Trends in Dioxin Emissions and Exposure in the United States

Introduction

Dioxins are byproducts of many types of combustion, both natural and anthropogenic, including industrial, municipal and medical incineration; domestic fireplace and wood stove use; backyard barrel burning; and forest and brush fires.

There are several encouraging developments regarding the issue of dioxin1 emissions and exposure in the United States. U.S. Environmental Protection Agency (EPA) data (2006) show recent dramatic declines in dioxin emissions to the environment from quantified sources. In addition to an overall decline in dioxin emissions, the pattern of emitted dioxin chemical forms, or congeners, which have toxicities varying over several orders of magnitude, has changed. On average, current dioxin emissions are composed of a greater percentage of lower toxicity congeners than in the past.

The 3-T Rule: Combustion temperature, time and turbulence conditions are adjusted to minimize dioxin formation.

Total environmental releases of dioxins from all quantified sources decreased by 90% between 1987 and 2000 (see Figure 1). This significant decrease in dioxin emissions can be attributed to successful US government regulation as well as to the voluntary application of control technologies by industry. Emissions from the major historic source of dioxin for the past 30 years, incineration (especially of municipal solid waste and medical waste), have been continually declining since at least 1987. According to EPA data, backyard burning of rubbish2, currently a largely unregulated source, is the largest identifiable source of dioxin emissions. ("Regulated sources" refers to those sources which have controlled emissions of dioxins, either by virtue of regulations designed specifically for that purpose or by virtue of regulations targeting other pollutants, which have also reduced dioxin emissions.)

US Environmental Protection Agency Dioxin Emission Data Show Declining Levels and a Changing Pattern of Sources

Declining emissions levels of dioxin to the environment are characterized by a changing pattern of emission sources over time. EPA data demonstrate that industry and regulatory controls on incineration have resulted in significantly diminishing contributions of dioxin from this source since 1987 (see the blue portions of the stack graphs in Figure 1). The "3-T Rule" is an industry "rule of thumb" stating that combustion temperature, time and turbulence conditions may be optimized to minimize dioxin formation. Application of this rule has proven that technology works to reduce dioxin emissions. As emissions from industry have declined, largely unregulated sources such as backyard barrel burning of rubbish and residential wood burning have risen in significance as contributors to overall emissions.


*Includes incineration of municipal solid waste, sewage sludge, and hazardous waste.

**Other category includes: leaded and unleaded gasoline, land applied 2,4-D, iron ore sintering, oil-fired utilities, EDC/vinyl chloride, lightweight aggregate kilns that combust hazardous waste, petroleum refinery, catalyst regeneration, cigarette smoke, boilers/industrial furnaces, crematoria and drum reclamation.


1 "Dioxin" here is defined as the totality of 7 dioxins and 10 furans. "TEQ" denotes "toxic equivalent," a quantitative measure of the combined toxicity of a mixture of dioxin-like chemicals.
2 U.S. EPA (Environmental Protection Agency). 2006. An inventory of sources and environmental releases of dioxin-like compounds in the United States for the years 1987, 1995, and 2000. National Center for Environmental Assessment, Washington, DC; EPA/600/P-03/002F. (http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=159286)

Backyard Barrel Burning is Estimated to be a Major Source of Dioxin Emissions to the Environment

Dioxin emissions have decreased dramatically as a result of regulations and voluntarily implemented controls. Today, unregulated sources such as backyard barrel burning of rubbish constitute the major sources of dioxin emissions to the environment. It is important to note that estimates of dioxin emissions from private rubbish burning are subject to great uncertainty. Quantities and types of dioxin congeners formed during backyard burning depend upon the amount of rubbish burning that occurs over a region, the composition of the rubbish burned and the physical conditions of burning.

*"Dioxin" here is defined as the totality of 7 dioxins and 10 furans. "TEQ" denotes "toxic equivalent," a quantitative measure of the combined toxicity of a mixture of dioxin-like chemicals.
Source: U.S. EPA (Environmental Protection Agency). 2006. An inventory of sources and environmental releases of dioxin-like compounds in the United States for the years 1987, 1995, and 2000. National Center for Environmental Assessment, Washington, DC; EPA/600/P-03/002F. (http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=159286)

EPA dioxin emissions data, divided into categories of industrial/municipal/transportation and non- industrial/municipal/transportation sources, are depicted in Figure 2, above. (Non- industrial/municipal/transportation sources are backyard barrel burning of rubbish, residential wood burning and cigarette smoke.) Dioxin emissions from industrial, municipal and transportation sources have been greatly reduced by regulations targeting (a) dioxin specifically and (b) other pollutants, the regulation of which, coincidentally, has reduced dioxin emissions. Figure 2 demonstrates that, overall, industrial, municipal and transportation dioxin emissions have declined dramatically as a result of regulation, whereas emissions from backyard barrel burning of rubbish and residential wood burning have remained essentially constant since 1987, and as of 2000 have overtaken industrial/municipal/transportation sources as more significant emitters of dioxin.

