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PETITIONED PUBLIC HEALTH ASSESSMENT

UNDERGROUND STORAGE TANKS
JACKSONVILLE, TOMPKINS COUNTY, NEW YORK


SUMMARY

A resident of Jacksonville, New York, petitioned the Agency for Toxic Substances and Disease Registry (ATSDR) to investigate groundwater contamination from several gasoline leaks at the Underground Storage Tank Site and to determine potential health effects. Residents were concerned with potential adverse health effects from intermittent exposures between 1971 and 1987 to benzene, toluene, ethylbenzene, and xylene in their private wells. Bottled water and carbon filters were provided to seven residences with contaminated wells. However, some filters were not adequate in preventing exposures, and residents were relocated in 1987. Toluene, xylene, ethylbenzene, and methyl tertiary butyl ether were reported to be below levels of health concern. Some residents were intermittently exposed above ATSDR's comparison values by ingestion and inhalation of, and dermal contact with benzene. However, no adverse health effects are likely to occur from drinking water at the contaminant levels identified during the time of exposure. To prevent future exposure, ATSDR recommends that residents should not relocate to contaminated residences at this time and should contact the local health department regarding relocation in the future.


BACKGROUND AND STATEMENT OF ISSUES

A resident of Jacksonville, New York, requested the Agency for Toxic Substances and Disease Registry (ATSDR) to investigate the exposure of residents to gasoline leaking from underground storage tanks located at a nearby gasoline station. Three releases were reported to have occurred from the gasoline station in 1971, 1979, and 1986 (1). The largest spill occurred in 1979, when approximately 4,000 to 10,000 gallons were reported to have been released. The release led to the contamination of groundwater and private wells serving approximately 23 individuals. Another leak was reported to have occurred in 1997 at the pump island on the site. The New York State Department of Environmental Conservation (NYSDEC) is conducting an investigation into the spill.

Jacksonville is a small community located in the town of Ulysses, Tompkins County, New York. The town is approximately 12 miles northwest of Ithaca on the western side of Cayuga Lake in the Finger Lakes region of upstate New York. Approximately 150 persons (the census tract contains a total population of 1,616) live in Jacksonville, with 65 developed properties in the town. No schools, hospitals, nursing homes, or child care centers are located in Jacksonville. The community obtains its water supply from domestic wells between 50 and 100 feet deep (2).

Residents were concerned with potential adverse health effects from exposure to benzene, toluene, ethylbenzene, and xylene (BTEX) in their water supply. Reports of ill health by residents included skin irritations, detectable levels of gasoline constituents in one resident's blood, severe headaches, febrile seizures, and respiratory problems. Initially a community group called the Jacksonville Clean Safe Water Association was formed but is not currently active. The community was also concerned with decreased property values, especially because of vacant homes bought by the Mobil Oil Corporation. The town of Ulysses has filed an application for municipal water to be provided for Jacksonville residents. According to the Tompkins County Department of Health Division of Environmental Health, the community continues to be concerned with the site and related health issues.


DISCUSSION

ATSDR obtains the community's concerns, and evaluates other medical, toxicologic, demographic, and environmental factors that may affect the health of a community exposed to hazardous substances. To determine whether health effects are likely to occur within the community, ATSDR health professionals consider the toxicity of the contaminant, the concentration (how much), the time of exposure (how long), and how the chemical gets into the body (breathing, eating, drinking, or skin contact). Other factors are also considered; occupation, personal habits, age, nutritional status, general health, and genetics. These factors affect how a contaminant is absorbed, distributed, metabolized, and eliminated from the body. Contaminants are evaluated in a health consultation to determine whether exposure to them has public health significance. These comparison values are based on animal studies and human exposures to determine safe levels of contaminants to which individuals may be exposed without harmful effects. They are conservative values and include a wide margin of safety. ATSDR selects and compares on- and off-site concentrations of contaminants with ATSDR comparison values for noncarcinogenic and carcinogenic effects. Comparison values are concentrations of contaminants in specific environmental media (air, soil, drinking water) that are not expected to produce adverse health effects in persons who are exposed. These values are used only for screening, and listing a contaminant in a table of "chemicals of concern" does not mean that it will necessarily cause adverse health effects if exposure occurs at that specified concentration. When the concentration of a contaminant detected on or off the site is above the comparison value, it is further evaluated to determine the potential for adverse health effects. The focus of the evaluation is on health effects that could plausibly result from exposures to site-related contaminants. ATSDR considers both adults and children when developing comparison values. The potential health effect on children is considered separately because in certain situations children may be more sensitive and more exposed to contaminants. Finally, ATSDR presents its conclusions and recommends appropriate actions.

