CRWI's Mission Statement
Why be a Member?
CRWI Positions on Selected Rules
Return to Main Page
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)
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.
||Annual dioxin emissions - g TEQ/year
|Backyard barrel burning
|Medical waste incineration
|Municipal solid waste incineration
|Coal combustion - utility boilers
|Diesel fuel combustion - on-road
|Wood combustion - industrial
|Diesel fuel combustion - off-road
|Ferrus metal smelting/refining
|Cement kilns burning hazardous waste
|Cement kilns not burning haz waste
|Oil combustion - industrial/utility
|Wood combustion - residential
|Sewage sludge incineration
|Secondary aluminum smelting
|Vehicle on-road gasoline combustion
|Hazardosu waste incineration
|Secondary copper smelting
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.
Needless to say, one nanogram per dry standard cubic meter is a very small concentration.
- 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.
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:
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?
- temperatures between 400 and 750
- residence time (time in the air pollution control device)
greater than 2 seconds;
- presence of chlorine;
- the presence of carbon molecules;
- the presence of a catalytic surface; and
- the absence of sulfur.
There are two good places where additional information on dioxins can be found. The first is EPA's dioxin website (www.epa.gov/dioxins). The second is the Chlorine Chemistry council's website (www.dioxinfacts.org). 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.
Return to Main Page