Toxicology and Industrial Health
|
VOL. 13, No. 6, pp. 759-768, 1997 |
Dioxin and Dioxin-Like Compounds in Soil, Part 1:
ATSDR Interim Policy Guideline
Christopher T. De Rosa, David Brown, Rosaline Dhara, Woodrow Garrett, Hugh Hansen,
James Holler, Dennis Jones, Denise Jordan-Izaguirre, Ralph O'Connor, Hana Pohl, and
Charles Xintaras
Agency for Toxic Substances and Disease Registry,
U.S. Department of Health and Human Services
Atlanta, Georgia
Approved:
_______/s/_______
Barry L. Johnson
Assistant Administrator
Effective Date:
August 21, 1997
Note: While the original publication dates on some ATSDR's documents may not appear to be current, the information in the document is valid and may still provide relevant information.
PURPOSE
The Agency for Toxic Substances and Disease Registry (ATSDR) has adopted this interim
policy guideline to assess the public health implications of dioxin and dioxin-like compounds
in residential soils near or on hazardous waste sites. These compounds include
- 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)
- Chlorinated dibenzofurans (CDFs)
- Other structurally related groups of chemicals from the family of halogenated aromatic
hydrocarbons.
These substances are defined under the Comprehensive Environmental Response,
Compensation, and Liability Act of 1980 (CERCLA), as amended, commonly known as
Superfund. This interim policy guideline will provide a clear and consistent understanding of
ATSDR's current approaches and judgments regarding hazards posed by the presence of TCDD
and its less toxic dioxin-like congeners, the CDDs and CDFs, in residential soils. Likely users
of this interim policy guideline include
- ATSDR and state-based health assessors
- ATSDR partners including relevant federal, state, and local health and environmental entities
- Concerned community groups.
INTERIM POLICY GUIDELINE
This interim policy guideline is based on a current understanding of the toxicology and
epidemiology associated with TCDD and its congeners (see "Background" section) and on exposure potential when soil is the primary medium of interest.
This guidance is consistent with the Technical Support Document for ATSDR Interim Policy
Guideline: Dioxin and Dioxin-Like Compounds in Soil (ATSDR 1997) and with the ATSDR
Public Health Assessment Guidance Manual (ATSDR 1992). They explain how to use
comparison values to select contaminants for further evaluation and then draw conclusions
about the public health implications of the contaminants. Assessments of public health
implications are based on considerations of site-specific factors affecting the extent and
characteristics of exposure and on the toxicology and epidemiology of the compounds selected
for evaluation.
This guidance for dioxin and dioxin-like compounds is unique because of the potency of TCDD
itself, and the need to consider the total potency of all dioxin and dioxin-like compounds
detected in soil. The toxicity of a dioxin-like compound is commonly referred to in terms of
its dioxin toxicity equivalency factor (TEF). See "Background" section for further information.
These guidelines and procedures apply to human exposure for direct ingestion of soils
contaminated with dioxin and dioxin-like compounds in residential areas and may not be
appropriate for other exposure scenarios. The guidance will be evaluated in view of new data
that may become available. The science basis for the guidance is outlined in the "Background"
discussion.
Step 1. Screening for contaminants of concern
Review soil sampling data and compare levels against dioxin comparison values (environmental
media evaluation guide or EMEG for children) that are not site-specific. If one or more soil
sampling values exceed the screening value of 50 parts per trillion (ppt) of toxicity equivalents
(TEQs), further site-specific evaluations are needed as described next and in Table 1.
If samples exceed this screening value, then ATSDR generally assumes that further evaluation
is required. However, even if samples are below these values, ATSDR policy states that it may
still be necessary to conduct a more detailed site-specific evaluation under the following
conditions:
- community health concerns
- health assessor's concerns about other combinations of contaminants.