ATSDR contacted the petitioner in February and May of 1988 to discuss health concerns related to the site. ATSDR also contacted the New York State Department of Health (May 20, 1988) and a private physician who tested and treated one resident in whose blood BTEX was detected. A site visit that included meetings with the petitioner, health and environmental officials, and data review was conducted from May 26 to 29, 1989 (3). Residents were potentially exposed over a period of 16 years, from approximately 1971 to 1987. BTEX were detected above comparison values in the past in several domestic wells located near the gas station. Mobil Oil Corporation entered into a consent agreement in 1988 to relocate persons living in seven of the residences. Currently, other homes in the community are located outside the area of the migrating plume, and the groundwater is not contaminated in these areas.

Groundwater

A recovery trench and air stripper were installed in February 1987 (4). Three underground gasoline storage tanks and approximately 2,000 tons of contaminated soil were removed near the tanks in July 1987 (5,6). Mobil Oil Corporation has been remediating the contaminated groundwater since 1988 by a recovery treatment system. Reports are sent quarterly to the New York State Department of Environmental Conservation (NYSDEC). The site contains 46 monitoring wells, three recovery wells, and eight private domestic wells (7). Mobil conducts monthly sampling and treatment-system monitoring, quarterly groundwater sampling, and annual groundwater sampling.

Between June 1992 and October 1996, approximately 45 pounds of BTEX were removed from treating 1,982,602 gallons of groundwater (8). Groundwater sampling results have demonstrated fluctuations in the concentrations of BTEX and methyl tertiary butyl ether (MTBE). In December 1994, Mobil Oil Corporation reported to the NYSDEC that BTEX had not been detected in two residential wells for two consecutive years. One family repurchased their home from Mobil Oil Corporation in 1990 and currently transports water from a neighboring town's municipal water supply. They use this alternate water source for drinking and cooking and use the domestic well for other activities. Due to an interest by some residents to repurchase their former homes, Mobil Oil Corporation has investigated whether deep wells would be adequate to provide safe drinking water. However, contaminants were found during deep-well investigations, and to date no other residents have relocated to their former homes.

From approximately 1971 to 1987, some residents were likely to have been intermittently exposed by ingestion and inhalation of and dermal contact with benzene (9) in drinking water, at levels above ATSDR's current adult RMEG of 100 ppb and EPA's maximum contaminant level (MCL) of 5 ppb. Toluene (10), xylene (11), MTBE (12) were reported to be below levels of health concern. Bottled water was provided to some residents as early as 1980, and filters were placed on water taps in affected homes between 1984 and 1987. However, some filters were not adequate in preventing exposures, and residents were relocated in 1987.

The highest concentrations of benzene, toluene, and xylene were detected in 1980 in private wells located at two homes down gradient of the service station. These concentrations of contaminants were sporadically detected in the private wells sampled from 1980 to 1987. From 1980 to 1988, one private well consistently showed benzene at a maximum concentrations (528 ppb in the shallow well and 260 ppb in the deep well) above the MCL (13).

Private wells were sampled at five residences from April 1980 to January 1988 (13). Wells were not sampled routinely each year during this period. The concentration of benzene ranged from less than 1 ppb to 5,128 ppb. Toluene concentrations ranged from less than 1 ppb to 5,815 ppb and xylene from less than 1 ppb to 7,047 ppb. The concentrations of benzene, toluene, and xylene in private wells varied over the sampling period, probably because of the seasonal variations in the water table. The highest concentration of contaminants was detected in April 1980 at the closest residence located down-gradient from the contaminated groundwater plume. From July 1985 to January 1988, 23 other residential wells were sampled. Benzene concentrations ranged from less than 1 ppb to 19 ppb. The concentration of toluene ranged from less than 1 ppb to 15 ppb, and for xylene concentrations ranged from less than 1 ppb to 14 ppb.

Table 1 provides a summary of the total BTEX, BTEX constituents, and MTBE concentrations for several 1996 sampling activities. Benzene continues to be detected in private wells above the MCL of 5 ppb. Toluene, ethylbenzene, xylene, and MTBE were not detected at levels of health concern. Although adverse health effects are unlikely, these domestic wells should not be used in the future as potable water sources.

