Although actions have been taken to reduce industrial emissions of PCDDs to the environment, an "environmental reservoir" of past releases remain. Thus, PCDD are considered persistent organic pollutants (POPs). PCDDs persist for a long time in the environment because their structure is resistant to chemical or biological degradation. They are subject to slow breakdown by sunlight (photolysis) and can be destroyed by hydroxyl radicals in the atmosphere. Because dioxins break down slowly, past emissions remain in the environment for years—even decades—before they diminish. The environmental reservoir of PCDDs stored in soil and sediment can contaminate food crops.
Dioxins are produced commercially for toxicological and chemical research only. Their occurrence in the environment is largely a result of the deposition of products of incomplete combustion as well as the unintended result of uncontrolled chemical reactions involving chlorine in manufacturing. Before they were banned, a major source of dioxins was the manufacture and application of phenoxy herbicides, which could contain 2,3,7,8-TCDD at a parts-per-million (ppm) level. These dioxin-containing herbicides were widely applied to plants and soil to control weeds on cropland, along fence rows, ditch banks, farm roadways, pastures, range land, rights-of-way, roadways, fire lanes, and railroads. Dioxin also are a byproduct in some current manufacturing of chlorinated organic compounds, including some non-banned herbicides, with the majority of the contamination found in oily residues at the bottom of process vessels. In one well-publicized instance, these "still bottom" oils were inadvertently used for dust control on dirt roads and parking lots. In eastern Missouri, 29 sites were identified as being contaminated with oil used for dust control. OctaPCDD can be produced during the manufacture of chlorophenols.
The manufacture of white paper using the kraft process, which commonly used elemental chlorine to bleach wood pulp, was once a major source of dioxins in wastewater discharged to surface water bodies. A switch to elemental chlorine-free and totally chlorine-free alternatives in the process along with stricter effluent guidelines reduced the amount of dioxin in sludges and effluents. However, the legacy contamination from past manufacturing can be significant in sediment.
The biggest contributor of dioxins to the environment is incomplete combustion. Dioxins are emitted to the air as byproduct of waste incineration; the burning of coal, oil, and wood (including natural sources, like forest fires); combustion engines, smoking, etc. Air emissions from combustion sources can result in widespread deposition of dioxins to the land and surface water. Hence, dioxins are found around the world at low levels in rural soils, as well as in sediments of otherwise pristine water bodies. Most of the dioxin deposits from wet and dry deposition ultimately become components of runoff that enter rivers, streams, and estuaries directly or through urban stormwater outfalls.
While people are exposed to dioxins mostly from eating commonly consumed foods (including fish and dairy products), limited exposure may result from breathing air containing trace amounts of dioxins, inadvertently ingesting soil containing dioxins, and absorbing minute levels through the skin by touching contaminated soil.
Information regarding levels of contaminants in specific geographic locations or water bodies may be available in a monitoring or characterization report. The U.S. Geological Survey Library catalog can be searched to locate reports for specific locales.
Toxicological Profile Chlorinated Dibenzo-p-Dioxins
Agency for Toxic Substances and Disease Registry, 1998.
For Further Information
Gas Phase Thermal Oxidation of Endosulfan and Formation of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans (Abstract)
Environmental Science & Technology. 50, 18, 10106-10113, 2016.
Nirmala K. Dharmarathne, John C. Mackie, Eric M. Kennedy*, and Michael Stockenhuber
This paper investigates the thermal decomposition of technical endosulfan under oxidative conditions and the subsequent formation of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/F, dioxins), and their precursors. Both quantum chemical calculations and laboratory experiments were employed to investigate the pathways of oxidation of endosulfan.
The Changing Sources of Polychlorinated Dibenzo-p-dioxins and Furans in Sediments from the Lower Passaic River and Newark Bay, New Jersey, USA (Abstract)
Environmental Toxicology and Chemistry, 35(3):550-562.
Khairy, M., K. Barrett, and R. Lohmann. 2016.
Understanding Dioxin-Like Compounds in Indoor Dust
Protect Gainesville's Citizens Inc., 99 pp, 2014
Study funded by EPA's Environmental Justice Small Grant Program evaluated the concentrations and patterns of dioxins and dioxin-like compounds in indoor dust from homes near the former Koppers wood treating Superfund site in Gainesville, Florida.
The History, Use, Disposition and Environmental Fate of Agent Orange
Springer Science, New York. ISBN: 978-0-387-87485-2, DOI: 10.1007/978-0-387-87486-9, 2009 Young, A.L.
See table of contents and chapter abstracts at https://www.springer.com/gp/book/9780387874852
See review of this book at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2898884/
Dioxins Sources and Current Remediation Technologies: A Review (Abstract)
Environment International 34:139-153(2008)
Kulkarni, P.S., J.G. Crespo, and C.A.M. Afonso.
Dioxins and PCBs in Rural Areas
ES&T, Science News-January 17, 2007
Human Exposure to Dioxins from Clay: A Case Report
Alfred Franzblau, et al. National Institute of Environmental Health Sciences. 27 pp, 2007.