Step 2. Evaluating potential exposure pathways
Further evaluation includes the most critical aspect of health hazard evaluations, that is, the
determination of likelihood, extent, and duration of exposure of populations. Thus, the health
assessor uses the following to determine the existence of a potential or completed exposure
pathway--past, present, or future:
- site visits and observations
- detailed review of data packages for land use scenarios, contaminant locations, and site
locations
- evaluation of receptor populations and potential points of contact.
If a completed or potentially completed exposure pathway is identified, then the extent of
exposure and public health implications are further evaluated.
Site-specific exposure scenarios based on site-specific factors are evaluated in conjunction with
relevant toxicologic, epidemiologic, and medical information. This involves assessing site-specific information about the likelihood, frequency, routes, and levels of exposure to
contaminants, and the populations that are likely to be exposed.
Where estimated levels of exposure in soil fall in the range of greater than 50 ppt to less than
1 part per billion (ppb) TEQs (Table 1), a weight-of-evidence approach is recommended to
evaluate the exposure and the public health implications of the exposure.
Health assessors must ask the following questions:
- How extensive is the contamination?
- Is the contamination isolated or widespread?
- Is the contamination in surface soils or areas easily accessible to children or adults? Is
it in areas with no vegetation or in any other areas?
- At this site, how often (daily, weekly, monthly) and for what length of time (months,
years, lifetimes) would exposures be likely to occur?
Many of these estimates depend on professional judgment and experience regarding the
likelihood of exposures from soils in different kinds of sites. For further information on the
evaluation process see ATSDR (1992).
Interpretation of health guidance values. The policy incorporates information on exposure
potential from residential soils and residential exposure scenarios. It should be noted that
the levels (in TEQs) < 50 ppt (0.05 ppb), > 0.05 ppb but < 1 ppb and >1 ppb in residential
soils are guidance values and should not be construed to indicate that actual health effects
will occur. The policy provides a protective framework for evaluating the health
implications of exposures to dioxin and dioxin-like compounds in residential soils on a site-specific basis. |
Table 1. ATSDR's decision framework for sites contaminated with dioxin and dioxin-like compounds
Because the toxicity of dioxin and dioxin-like compounds is assumed to be elaborated through a
common receptor-mediated mechanism, levels greater than 50 ppt TEQs* are used to determine
whether further site-specific evaluation for dioxins is to occur based on the maximum soil
concentrations identified at the site. A level of 1 ppb TEQs is used to determine the potential need
for public health actions on a site-specific basis and on the basis of adequate sampling and measured
or projected human exposure--past, present, or future--as determined by the health assessor.
SCREENING LEVEL |
EVALUATION LEVELS |
ACTION LEVEL** |
< 50 ppt (0.05 ppb) TEQs |
> 0.05 ppb but < 1 ppb TEQs |
>1 ppb TEQs |
- The EMEG for
TCDD is 50 ppt
- This is based on
an MRL of
1 pg/kg/day for
TCDD (ATSDR
1989).
- For screening
purposes 50 ppt
TCDD is assumed
to be equivalent
to 50 ppt TEQs
|
Evaluation of site-specific factors, such as
- Bioavailability
- Ingestion rates
- Pathway analysis
- Soil cover
- Climate
- Other contaminants
- Community concerns
- Demographics
- Background
Exposures
|
Potential public health
actions considered, such as
- Surveillance
- Research
- Health studies
- Community education
- Exposure investigations
|
* The toxicity equivalent (TEQ) of TCDD is calculated by multiplying the exposure level of a particular dioxin-like compound by its toxicity equivalency factor (TEF). TEFs are based on congener-specific data and the assumption that Ah receptor-mediated toxicity of dioxin-like chemicals is additive. The TEF scheme compares the relative toxicity of individual dioxin-like compounds to that of TCDD, which is the most toxic halogenated aromatic hydrocarbon.
** A concentration of chemicals at which consideration of action to interdict/prevent exposure occurs, such as surveillance, research, health studies, community education, physician education, or exposure investigations. Alternatively, based on the evaluation by the health assessor, none of these actions may be necessary.
