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Common Questions Asked About Dioxins

What are Dioxins and Furans?

The term "dioxin" is commonly used to describe a family of chemical compounds containing 75 dioxins and 135 furans. A dioxin consists of two benzene rings connected by two oxygen atoms. A furan consists of two benzene rings connected by one oxygen atom. Often chlorine molecules are attached at different positions on the dioxin compound. The number and position of these chlorine atoms are very important in the toxicity of dioxins. The 75 dioxin compounds differ among themselves only by the location and number of chlorine atoms attached to the molecule.

Dioxin does not dissolve readily in water, it has a high melting and boiling point and only slowly evaporates. e.g., from surfaces of soil, water, or plants. Dioxin binds tightly with other organic compounds and thus, can readily accumulate in soils and sediments. In humans and animals, dioxins are stored in fat tissue, slowly metabolized, and eventually eliminated from the body. Studies of dioxin levels in humans suggest that an amount of dioxin stored in fat tissue decreases by one-half every seven years (Gough 1993).

What is Known about the Toxicity of Dioxin?

There are two key factors in determining the toxicity of dioxin compounds: the number of chlorine atoms present and the position of those atoms. Dioxin compounds with four chlorine atoms in the 2,3,7,8 position of the dioxin molecule exhibit the greatest toxicity. Dioxin compounds with less than four chlorine atoms are not toxic (EPA, 1994). Dioxin compounds that do not have chlorine atoms in the 2,3,7,8 positions are also not toxic (EPA, 1994), no matter how many chlorine atoms are present. In addition, compounds with more than four chlorine atoms exhibit less toxicity when compared to 2,3,7,8-TCDD (see Table below). The scientific community has developed and accepted a scheme for comparing the toxicity of different dioxin compounds, referred to as a toxicity equivalency factor (TEF). Some of the TEFs (EPA 1998a) are:

Compound TEF
2,3,7,8-TCDD 1
1,2,3,7,8-PeCDD 0.5
1,2,3,4,7,8-HxCDD 0.1
1,2,3,6,7,8-HxCDD 0.1
1,2,3,7,8,9-HxCDD 0.1
1,2,3,4,6,7,8-HpCDD 0.01
OCDD 0.001

A similar scheme is developed for furans, but with lower TEFs. Thus, as the number of chlorine atoms present on the molecule increases above four, the toxicity of the dioxin/furan decreases. Most of the information about dioxin toxicity is derived from studies of the 2,3,7,8-TCDD and simple screening tests for the others.

What is TEQ and how is it Calculated?

TEQ is toxic equivalents. It is a method developed by scientists and regulators to estimate the toxicity of all dioxins and furans relative to 2,3,7,8-TCDD. TEQs are calculated by multiplying the amount of a specific dioxin times its TEF. Thus, one gram of 2,3,7,8-TCDD is also one gram TEQ dioxin (because it has a TEF of 1). However, 1 gram of 2,3,5,7,8-PCDD has a TEQ of 0.5 (one gram times 0.5 TEF). The two could be combined into 2 grams of dioxin or 1.5 grams TEQ of dioxin. Both numbers are correct. The two grams accurately describes the total amount of dioxin present but does not give any information on the total toxicity of those two grams. The 1.5 grams more accurately represents the toxicity but does not give any indication on the total amount of dioxin present.

What are the Major Sources of Dioxin?

Dioxin is not intentionally produced but is a by-product of many industrial activities and natural processes. Dioxin can be produced naturally during forest fires and volcanic eruptions, through chemical and photochemical reactions occurring in air and water, and during enzymatic reactions in natural organisms. Anthropogenic sources comprise a broad range of industrial and residential activities, including motor vehicle use, industrial processes, incineration of waste material, and burning of wood and coal.

EPA initially developed a list of sources of dioxins in 1994 (EPA, 1994). The source list was updated in 1998 (EPA, 1998a) and in 2006 (EPA, 2006). The 2006 document revised the previous estimates of dioxin emissions to the air and added estimates for 2000. The estimated emissions from the largest sources are summarized in the following table.

Source Annual dioxin emissions - g TEQ/year
Backyard barrel burning 572595473
Medical waste incineration 2440459357
Municipal solid waste incineration 7860110176
Coal combustion - utility boilers 516170
Diesel fuel combustion - on-road 263262
Wood combustion - industrial 252539
Diesel fuel combustion - off-road 182231
Ferrus metal smelting/refining 292524
Cement kilns burning hazardous waste 11014517
Cement kilns not burning haz waste 121617
Oil combustion - industrial/utility 312315
Wood combustion - residential 221611
Sewage sludge incineration 6149
Secondary aluminum smelting 10188
Vehicle on-road gasoline combustion 347
Hazardosu waste incineration 563
Secondary copper smelting 9662661
Total 12,2302,8571244

As can be seen from this list, there have been significant changes in the total amounts of emissions as well as significant changes in many individual source categories. For example, secondary copper smelting was one of the highest emitters of dioxin in 1987 but one of the smallest in 2000. This source category virtually eliminated their dioxin emissions. Industry has done such a good job at reducing dioxin emissions that the largest single source for dioxin emissions is now backyard barrel burning, essentially an uncontrolled source. Most of this is the result of EPA regulations that set limits on the amount of dioxin emissions from each source category. Hazard waste incineration contributes about 0.2% of the annual estimated dioxins emissions to the atmosphere.