Based on the EPA's inventory of dioxin sources, more stringent restrictions on the regulated community will not significantly reduce dioxin emissions. Instead, the mostly unregulated practice of backyard barrel burning of rubbish is a likely candidate for effecting further emissions reductions. In fact, Maine, New Hampshire, California, and other states have recently enacted legislation to control dioxin emissions from burn barrels.

A Changing Pattern of Congener Emissions Results in Changing Exposure

Overall environmental levels of dioxin have fallen over the past 30 years. In addition, the relative proportions of dioxin congeners emitted to the environment have changed such that greater proportions of dioxin congeners of lower toxicity are emitted. This pattern of decline has resulted in significant reductions in average human exposure to 2,3,7,8-TCDD, the dioxin congener of greatest toxicity and concern, and current exposures to dioxin are typically composed of smaller amounts of lower toxicity congeners.

Hays and Aylward (2001) examined mean serum lipid levels of 2,3,7,8-TCDD in 1,419 US Air Force Vietnam era veterans not occupationally exposed to Agent Orange. These researchers back-calculated the amount of 2,3,7,8-TCDD the veterans would have had to be exposed to in order to produce the measured serum lipid levels of this compound. Their model predicts that 2,3,7,8-TCDD intake must have declined to very low levels by 1992 in order to produce the mean measured serum lipid 2,3,7,8-TCDD decrease seen over the decade 1986-1996.

Thus, not only are overall environmental levels of dioxin declining through decreased emissions, but also, the total toxicity of the congeners to which humans are exposed are decreasing. As environmental levels of 2,3,7,8-TCDD have dropped, average body levels of this chemical also have decreased substantially. Indeed, Petreas et al. (2001) in a study of persistent halogenated contaminants, recently documented a decrease in Californians' dioxin body burdens which demonstrated statistically significant decreases in all but one major congener (2,3,4,7,8-PeCDF). The authors claim the decline is ".consistent with decreases observed worldwide and probably reflects successful measures of pollution control" (p. 878).

 

Summary

In conclusion, the application of control technologies, especially to incineration processes in the US, has led to significant decreases in dioxin emissions to the environment. Furthermore, remaining emissions are characterized by a less toxic array of dioxin congeners. Given the documented substantial declines in serum 2,3,7,8-TCDD for the decade preceding 1996, and the application of control technologies by industry, it is likely that 2,3,7,8-TCDD body levels have continued to decrease, reflecting further declines in exposure to this chemical.

Unregulated domestic burning has supplanted incineration as a major source of dioxins and may become increasingly regulated. Although natural combustion processes, such as forest fires, ensure the enduring presence of a baseline of dioxins in the environment which can never be eliminated, the use of technology to minimize anthropogenic dioxin emissions is a documented success story.

References

Hays, S.M. and Aylward, L.L. (2001). Temporal trends in body-burden suggest that dioxin exposure in the general population have declined significantly. Organohalogen Compounds, 52, 214.

Petreas, M., She, J., Visita, P., Winkler, J., McKinney, M., Brown, F.R., Dhaliwal, J., Denison, G., and Mok, M. (2201). Trends in persistent contaminants in California biota, Symposia Papers Presented Before the Division of Environmental Chemistry, American Chemical Society, Sand Diego, CA, April 1-5, 2001.

U.S. EPA (Environmental Protection Agency). 2006. An inventory of sources and environmental releases of dioxin-like compounds in the United States for the years 1987, 1995, and 2000. National Center for Environmental Assessment, Washington, DC; EPA/600/P-03/002F. (http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=159286)

U.S. EPA (September, 2000 Draft). Exposure and Human Health Reassessment of 2.3.7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. Part III: Integrated Summary and Risk Characterization for 2.3.7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and Related Compounds.

World Health Organization, (June, 1999). "Dioxins and their effects on human health". [On-Line]. Available: http://www.who.int/inf-fs/en/fact225.html.

World Health Organization (1998). Executive Summary. Assessment of the health risk of dioxins: re-evaluation of the tolerable daily intake (TDI). Available: http://www.who.int/pcs/pubs/dioxin-exec-sum/exe-sum-final.html.

Notes

1"Dioxin" here is defined as the totality of 7 dioxins and 10 furans (of a much larger family of similar but less toxic compounds) of environmental concern.
2"Backyard barrel burning of rubbish" is the mostly rural practice of burning domestic household and garden refuse in open air, often in steel barrels.

 

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