After reviewing the environmental data and considering the exposure to some residents to benzene in private well water, ATSDR has determined that no adverse health effects are likely to occur as a result of drinking-water exposures at Jacksonville. The maximum human doses calculated from the maximum concentrations detected in private wells in Jacksonville from 1980 to 1988 (5,128 ppb) would be 0.15 mg/kg/day for a 70-kg adult drinking 2 liters/day, i.e., less than 2% of the lowest recorded effect level (8 mg/kg/day) for ingested benzene. All doses of ingested benzene that have produced effects of any kind (carcinogenic or non-carcinogenic) in laboratory animals have exceeded 1 mg/kg/day, and most doses were actually 10s, 100s, or even 1000s of times higher and involved forced (gavage) feeding.

However, no one in Jacksonville is likely to have actually been exposed at this maximal level. Most past exposures would have been 10 to 1,000 times lower because the levels detected in other private wells were 10 to 1,000 times lower. In addition, the default of 2 liters/day (eight 8-oz glasses of water a day) is probably an over-estimate, because 1) few people in temperate climates actually drink this much water, and 2) only some fraction of an individual's total daily fluid consumption is actually derived from tap water at home. Furthermore, because most people can begin to taste benzene in water at 0.5 to 4.5 ppm (9), the peculiar taste of well water containing the highest detected concentrations of benzene at Jacksonville (528 ppb and 5,128 ppb) would probably have further limited consumption of water from these private wells. Therefore, using ATSDR's current adult RMEG of 100 ppb benzene in drinking water, few past exposures and none of the present exposures to benzene in drinking water would represent a public health concern.

Table 1. Groundwater Monitoring Results of the Underground Storage Tank Site. Jacksonville, New York, 1996.

Chemical Contaminant

Concentration Range (ppb)1

Sample Location

Monitoring and Domestic Wells2 Recovery Wells3 Treatment System Effluent3 Comparison Values4 (ppb)
Benzene ND5-43 ND-3,160 ND-2.6 MCL6=5
Toluene ND-2 ND-397 ND-1.2 IEMEG7=
200(child),
700 (adult)
Ethylbenzene ND-21 ND-1,340 ND-4.2 RMEG8=
1,000 (child),
4,000 (adult)
Xylene ND-16 ND-5,028 ND-26.5 IEMEG=
2000 (child),
7,000 (adult)
MTBE ND-112 ND-4,940 ND IEMEG=
3000 (child),
10,000 (adult)
1 ppb= parts per billion.
2 Report dated September 17, 1996.
3 Results reported during monthly sampling events conducted between January and October 1996.
4 ATSDR health-based comparison values. See Appendix A, list of terms.
5 ND=contaminant not detected during sampling.
6 MCL=maximum contaminant level. See Appendix A, list of terms.
7 IEMEG=intermediate environmental media evaluation guide (ATSDR). See Appendix A.
8 EMEG= environmental media evaluation guide (ATSDR), See Appendix A.

EPA's MCL of 5 ppb is lower than ATSDR's RMEG because the MCL represents an effort to take into account potential carcinogenic effects, as well as non-carcinogenic effects. However, available epidemiological data indicate that the carcinogenicity of benzene in humans is limited to occupational, inhalation exposures of 10s or 100s of ppm, suggesting a threshold effect (9). No data suggest that oral exposures below 1 mg/kg/day have any carcinogenic potential in either animals or humans (ATSDR). Lower cancer risk estimates for benzene are based on conservative regulatory assumptions (e.g., the zero-threshold concept), which are not supported by existing data.

In conclusion, the available data do not suggest the existence of a current public health hazard. Although data is limited and represents an indeterminate health hazard for past exposures, adverse health effects are not likely to occur. Notwithstanding ATSDR's conclusion that none of the exposures at Jacksonville are or were likely to produce adverse health effects, the elimination of those excess exposures was an appropriate and justifiable public health action.

Air

Residents complained of gasoline fumes indoors and outdoors during and after the spill. Mobil Oil Corporation conducted limited indoor sampling before 1991. Trace amounts of toluene were detected (8, 14).


CONCLUSIONS

1. Residents were potentially exposed to gasoline constituents through inhalation and ingestion of and dermal contact with contaminated groundwater intermittently for approximately 16 years (1971 to 1987). No indoor air data or private well data are available to evaluate past exposures (before 1979) to volatile and liquid contaminants from the gasoline release.

