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Comparison Values

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. The latter is, perhaps, an unfortunate term since the word "concern" may be misinterpreted as an implication of "hazard." As ATSDR uses the phrase, however, a "contaminant of concern" is merely a site-specific chemical substance that the health assessor has selected for further evaluation of potential 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. However, it cannot be emphasized strongly enough that comparison values are not thresholds of toxicity. While concentrations at or below the relevant comparison value may reasonably be considered safe, it does not automatically follow that any environmental concentration that exceeds a comparison value would be expected to produce adverse health effects. Indeed, the whole purpose behind highly conservative, health-based standards and guidelines is to enable health professionals to recognize and resolve potential public health problems before they become actual health hazards. The probability that 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 noncancer 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 simply do not exist, and probably never will. In neither case can the resulting screening values (i.e., EMEGs or CREGs) 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 Guide

Minimal Risk Level

IMRL = Intermediate Risk Level
CMRL = Chronic Risk Level
EMEG = Environmental Media Evaluation Guide
aEMEG = Environmental Media Evaluation Guide based on acute Minimal Risk Level
IEMEG = Intermediate Environmental Media Evaluation Guide
RMEG = Reference Dose Media Evaluation Guide
RfD = Reference Dose
RfC = Reference Dose Concentration
DWEL = Drinking Water Equivalent Level
CLHA = Child Longer-Term Health Advisory
LTHA = Drinking Water Lifetime Health Advisory
MCL = Maximum Contaminant Level
MCLG = Maximum Contaminant Level Goal (µg/L)
MCLA = Maximum Contaminant Level Action
NAAQS = National Ambient Air Quality Standards
PEL = Permissible Exposure Limit (OSHA)

Recommended Exposure Limit (NIOSH)

TLV = Threshold Limit Value (ACGIH)
FDA = Food and Drug Administration
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 (MRLs) 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.

Intermediate Environmental Media Evaluation Guides (IEMEGs) are media-specific concentrations that correspond to a minimal risk level, factoring in body weight and ingestion rates for intermediate exposures (i.e., >14 days and <1 year).

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 Concentration (RfC) is a concentration in air expected to be associated with no deleterious health effects over a lifetime of exposure, assuming default body weights and inhalation rates.

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

Drinking Water Equivalent Levels (DWELs) are based on EPA's oral RfD and represent corresponding concentrations of a substance in drinking water that are estimated to have negligible deleterious effects in humans over a lifetime of exposure, at an intake rate of 2 L/day, and assuming that drinking water is the sole source of exposure to the contaminant. Similar to ATSDR's RMEG for drinking water.

Child Longer-Term Health Advisories (CLHAs) are contaminant concentrations in water that the Environmental Protection Agency (EPA) deems protective of public health (taking into consideration the availability and economics of water treatment technology) over a period of about 7 years, using a child's weight (10 Kg) and ingestion rate (1 L/day).

Lifetime Health Advisories (LTHAs) are calculated from the DWEL and represent the concentration of a substance in drinking water estimated to have negligible deleterious effects in humans over a lifetime of 70 years, assuming 2 L/day water consumption for a 70-kg adult, and taking into account other sources of exposure. In the absence of chemical-specific data, the assumed fraction of total intake from drinking water is 20%. Lifetime HAs are not derived for compounds that are potentially carcinogenic for humans.

Maximum Contaminant Levels (MCLs) represent contaminant concentrations in drinking water that EPA deems protective of public health (considering the availability and economics of water treatment technology) over a lifetime (70 years) at an exposure rate of 2 liters of water per day.

Maximum Contaminant Level Goals (MCLGs) are drinking water health goals set at levels at which no known or anticipated adverse effect on the health of persons occurs, and which allow an adequate margin of safety. Such levels consider the possible impact of synergistic effects, long-term and multi-stage exposures, and the existence of more susceptible groups in the population. When there is no safe threshold for a contaminant, the MCLG should be set at zero.

Maximum Contaminant Level Action (MCLA) are levels set by EPA under Superfund that trigger a regulatory response when the contaminant concentration exceeds this value.

National Ambient Air Quality Standards (NAAQS) are established by the EPA, as mandated in the Clean Air Act, for six criteria pollutants (carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone, particulate, and lead). NAAQS are classified as either primary, which define levels deemed protective of public health, or secondary, which in some instances establish lower levels to prevent adverse effects on vegetation, property, or other elements of the environment.

