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:   Time Period Daily Exposure (pg-TEQ/kg-body weight) Early 1900s 0.5 Mid 1900s 6 Late 1900s 0.5

Going Forward

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.

References:

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.
⁴Ibid.
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.