2. Benzene was detected at concentrations above ATSDR's comparison value. However, because no adverse health effects would be expected as a result of these exposures, they represent no apparent public health hazard today. They would have represented an indeterminate public health hazard in the past, because of incomplete exposure data.

3. Groundwater is currently contaminated. Residents living above the plume who use private well water are at risk of exposures to BTEX and MTBE. Citizens who have repurchased their former residences may currently be exposed through inhalation, ingestion and dermal contact while showering or using contaminated water and in the future if the carbon filter is not properly maintained.

4. People who wish to relocate to former residences that lie above the contaminated groundwater plume and use domestic wells for potable water sources, are potentially at risk for exposure to benzene in the future.

5. The undeveloped property overlying or in the path of the contaminated groundwater plume may be contaminated with BTEX now or in the future. The groundwater plume is currently moving in a north to northeasterly direction, away from existing homes and toward presently undeveloped areas.


RECOMMENDATIONS

1. To prevent future exposure, residents should not relocate to contaminated residences until a safe source of drinking water is available and indoor air is sampled for potential volatile organic compounds.

2. Alternate sources of water (municipal) should be provided to homes with domestic wells that have contaminants detected above health comparison values.

3. The NYSDEP and the Mobil Oil Corporation should continue to monitor the private wells and treat the contaminated ground water plume.

4. ATSDR will review groundwater sampling data collected during the reported 1997 spill as it becomes available.


PUBLIC HEALTH ACTION PLAN

ATSDR Actions Completed

1. Meeting with the petitioner (February and May 1988)

2. Meetings with the New York Department of Health and private physician (May 20, 1988)

3. Site visit (May 20, 1988)

ATSDR Actions Planned

1. Review the 1997 gasoline leak when groundwater sampling data becomes available.


SITE TEAM/AUTHORS

Adele M. Childress, PhD, MSPH
ATSDR/DHAC/EICB/PRS

Frank Schnell, Ph.D., DABT
Toxicologist
ATSDR/DHAC/EICB/PRS


REFERENCES

1. Thomsen Associates. Hydrogeologic Investigation. Jacksonville Oil Spill. September 1986. In the Ulysses Town Hall, Jacksonville, NY, file.

2. J. M. Anderson. P.E. Tompkins County Health Department. Report on the "Individual Water Systems Serving the Residents of Jacksonville", Town of Ulysses. January 1987.

3. Agency for Toxic Substances and Disease Registry. Trip Report. May 15, 1992.

4. Thomsen Associates. 1987. Miscellaneous reports regarding hydrogeological evaluation and water and soil sampling and analysis.

5. NYSCEC. February 24, 1988. New York State Department of Environmental Conservation. Memo.

6. Thomsen Associates. April 6, 1987. Letter to New York State Department of Environmental Conservation.

7. Thomsen Associates. Geotechnical and Materials Engineering. Geologic and Environmental Geoscience Services. 105 Corona Avenue, Groton, NY 13073. Report dated June 9, 1986, sent to NYSDEC.

8. Eastern Research Group, Inc. April 8, 1997. Report on the Underground Storage Tank Site, Jacksonville, Tompkins County, New York, prepared for ATSDR. ERG, 110 Hartwell Avenue, Lexington, MA 02173-3198.

9. Agency for Toxic Substances and Disease Registry. Toxicological profile for benzene. Draft for Public Comment. Atlanta, GA: Agency for Toxic Substances and Disease Registry. February 1996.

10. Agency for Toxic Substances and Disease Registry. Toxicological profile for toluene. Draft for Public Comment. Atlanta, GA: Agency for Toxic Substances and Disease Registry. February 1999.

11. Agency for Toxic Substances and Disease Registry. Toxicological profile for xylene. Update. Atlanta, GA: Agency for Toxic Substances and Disease Registry. August 1995.

12. Agency for Toxic Substances and Disease Registry. Toxicological profile for methyl tertiary butyl ether. Atlanta, GA: Agency for Toxic Substances and Disease Registry. August 1996.

13. Empire Soils Investigation Inc. Jacksonville Gasoline Spill, Supplemental Hydrogeologic Report , Jacksonville, NY, for New York State Department of Environmental Conservation, April 1988, Job No. GTA-86-1D. 105 Corona Avenue. Groton, NY 13073.