Permissible Exposure Limits (PELs) are air standards developed by the Occupational Safety and Health Administration (OSHA) for the workplace. They are time-weighted average concentrations of contaminants considered safe for healthy workers over the course of an 8-hr workday and a 40-hr workweek. A PEL may be exceeded for brief periods, but the sum of the exposure levels averaged over 8 hours must be equal to or below the PEL.

Recommended Exposure Limits (RELs) are established by the National Institute for Occupational Safety and Health (NIOSH) and are similar to OSHA's PELs. They are time-weighted average concentrations for the workplace deemed to be safe for up to 10 hours/day, for 40-hours/week.

Threshold Limit Values (TLVs) are established by the American Conference of Governmental Industrial Hygienists (ACGIH). The TLV is the time-weighted average concentrations for a normal 8-hour workday and a 40-hour workweek, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect. Many of ACGIH's TLVs were adopted by OSHA for use as PELs. TLVs and PELs, which were designed to protect healthy workers, are usually much higher than the health-based values of ATSDR and EPA, which were designed to protect the health of the general population, including the very young and the elderly. Although the ATSDR does not base any of its community health decisions on TLVs or PELs, it sometimes cites such values in Public Health Assessments merely as a means of putting concentrations of site-specific contaminants into a meaningful perspective for the reader.

The Food and Drug Administration (FDA) has recommended concentration levels for certain substances in food, including fish. Levels above the FDA levels mean the food may be unsafe for human consumption.


  1. Agency for Toxic Substances and Disease Registry. Health Assessment Guidance Manual. Atlanta: ATSDR, October 1992.
  2. National Institute for Occupational Safety and Health. Pocket Guide to Chemical Hazards. Washington D.C.: U.S. Department of Health and Human Services, June 1994.
  3. U.S. Environmental Protection Agency. New Interim Region IV Guidance for Toxicity Equivalent Factors Methodology for Carcinogenic PAHs. February 11, 1992.


Public Comments

Public Comments

Comment 1. We empathize with the petitioner concerning the impact cancer has had on his or her life. However, we concur with the overall conclusion ATSDR has reached regarding the lack of increased brain cancer incidence in southwest Butler County and note that the conclusion matches those of previous studies.

Response: As stated in the draft Public Health Assessment, dated September 6, 1999, ATSDR reviewed the documents prepared by the Pennsylvania Department of Health, Bureau of Epidemiology, that assessed the risk of communities living in southwestern Butler County developing cancer. While these documents address the community's concern regarding a potential increase in cancers, it does not evaluate other potential health end points that may be plausible. A review of available health statistics, to identify any obvious areas of health concern while environmental data are being reviewed or obtained, is only one part of ATSDR's health assessment process.

Comment 2. Given that ATSDR has concluded that the concerns raised by the Petitioner cannot be scientifically validated and do not demonstrate potential risk to human health, we do not understand why ATSDR wishes to continue its assessment.

Response: ATSDR has not concluded in the draft Public Health Assessment that all the concerns raised by the petitioner cannot be scientifically validated and do not demonstrate potential risk to human health. Based on a review of health statistics for the period of years examined and for defined cancer end-points, a significant increase in the incidence of cancer in this community was not observed compared with other communities. However, ATSDR is committed to addressing the other concern raised in the petition, regarding potential exposures from the Callery Chemical site. Following a review of available environmental data, ATSDR will reevaluate the plausible health outcomes based on contaminants identified. This evaluation will be provided in a supplemental document once the data are obtained and reviewed.

Comment 3. Nothwithstanding Comment 2, if ATSDR wishes to further study public health in southwest Butler county, it should expand the scope of its study beyond Callery Chemical Company to include all potential sources of impact to the community.

Response: It is not feasible and within good public health policy to expand the scope of review to include the area defined in the previous comment. ATSDR's mandate is to address specific and defined releases of concern within the community and to make comments and recommendations to mitigate exposures to these hazardous substances.

Comment 4. Notwithstanding Comments 2 and 3, we suggest that ATSDR evaluate the vast volume of Callery Chemical Company data that is already available.