Step 3. Defining public health implications/actions
Where exposures to concentrations in residential soils exceeding 1 ppb TEQs are significant,
ATSDR health assessors should consider judging the site a public health hazard and consider site-specific public health recommendations/actions to prevent or interdict exposures (Table 1).
BACKGROUND FOR INTERIM POLICY GUIDELINE
Dioxin and dioxin-like compounds. Dioxin and dioxin-like compounds are structurally related groups of chemicals from the family of halogenated aromatic hydrocarbons. Depending on the number of chlorine-substituted positions, there are several congeners in each group. The most toxic and the most studied congener is TCDD.
Toxicity equivalency factors (TEFs) were developed to compare the relative toxicity of individual dioxin-like compounds to that of TCDD (Tables 2 and 3). This comparison is based on the assumption that dioxin and dioxin-like compounds act through the same mechanism of action. The TEF for TCDD is defined as one, whereas TEF values for all other dioxin-like compounds are less than one. Toxicity equivalents (TEQs) are used to assess the risk of exposure to a mixture of dioxin-like compounds. A TEQ is defined as the product of the concentration, Ci, of an individual "dioxin-like compound" in a complex environmental mixture and the corresponding TCDD toxicity equivalency factor (TEFi) for that compound. The total TEQs is the sum of the TEQs for each of the congeners in a given mixture:
n
Total TEQs = Sum (CI x TEFi)
i=1
Adverse health effects. Studies in animals demonstrated a wide range of effects associated with dioxin exposure including death, cancer, and wasting, as well as hepatic, immunologic, neurologic, reproductive, and developmental effects. In contrast to laboratory results, direct exposure information is not available in human studies; therefore, body burden is used as a surrogate. Body burdens in some animal studies were in the same range as those associated with adverse health effects in human studies. For more information, see Technical Support Document for ATSDR Interim Policy Guideline: Dioxin and Dioxin-Like Compounds in Soil (ATSDR 1997). These results underscore the need for research to elucidate the toxicity at low doses to human populations and to evaluate exposures in at-risk populations (see Appendix 1) in view of total body burdens of dioxin and dioxin-like compounds.
Screening level for dioxin and dioxin-like compounds in soil. While identifying levels of potential concern to human health, ATSDR considers a spectrum of contaminant concentrations. In general, screening levels are concentrations used to select contaminants of concern at hazardous waste sites that are taken forward in the health assessment process for further evaluation (screening levels are also called comparison values; see Appendix 1 - Glossary).
Table 2. Recommended Toxicity Equivalency Factors (TEFs) for CDDs and CDFs
CDDs |
EPA current recommended values |
|
CDFs |
EPA current recommended values |
monoCDDs |
0 |
|
monoCDFs |
0 |
diCDDs |
0 |
|
diCDFs |
0 |
triCDDs |
0 |
|
triCDFs |
0 |
2,3,7,8-TCDD
other tetraCDDs |
1
0 |
|
2,3,7,8-tetraCDF
other tetraCDFs |
0.1
0 |
2,3,7,8-pentaCDDa
other pentaCDDs |
0.5 0 |
|
1,2,3,7,8-pentaCDF
2,3,4,7,8-pentaCDF
other pentaCDFs |
0.05
0.5
0 |
2,3,7,8-hexaCDDa
other hexaCDDs |
0.1
0 |
|
2,3,7,8-hexaCDFa
other hexaCDFs |
0.1
0 |
2,3,7,8-heptaCDD,aother heptaCDDs |
0.01
0 |
|
2,3,7,8-heptaCDFa
other heptaCDFs |
0.01
0 |
octaCDD |
0.001 |
|
octaCDF |
0.001 |
a Any isomer that contains chlorine in the 2,3,7,8-positions
Source: derived from EPA 1989.