14. Galson Technical Services, Inc. 1990. Residential Air Sampling. Jacksonville, NY. Re-sampling of residential homes. June 1990.


PUBLIC COMMENTS

ATSDR released the Underground Storage Tank Site Public Health Assessment for public review and comment during the period from April 16 thru May 16, 1999. ATSDR appreciates written comments provided by residents, state and local government and health agencies. This section contains the comments received and ATSDR's response to these comments. General editorial comments were addressed, where appropriate, within the final document.

Comment 1. Summary-Line 6: I believe bottled water and activated carbon filters were maintained until the residences were vacated, and there was no gap from 1984 to 1987.

Response: ATSDR has clarified line six and seven in the summary to, "Bottled water and carbon filters were provided to seven residences with contaminated wells. However, some filters were not adequate in preventing exposures, and residents were relocated in 1987."

Comment 2. Summary-the recommendation should reflect Mobil's agreement with the New York State Attorney General's office which has more severe restrictions on reoccupying the houses than the summary implies. The summary implies reoccupation could occur in the near future, when it can not.

Response: ATSDR has clarified the recommendations in the summary to, "To prevent future exposure, ATSDR recommends that residents should not relocate to contaminated residences at this time and should contact the local health department regarding relocation in the future."

Comment 3. Background-P1, line 7: The alleged 1997 leak is still under investigation and the source and volume is not confirmed. Also, it may be worth noting that according to the Site Monitoring Report of 23 December 1998 by GES, Inc., for Mobil, that from June 1992 to November 1998, 2.3 million gallons of groundwater were treated but only 46.77 pounds of hydrocarbons were removed. This indicates to me that it will be years before the concentration of hydrocarbons are significantly reduced in the groundwater.

Response: Thank you for your information, ATSDR will review additional environmental data for potential health implications as it becomes available.

Comment 4. Background-P3, last sentence: Community interest in this problem is still high.

Response: The last sentence of paragraph 3 in the background section has been modified to relate on-going community interest.

Comment 5. Groundwater-P2, line 6: the wording implies a neighboring town's water is piped to this residence; in fact water is hauled from another municipality to this residence.

Response: The important message in the sentence is that the resident is provided an alternate source of water and is not drinking or cooking with contaminated water. The word "receives" has been replaced with the word "transports".

Comment 6. Table: You and the report readers should be aware the New York State Maximum Contaminant Levels in drinking water for benzene, toluene, ethylbenzene and xylene is 5 ppb each and for MTBE it is 50 ppb.

Response: Thank you. ATSDR is aware that different agencies in different states and even different countries often establish different comparison values for the same substance. However, since all comparison values, regardless of source, are designed to be conservatively protective of human health, rather than predictive of toxic levels in humans, their only proper use is as screening tools for the selection of contaminants for further evaluation. That "further evaluation" is based on the best available toxicological and biomedical information relevant to the expression of toxicity in humans. Consequently, the choice of comparison value should, by itself, have no impact on ATSDR's final evaluation of the likelihood that a substance will or will not actually produce adverse health effects in exposed humans, under site-specific conditions of exposure.

Comment 7. Despite Dermal Contact being a potential exposure route, no section discusses this.

Response: Please refer to Page 4, the third paragraph under the groundwater heading for a discussion of dermal contact and potential health effects.

Comment 8. Recommendation 1: the best recommendation would be to prevent residential use of the land above the plume until it could be shown that no vapors could enter the buildings (either by building techniques or reduced values in the soil), air monitoring confirms this, and drinking water is provided free from gasoline contaminants without individual treatment units. The report recommendation implies that people should be able to reoccupy the homes now.

Response: ATSDR has modified the recommendations, "To prevent future exposure, residents should not relocate to contaminated residences at this time and should contact the local health department regarding relocation in the future.

Comment 9. Recommendation 2: the health comparison values are not protective enough; no one should be provided drinking water above the New York State Health Department Maximum Contaminant Levels.

Response: See response to Comment #6, above. The decision as to precisely what level of contaminants is "acceptable" in public drinking water is a regulatory decision, which is ultimately based on political and economic considerations, as well as on toxicological ones. ATSDR's responsibility is to determine whether or not site-specific exposures are likely to produce adverse health effects in local residents and/or workers, and to make the appropriate recommendations to those agencies authorized to implement them. See Appendix A for a description of the conservatively protective nature of comparison values.