What are the Dioxin Emissions Standards for Hazardous Waste Incinerators?

The emission standards for hazardous waste incinerators can be found in the Code of Federal Regulations, specifically 40 CFR 63.1219. The dioxin standards are in paragraph (a)(1). There are different dioxin standards depending upon whether the unit has a waste heat boiler and/or the type of air pollution control devices used to clean up the gas stream. For incinerators equipted with a waste heat boiler or a dry air pollution control system, their standards are either 0.20 ng TEQ/dscm or 0.40 ng TEQ/dscm providing the air temperature at the inlet to the air pollution control equipment is below 400 degrees F. For incinerators not equipted with a wet air pollution control system, the standard is 0.40 ng TEQ/dscm.

What is a ng/dscm?

The first part - ng - is nanogram. A nanogram is one billionth of a gram. Said another way, one nanogram is 0.000000001 gram or 0.000000000002 pounds - a very small number. The dscm part stands for dry standard cubic meters. This is a volume of one cubic meter corrected for moisture and temperature. Taken together, they represent the mass of dioxins found in a specific volume of air. So how does this relate to something more understandable. Here are a three examples that might help put this number in perspective.

  • Each human cell has a mass of approximately 1 ng. Measuring 0.40 ng/dscm is about the same as trying to find a half of a human cell in a box that is three feet high, three feet wide, and three feet deep.
  • The popululation of China and India is approximately 2.5 billion people. Trying to find one persion in those two countries is approximately equivalent to 0.4 ng/dscm.
  • There are approximately 2.3 billion acres of land in the United States. Finding one specific acre in the United States is approximately equivalent to 0.40 ng/dscm.
Needless to say, one nanogram per dry standard cubic meter is a very small concentration.

How do Dioxins get into the Emissions from Combustors?

There are only a few combustion devices in the United States that are allowed to burn dioxin-contaminated waste. To obtain a license to burn dioxin-contaminated waste, a facility must pass a series of rigid tests that show that 99.9999% of the dioxins are destroyed in the process. Since there are only a few facilities that have this license and the vast majority of the dioxin is destroyed, most of the dioxin found in the emissions of waste combustors is newly formed (de novo synthesis) in the post-combustion air cleaning process.

A considerable amount of scientific research has gone into determining the conditions where de novo synthesis of dioxins occurs. If the following conditions are met, dioxins can be formed:

  1. temperatures between 400 and 750 °F;
  2. residence time (time in the air pollution control device) greater than 2 seconds;
  3. presence of chlorine;
  4. the presence of carbon molecules;
  5. the presence of a catalytic surface; and
  6. the absence of sulfur.
Remove any one of these factors, and dioxin formation in the air pollution control device is much slower. The presence of sulfur will effectively kill any reactions to make dioxins. Combustors that have dioxin formation problems can often change their air pollution control operating parameters and eliminate the problem. Where this is not possible, the use of some form of activated carbon will efficiently remove most dioxins formed during the combustion process. There is a good discussion of the mechanisms of dioxin synthesis in Chapter 4 of EPA's guidance document on collecting data to support a site-specific risk assessment (EPA, 1998b).

Where can I Find Additional Information on Dioxins?

There are two good places where additional information on dioxins can be found. The first is EPA's dioxin website ( The second is the Chlorine Chemistry council's website ( Both will give detailed information on dioxins.


Gough, M. 1993. Dioxin: Perception, Estimates, and Measures. Phantom Risk: Scientific Inference and the Law Eds. K.R. Foster. D.E. Bernstein and P W. Hubber. Cambridge, MA: MIT Press.

U. S. Environmental Protection Agency (EPA). 1994. Estimating Exposure to Dioxin-Like Compounds, Volume 1: Executive Summary. EPA/600/6-88/005Ca. June 1994.

EPA. 1998a. The Inventory of Sources of Dioxin in the United States. EPA/600/P-98/002Aa. April 1998.

EPA. 1998b. Guidance on Collection of Emissions Data to Support Site-Specific Risk Assessments at Hazardous Waste Combustion Facilities. EPA530-D-98-002. August 1998.

EPA. 2006. An Inventory of Sources and Environmental Releases of Dioxin-Like Compounds in the United States for the Years 1987, 1995, and 2000. EPA/600/P-03/002F, November 2006.

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