CDDs = chlorinated dibenzo-p-dioxins; CDFs = chlorinated dibenzofurans;
TCDD = tetrachlorodibenzo-p-dioxin
Table 3. Recommended Toxicity Equivalency Factors (TEFs) for Dioxin-Like PCBs
PCB |
WHO proposed interim values a |
|
PCB |
WHO proposed interim values a |
3,3',4,4'-TCB |
0.0005 |
|
2,3,3'4,4',5-HxCB |
0.0005 |
3,3',4,4',5-PeCB |
0.1 |
|
2,2,3',4,4',5'-HxCB |
0.0005 |
3,3',4,4',5,5'-HxCB |
0.01 |
|
2,3',4,4',5,5'-HxCB |
0.00001 |
2,3,3',4,4'-PeCB |
0.0001 |
|
2,3,3',4,4',5,5'-HpCB |
0.0001 |
2,3,4,4',5-PeCB |
0.0005 |
|
2,2',3,3',4,4',5-HpCB |
0.0001 |
2,3',4,4',5-PeCB |
0.0001 |
|
2,2',3,4,4',5,5'-HpCB |
0.00001 |
2',3,4,4',4-PeCB |
0.0001 |
|
- |
- |
a Interim values proposed by World Health Organization/International Programme on
Chemical Safety
Source: derived from Ahlborg et al. 1994
PCB = polychlorinated biphenyl; TCB = tetrachlorinated biphenyl; PeCB = pentachlorinated biphenyl;
HxCB = hexachlorinated biphenyl; HpCB = heptachlorinated biphenyl
A minimal risk level (MRL) is an estimate of the daily human exposure to a hazardous substance
that is likely to be without appreciable risk of adverse noncancer health effects over a specified
duration and route of exposure. These substance-specific estimates, which are intended to serve as
screening levels, are used by ATSDR health assessors and others to identify contaminants and
potential health effects that may be of concern at hazardous waste sites. The intermediate-duration
oral MRL of 1 picogram/kilogram/day or pg/kg/day for TCDD (ATSDR 1989) was based on
reproductive effects in rats. The intermediate-duration oral MRL was also adopted as a chronic oral
MRL. Based on this value, an EMEG of 50 ppt (0.05 ppb) TCDD, which is equivalent to 50 ppt
(0.05 ppb) TEQs, was derived for exposure from contaminated soil. Uncertainty factors of 1000
(total) were used in the calculations of the MRL (for further details, see Appendix 3 of the
Technical Support Document). Based on a review of more recent literature, ATSDR scientists
conclude that the MRL of 1 pg/kg/day is approximately two orders of magnitude below the
noncancer health effect levels observed in recent studies. This is also true for cancer effect levels.
Evaluation levels for dioxin and dioxin-like compounds in soil. Evaluation levels are
concentrations > 50 ppt (0.05 ppb) but < 1 ppb TEQs at which site-specific factors, including, but not
limited to, bioavailability, ingestion rates, pathway analysis, soil cover, climate, other
contaminants, community concerns, demographics, and background exposure, are considered in
a deliberative process to assess the nature and extent of contamination and its impact on the
community. Such an evaluation process may prompt further assessment at the next level where
actions are considered. The evaluation levels are to be used as a framework to guide procedures
for that judgment process. Thus, judgments in the evaluative phase are linked to actions where
consideration is given to interventions from a public health perspective.
Action level for dioxin and dioxin-like compounds in soil. Action levels are concentrations of
chemicals at which consideration of action to interdict exposure occurs; 1 ppb TCDD in residential
soil was identified by Kimbrough et al. (1984) as a "level of concern," and recommended as "a
reasonable level to begin consideration of action to limit exposure." Kimbrough et al.'s (1984)
conclusions were derived in part from an evaluation of the carcinogenic potential of TCDD, based
on a 2-year oral chronic toxicity and oncogenicity study in rats (Kociba et al. 1978). With the
advancement of knowledge about dioxin-like chemicals and their assumed common mechanism of
toxicity, the TEQs were introduced into the risk assessment process. Since then, 1 ppb of total
dioxins (expressed as TEQs) in soil has been used as an action level by ATSDR.