Comment 10. I believe Mobil is spelled without an "e" at the end.

Response: Yes, thank you, the change has been made where the "e" has been omitted.


APPENDIX A: SITE MAP AND ATSDR COMPARISON VALUES

Intro Map
Figure 1. Intro Map

ATSDR's Comparison Values

ATSDR comparison values are media-specific concentrations that are considered to be "safe" under default conditions of exposure. They are used as screening values in the preliminary identification of "contaminants of concern" at a site. A "contaminant of concern" is a site-specific chemical substance that the health professional has selected for further evaluation of potential human health effects.

Generally, a chemical is selected as a contaminant of concern because its maximum concentration in air, water, or soil at the site exceeds one of ATSDR's comparison values. Comparison values are not thresholds of toxicity. It does not necessarily follow that any environmental concentration that exceeds a comparison value would be expected to produce adverse health effects. Whether adverse health outcomes will actually occur as a result of exposure to environmental contaminants depends on site specific conditions and individual lifestyle and genetic factors that affect the route, magnitude, and duration of actual exposure, and not on environmental concentrations alone.

Screening values based on non-cancer effects are obtained by dividing NOAELs or LOAELs determined in animal or (less often) human studies by cumulative safety margins (variously called safety factors, uncertainty factors, and modifying factors) that typically range from 10 to 1,000 or more. By contrast, cancer-based screening values are usually derived by linear extrapolation from animal data obtained at high doses, because human cancer incidence data for very low levels of exposure do not exist. The resulting screening values (i.e., EMEGs or CREGs) can be used to make realistic predictions of health risk associated with low-level exposures in humans.

Listed and described below are the various comparison values that ATSDR uses to select chemicals for further evaluation, along with the abbreviations for the most common units of measure.

CREG = Cancer Risk Evaluation Guides
MRL = Minimal Risk Level
EMEG = Environmental Media Evaluation Guides
RMEG = Reference Dose Media Evaluation Guide
ppm = parts per million, e.g., mg/L or mg/kg
ppb = parts per billion, e.g., µg/L or µg/kg
kg = kilogram (1,000 grams)
mg = milligram (0.001 grams)
µg = microgram (0.000001 grams)
L = liter
m3 = cubic meter (used in reference to a volume of air equal to 1,000 liters)

Cancer Risk Evaluation Guides (CREGs) are estimated contaminant concentrations in water, soil, or air that would be expected to cause no more than one excess cancer in a million persons exposed over a lifetime. CREGs are calculated from EPA's cancer slope factors.

Minimal Risk Levels (MRL) are estimates of daily human exposure to a chemical (i.e., doses expressed in mg/kg/day) that are unlikely to be associated with any appreciable risk of deleterious noncancer effects over a specified duration of exposure. MRLs are derived for acute (< 14 days), intermediate (15-364 days), and chronic (> 365 days) exposures, and are published in ATSDR's Toxicological Profiles for specific chemicals.

Environmental Media Evaluation Guides (EMEGs) are concentrations of a contaminant in water, soil, or air that are unlikely to be associated with any appreciable risk of deleterious noncancer effects over a specified duration of exposure. EMEGs are derived from ATSDR minimal risk levels by factoring in default body weights and ingestion rates. Separate EMEGS are computed for acute (< 14 days), intermediate (15-364 days), and chronic (>365 days) exposures.

Environmental Protection Agency (EPA) values are similar to ATSDR's CREGs and EMEGs in that they are risk-based concentrations derived for carcinogens and non-carcinogens from RfDs and Cancer Slope Factors, respectively, assuming default values for body weight, exposure duration and frequency, etc. Unlike ATSDR values, however, they are available for fish, as well as for water, soil, and air.

Reference Dose Media Evaluation Guide (RMEG) is the concentration of a contaminant in air, water, or soil that corresponds to EPA's RfD of RfC for that contaminant when default values for body weight and intake rates are taken into account.

(EPA's) Reference Dose (RfD) is an estimate of the daily exposure to a contaminant unlikely to cause noncarcinogenic adverse health effects over a lifetime of exposure. Like ATSDR's MRL, EPA's RfD is a dose expressed in mg/kg/day.

Reference Dose Concentrations (RfD-C) is a concentration derived from an EPA Reference Dose with assumed body and ingestion rates factored into the calculation.


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