human
body, a true exposure) of this compound that would have produced
the measured historic body level trends. Aylward and Hays
found that absorbed intake must have fallen by more than
95% since 1972, with the bulk of the decline occurring
prior to 1980 (see Figure 2, p. 3).
Lowered emissions and exposures do not produce
immediate declines in 2,3,7,8-TCDD body levels; it takes years
for body concentrations to equilibrate to these changes. Since
body levels have not had time to respond to recent emissions
and exposure reductions, further declines in human 2,3,7,8-TCDD
levels from 2 ppt to between 0.5 and 1 part per trillion over
the next 15 years are expected even if average intake levels
do not decline further2 (see Figure 2, p. 3).
The curve labeled "0.9 ppt" in Figure 2 represents declining
2,3,7,8-TCDD body levels assuming the current intake rate
(or absorbed dose) remains constant. Interestingly, the model
predicts that a 10-fold decrease in 2,3,7,8-TCDD intake beginning
in 2003 (curve labeled "0.5 ppt") would have only a small
effect on further lowering body levels.
Fig. 1: Evidence for Decreasing Exposure3
|Mean lipid- adjusted TCDD levels from general population
in the United States, Canada, and Western Europe.
Fig. 2: Implications for Intake Estimates4
|Predicted time course of lipid - adjusted TCDD levels
due to immediate decline to 0.05 pg/kg/day in 1974 and
showing the effect of an additional 10-fold reduction
in in-take levels begining in 2003. The effect of future
reductions in TCDD intake rates on body burden will be
small compared to reductions that have already occured.
Body Levels of Dioxin-Like Chemicals Could
be Halved by 2020
Lorber (2002) examined not only 2,3,7,8-TCDD,
but also sixteen similar chemicals frequently grouped together
as "dioxin-like" compounds. Lorber used the concept of toxic
equivalents (TEQs) to model the compounds jointly as though
they were a single compound. TEQ units account for total toxicity
of a mixture of similar, but variously toxic compounds. Lorber
concluded that TEQ body levels fell from the 1970s (50-80
ppt) through the 1980s (30-50 ppt) and into the 1990s (10-20
ppt). Lorber fit TEQ body level data into a modeling framework
in which he assumed a low exposure of 0.5 picograms5
per kilogram of body weight per day (pg-TEQ/kg-body weight/day6
) in the early 20th century, a high exposure of 6 pg-TEQ/kg-body
weight/day in the middle decades of the century, and a current
return to 0.5 pg-TEQ/kg-body weight/day (see Figure 3, p.
4). Lorber thus demonstrated declining body levels of 17 dioxin
compounds in terms of TEQ and was able to model the decline
in a way that is similar to the method used by Aylward and
Hays to model 2,3,7,8-TCDD declines. Lorber states that if
current exposure stays constant at approximately 0.50 pg-TEQ/kg-body
weight/day, current body levels of approximately 20 ppt-TEQ
could go below 10 ppt-TEQ by 2020.
Fig.3: Dioxin TEQ Levels: Measured and Predicted
|Predicted adult population body concentration
of TEQs, pg/g lipid (solid lines) compared against
the measured population body concentration of TEQs,
pg/g lipid (squares).
Lorber's predicted TEQ body level
over time (solid line of the curve at left) is
based on the following assumed exposure rate:
The scientific investigations of Aylward
and Hays (2002) and Lorber (2002) demonstrate the positive
impact that dioxin emission reductions have had on human exposure
and body levels. Whether examining 2,3,7,8-TCDD alone, or
the 17 dioxin-like compounds together in terms of the TEQ
method, the conclusion remains the same: falling body levels
reflect steadily decreasing exposures to these compounds.
Aylward, L.L. and Hays, S.M. (2002). Temporal
trends in human TCDD body burden: Decreases over three decades
and implications for exposure levels. Journal of Exposure
Analysis and Environmental Epidemiology 12, p. 319-28.
Lorber, M. (2002). A pharmacokinetic model
for estimating exposure of Americans to dioxin-like compounds
in the past, present, and future. The Science of the Total
Environment 288, p. 81-95.
1Includes emissions from open
burning (trash, residential woodstove and fireplace), controlled
burning (waste combustion units) and various industrial facilities
as well as natural sources.
2Since dioxins are produced naturally in many combustion
environments, including forest fires and volcanoes, humans
have probably always been exposed to low levels of dioxins,
and it is not possible to eliminate them completely from the
realm of human exposure.
3Reprinted from the Journal of Exposure Analysis
and Environmental Epidemiology, Vol. 12, Aylward, L.L. and
Hays, S.M., Temporal trends in human TCDD body burden: Decreases
over three decades and implications for exposure levels, 319-328,
Copyright 2002, with permission from Nature Publishing Group.
51 picogram = 0.000000000001g
pg-TEQ/kg-body weight per day is a measure of exposure; it
is the number of picograms of dioxins in TEQ units that are
exposed to each kilogram of a person's body weight per day.
6Reprinted from The Science of the Total Environment,
Vol. 288, Lorber, M., A pharmacokinetic model for estimating
exposure of Americans to dioxin-like compounds in the past,
present, and future, 81-95, Copyright 2002, with permission
from Elsevier Science.