The Kociba et al. (1978) study also served as the basis for the Food and Drug Administration's
(FDA's) derivation of a risk-specific dose of 0.057 pg/kg/day dioxin for a l in a million (10-6)
upper-bound risk estimate for cancer (FDA 1990). Using a typical default value of 70 kg for
average body weight and 100 milligrams/day (mg/day) for soil consumption, FDA's 0.057
pg/kg/day risk-specific dose corresponds to a soil concentration of 40 ppt. This value is marginally
lower, but from a risk assessment perspective, it is essentially equivalent to the ATSDR media-specific screening level/comparison value (EMEG) of 50 ppt.
As noted previously, ATSDR's EMEG is based on the MRL of 1 pg/kg/day TCDD, which is
approximately two orders of magnitude below any health effect levels demonstrated either
experimentally or in epidemiologic studies for both cancer and noncancer health end points. The
conservative (i.e., protective) nature of both the MRL and the EMEG reflects adjustments made
for recognized areas of uncertainty, perhaps spanning two to three orders of magnitude. As such,
the EMEG and the MRL, on which the EMEG is based, are below levels of exposures associated
with demonstrated health effects and are therefore considered to be protective of human health.
The EMEG of 50 ppt (0.05 ppb) is at the low end of the range reflecting currently recognized areas
of scientific uncertainty; this range is 50-50,000 ppt (or 0.05-50 ppb), which is based on the
1,000-fold uncertainty factor used to derive the MRL.
Conclusions. ATSDR concludes that the action level of 1 ppb (TEQ) for dioxin and dioxin-like
compounds, when coupled to a site-specific context of evaluation for the range > 50 ppt (0.05 ppb)
to < 1 ppb TEQs in residential soil, is protective of public health and continues to represent a level
at which consideration of health action to interdict exposure, including cleanup, should occur. This
conclusion is based on ATSDR's review and evaluation of
- more recent experimental and epidemiologic research findings
- ATSDR's historical use of the term "action level"
- the range of health guidance values developed by ATSDR including the MRL and EMEG
- the limitations and uncertainties of ATSDR's health guidance values and the scientific data
- on which these values are based.
ATSDR considers this action level to be both reasonable and protective for the following reasons:
- ATSDR's MRL is approximately two orders of magnitude below effect levels in
- experimental and epidemiologic studies.
- Cancer risk-specific doses and screening values for end points other than cancer are
- essentially equivalent from a risk assessment perspective.
WHERE TO FIND MORE INFORMATION
For more information on the historical and scientific background of dioxin in soil values, their
proper use, and data on limitations associated with these numbers, please refer to Dioxin and Dioxin-Like
Compounds in Soil, Part II: Technical Support Document for ATSDR Interim Policy Guideline
(De Rosa et al., Toxicology and Industrial Health VOL. 13, No. 6, pp. 769-804, 1997)
Alternatively, this and related material are available through the ATSDR Information Center by
calling 1-800-447-1544 or by Faxing 1-404-639-6359. You can also e-mail the ATSDR Information Center directly.
REFERENCES
Ahlborg UG, Becking GC, Birnbaum LS, et al. 1994. Toxic equivalency factors for dioxin-like
PCBs. Chemosphere 28(6): 1049-67.
ATSDR (Agency for Toxic Substances and Disease Registry). 1989. Toxicological
profile for 2,3,7,8-tetrachlorodibenzo-p-dioxin. Atlanta, GA: US Department
of Health and Human Services, Public Health Service, Agency for Toxic Substances
and Disease Registry.
ATSDR (Agency for Toxic Substances and Disease Registry). 1992. Public health
assessment guidance manual. Atlanta, GA: US Department of Health and Human Services,
Public Health Service, Agency for Toxic Substances and Disease Registry. NTIS
PB 92-147164.
ATSDR (Agency for Toxic Substances and Disease Registry). 1997. Technical
support document for ATSDR interim policy guideline: dioxin and dioxin-like
compounds in soil. Atlanta, GA: US Department of Health and Human Services,
Public Health Service, Agency for Toxic Substances and Disease Registry.