Response: ATSDR is currently working with the Callery Chemical Company, the U.S. Environmental Protection Agency and the Pennsylvania Department of Environmental Protection to obtain available environmental sampling data. Once these data are obtained they will be reviewed for plausible exposures and health outcomes.

Comment 5: Much of the background information regarding Callery Chemical Company is outdated, speculative, and potentially misleading, particularly in the "Background" and "Discussion" sections (Pages 3 through 7).

  • The statement that "MSA received hazardous wastes from sources off-site..." (Page 4, emphasis added) is misleading, and we believe ATSDR may be referring to Callery Chemical company's receipt of customer returned sodium-potassium alloy for reuse. Similarly, statements regarding "reported incidences of hazardous waste violation" and " a pentaborane gas cylinder explosion" require clarification and a more accurate description.
  • Although "fish kills have been observed in the creek" (Page 4), the language implies that such impacts are routine. MSA is aware of two incidents that resulted in fish kills (one in April 1989 and one in July 1993), both of which were promptly and openly reported to federal, state, and/or local authorities. Each was attributable to an accidental release of sodium or potassium hydroxide and its temporary impact on the pH of the water, not due to poisoning or a chronic water quality problem. The only other significant incident impacting Breakneck Creek of which we are aware is a 1981 oil spill caused by a contractor's improper installation of a fuel oil tank. This incident also was managed in concert with regulatory authorities. Recent state studies of Breakneck Creek indicate abundant aquatic life and good water quality upstream and downstream of our facility.

Response: Thank you for the additional information. ATSDR has made suggested changes to the final Public Health Assessment where appropriate in the document.

On page III-48, bullet 2, of the Draft Phase II RCRA Facility Assessment for Mine Safety Appliances Company, Evans City, Pennsylvania, EPA I.D. No. PaD004322913, dated June 1993, it is stated that EPA Region III requested information from MSA regarding hazardous wastes received from off-site sources. The Mine Safety Appliances Company's response indicated that hazardous wastes were received from off-site sources on several occasions (approximately 20 times) between 1983 and 1985. On the same page, reference is made to additional information regarding waste materials containing potassium and sodium that were transported to the MSA facility from off-site sources (Letter to Bruce P. Smith, Chief, Hazardous Waste Enforcement Section, from Garry L. Goehring, Plt. Mgr., MSA, Re: Attachments in response to letter dated November 21, 1986, December 9, 1986).

On pages 58 through 60 in the Draft Phase II RCRA Facility Assessment for Mine Safety Appliances Company, Evans City, Pennsylvania, EPA I.D. No. PaD004322913, dated June 1993, is a summary of spills reported to EPA by MSA (from Reference 124 in the document, Spill Reports, January 28, 1993). This information is a summary of releases to Breakneck Creek primarily from breaks and leaks in the Acid Sewer Line (SWMU 17) or the Sanitary/Acid Line (SWMU 19); in August 1982, it was reported that 500 gallons of sulfuric acid waste were released and caused a fish kill downstream and in March 1984, a mixture of sanitary waste and city water was released into the creek. Reported releases pertaining to breaks and leaks in the Caustic Sewer Line (SWMU 18) occurred in February 1982 (100 gallons of caustic wastewater), April 1989 (approximately 350 gallons of potassium hydroxide solution were released and resulted in a fish kill), and February 1991(potassium hydroxide was released onto subsurface soils from the Caustic Sewer Line). Additional spills and leaks were reported to have occurred in January 1986, February 1988 (release to soil), August 1989 (on-site spill), October 1991, and September 1981 (approximately 12, 000 gallons of fuel oil leaked resulting in a fish kill in Breakneck Creek). Many of these spills were reported to regulatory authorities and rapidly contained.

Comment 6: We are disappointed that ATSDR did not notify the company of its 1996 determination to conduct a health assessment in response to a petitioner's request.

Response: No record of an official contact person for Callery Chemical Company is listed in the files in Atlanta. However, we have contacted our regional office to determine if there is a record of past communications. No record was found. ATSDR has communicated with EPA, PADEP, and the Pennsylvania Department of Health since 1994 regarding residents concerns of potential hazardous exposures.

Recently, ATSDR headquarters and our regional office have contacted Callery Chemical to discuss our health assessment process and request available environmental sampling data. We look forward to continuing to work with Callery Chemical Company to address concerns expressed by community members.

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