A study by University of Michigan researchers attributes high levels of dioxin in a woman's blood to exposure to off-venting of a ceramics kiln.
The History of the US Department of Defense Programs for the Testing, Evaluation, and Storage of Tactical Herbicides
Office of the Under Secretary of Defense, 83 pp, 2006
An Inventory of Sources and Environmental Releases of Dioxin-Like Compounds in the United States for the Years 1987, 1995, and 2000
U.S. EPA, National Center for Environmental Assessment. EPA 600-P-03-002A, 677 pp, 2006.
Human Exposure to Persistent Organic Pollutants, Illustrated by Four Case Studies in Europe
Jana Weiss, Ph.D. thesis, Stockholm University, Sweden. 89 pp, 2006.
Expert Workshop on Dioxin and Furan Releases from Uncontrolled Combustion Melbourne, Australia, 7-9 December 2005
UNEP Chemicals / Australian Government Department of the Environment and Heritage
Third National Report on Human Exposure to Environmental Chemicals, 2005
Department of Health and Human Services, Centers for Disease Control and Prevention. NCEH Pub. No. 05-0570, 475 pp, 2005.
The report provides information about levels of 29 specific dioxins, furans, and dioxin-related PCBs in the U.S. population based on serum samples tested from people who took part in CDC's National Health and Nutrition Examination Survey.
The Incidence and Severity of Sediment Contamination in Surface Waters of the United States, National Sediment Quality Survey: Second Edition
U.S. EPA, Office of Science and Technology, EPA-823-R-04-007, 280 pp, November 2004
Provides results of analyses of various food products tested for dioxin like compounds in the years 2000-2004.
Dioxins and Dioxin-Like Compounds in the Food Supply: Strategies to Decrease Exposure
National Research Council, National Academies Press, 340 pp, 2003.
Exposure and Human Health Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds, Part 1: Estimating Exposure to Dioxin-Like Compounds, Volumes 1-3 (Archived).
U.S. EPA, National Center for Environmental Assessment. EPA 600-P-00-001Cb, 2003
In October 2004, the EPA's draft dioxin reassessment was delivered to the National Academy of Sciences (NAS) for review. This document is a DRAFT FOR REVIEW PURPOSES ONLY and does not constitute U.S. EPA policy.
Research Priorities for Dioxins and Polychlorinated Biphenyls (PCBs): A Report to the Chemicals and GM Policy Division of the Department of the Environment, Food and Rural Affairs (Defra)
Kevin C. Jones and Andy J. Sweetman. Department for Environment, Food & Rural Affairs, London, UK. 80 pp, 2003.
Science Dossier: Dioxins and Furans in the Environment
Euro Chlor, 46 pp, 2003.
Guidance Document: Potential for Exposure to Polychlorinated Dibenzo-p-dioxins and Dibenzofurans when Recycling Sewage Biosolids on Agricultural Land
British Columbia Ministry of Water, Land, and Air Protection, 71 pp, 2002
Survey of Dioxin and Furan Compounds in Sediments of Florida Panhandle Bay Systems
Jon Hemming, Michael Brim, and Robert Jarvis.
U.S. Fish and Wildlife Service, Publication No. PCFO-EC 02-01, 92 pp, 2002.
The Atmospheric Transport and Deposition of Dioxin to the Great Lakes for 1996 (revised 2001)
NOAA, Air Resources Laboratory
User's Manual for the Database of Sources of Environmental Releases of Dioxin-Like Compounds in the United States
U.S. EPA, National Center for Environmental Assessment. EPA 600-R-01-012, 58 pp, 2001.
The site also provides access to the database via download or CD-ROM order.
Compilation of EU Dioxin Exposure and Health Data Summary Report
Deborah Buckley-Golder. et al.
European Commission DG Environment, UK department of the Environment Transport and the Regions, 629 pp, 1999
Polychlorinated Dibenzo-p-dioxins and Related Compounds Update: Impact on Fish Advisories
U.S. EPA, Office of Water. EPA 823-F-99-015, 6 pp, 1999.
Potential for Human Exposure
Toxicological Profile for Chlorinated Dibenzo-p-Dioxins, Chapter 5. Agency for Toxic Substances and Disease Registry, 129 pp, 1998.
Washington State Dioxin Source Assessment
Bill Yake, Stacie Singleton, and Karol Erickson
Washington State Department of Ecology, Publication No. 98-320, 102 pp, 1998.
Locating and Estimating Air Emissions from Sources of Dioxins and Furans
U.S. EPA, Office of Air and Radiation. EPA 454-R-97-003, 318 pp, 1997.
EPA's Dioxin Exposure Initiative was designed to fill critical data gaps regarding the sources of dioxin that contribute to human exposure. Illustrations on this website show that the cycling of dioxin through the environment is a complex process, involving multiple sources, flows, reservoirs, and sinks.