EPA (US Environmental Protection Agency). 1989. Interim procedures for estimating
risks associated with exposure to mixtures of chlorinated dibenzo-p-dioxins
and dibenzofurans (CDDs and CDFs) and 1989 update. Risk Assessment Forum. Washington,
DC: US Environmental Protection Agency. EPA 625/3-89/016. NTIS PB90-145756.
FDA (US Food and Drug Administration). 1990. Carcinogenic risk assessment
for dioxins and furans in fish contaminated by bleached-paper mills. Report
of the Quantitative Risk Assessment Committee. Washington, DC: Food and Drug
Administration.
Kimbrough RD, Falk H, Stehr P, et al. 1984. Health implications of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) contamination of residential soil. J Toxicol Environ Health 14:47-93.
Kociba RJ, Keyes DG, Beyer JE, et al. 1978. Results of a two-year chronic toxicity and
oncogenicity study of 2,3,7,8-TCDD in rats. Toxicol Appl Pharmacol 46:281-287.
APPENDIX 1 - GLOSSARY
- Action level
- A concentration of chemicals at which consideration of action to interdict/prevent exposure occurs, such as surveillance, research, health studies, community education, physician education, or exposure investigations. Alternatively, based on the evaluation by the health assessor, none of these actions may be necessary.
- "At-risk" population
- A population at a potentially elevated risk due to physiological sensitivity and/or increased exposure to a hazardous chemical.
- BDDs
- Brominated dibenzo-p-dioxins
- BDFs
- Brominated dibenzofurans
- CDDs
- Chlorinated dibenzo-p-dioxins
- CDFs
- Chlorinated dibenzofurans
- Comparison value
- A concentration used to select contaminants of concern at hazardous waste sites that are taken forward in the health assessment process for further evaluation (The terms comparison value and screening level are often used synonymously.)
- Dioxin
- A term used interchangeably with 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD
- Dioxin-like compounds
- Compounds from a group of halogenated aromatic hydrocarbons that have molecules shaped like TCDD and produce similar toxic effects, such as certain other chlorinated dibenzo-p-dioxins (CDDs) and certain chlorinated dibenzofurans (CDFs), polychlorinated biphenyls (PCBs), polybrominated biphenyls (PBBs), brominated dibenzo-p-dioxins (BDDs), and brominated dibenzofurans (BDFs).
- Dioxins
- A term used interchangeably with chlorinated dibenzo-p-dioxins
- EMEG
- An environmental media evaluation guide (EMEG) is a media-specific comparison value that is used to select contaminants of concern at hazardous waste sites.
- HazDat
- ATSDR's Hazardous Substance Release/Health Effects Database
- MRL
- A minimal risk level (MRL) is an estimate of the daily human exposure to a hazardous substance that is likely to be without an appreciable risk of adverse noncancer health effects over a specified route and duration of exposure.
- PBBs
- Polybrominated biphenyls
- PCBs
- Polychlorinated biphenyls
- Screening
- The process of initially identifying potentially important chemical contaminants and exposure pathways by eliminating those of known lesser significance.
- TCDD
- 2,3,7,8-Tetrachlorodibenzo-p-dioxin
- TEFs
- Toxicity equivalency factors (TEFs) are based on congener-specific data and the assumption that the toxicity of dioxin and dioxin-like compounds is mediated by the Ah receptor and is additive. The TEF scheme compares the relative toxicity of individual dioxin-like compounds to that of TCDD, which is the most toxic halogenated aromatic hydrocarbon.
- TEQs
- Toxicity equivalent (TEQ) is defined as the product of the concentration,
CI, of an individual "dioxin-like compound" in a complex environmental
mixture and the corresponding TCDD toxicity equivalency factor (TEFi)
for that compound. The total TEQs is the sum of the TEQs for each of the congeners
in a given mixture:
n
Total TEQs = Sum (CI x TEFi)
i=1
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This page last updated on June 25, 2001
Joanne Cox / vzc6@cdc.gov
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