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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 the 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. In fact, 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 depends on site-specific conditions and individual lifestyle and genetic factors that affect the route and duration of actual exposure, and not on environmental concentrations alone.

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.

CLHA = Child Longer-Term Health Advisory
CREG = Cancer Risk Evaluation Guides
MRL = Minimal Risk Level
EMEG = Environmental Media Evaluation Guides
IEMEG = Intermediate Environmental Media Evaluation Guides
RMEG = Reference Dose Media Evaluation Guide
RfD = Reference Dose
RfD-C = Reference Dose Concentration
DWEL = Drinking Water Equivalent Level
LTHA = Drinking Water Lifetime Health Advisory
MCL = Maximum Contaminant Level
MCLA = Maximum Contaminant Level Action
PEL = Permissible Exposure Limit
REL = Recommended Exposure Limit
TLV = Threshold Limit Value
ppm = parts Per million, e.g., mg/L water or mg/kg soil)
ppb = parts per billion, e.g., ug/L water or ug/kg soil
kg = kilogram (1,000 grams)
mg = milligram (0.001 grams)
ug = microgram (0.000001 grams)
L = liter
m3 = cubic meter (used in reference to a volume of air equal to 1,000 liters)

Child Longer-Term Health Advisories (CLHAs) are estimated 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 lifetime (70 years) using a child's weight and ingestion rate (10 kilograms and 1 liter per day).

Cancer Risk Evaluation Guides (CREGs) are estimated contaminant concentrations 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 calculated using data from human and animal studies and are reported for acute (< 14 days), intermediate (15-364 days), and chronic (> 365 days) exposures. MRLs are published in ATSDR Toxicological Profiles for specific chemicals.

Environmental Media Evaluation Guides (EMEGs) are concentrations that are calculated from ATSDR minimal risk levels by factoring in default body weights and ingestion rates.

Intermediate Environmental Media Evaluation Guides (IEMEG) are calculated from ATSDR minimal risk levels; they factor in body weight and ingestion rates for intermediate exposures (i.e., those occurring for more than 14 days and less than 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 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. 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.

Environmental Protection Agency Region III (EPAIII) values are risk-based concentrations which take into account factors such as body weight, toxicity, and exposure duration and frequency for non-carcinogens and carcinogens, when applicable.

Drinking Water Equivalent Levels (DWEL) 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 at an intake rate of 2 L/day, assuming that drinking water is the sole source of exposure.

Lifetime Health Advisories (LTHA) are calculated from the DWEL and represents 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 which 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 (for an adult).

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

Occupational Safety and Health Administration's Permissible Exposure Limit (PEL) for air is an 8-hour, time-weighted average developed for the workplace. The level of exposure may be exceeded (for brief periods), but the sum of the exposure levels averaged over 8 hours must not exceed the limit.

National Institute for Occupational Safety and Health's Recommended Exposure Limit (REL) is similar to a PEL since it is developed for the workplace; however the REL is based on a time-weighted average for up to a 10 hour workday during a 40-hour work week. A PEL is regulatory and legally enforceable, while a REL is not.

Threshold Limit Value (TLV), according to the American Conference of Governmental Industrial Hygienists (ACGIH), 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, agency health assessors and toxicologists may sometimes mention such values in Public Health Assessments or consultations as a means of putting site-specific concentrations of contaminants into some kind of meaningful perspective for the reader.

APPENDIX 3A. Comparison Value Calculations

The following formula was used to calculate soil comparison values from RfDs for volatile and semivolatile organic compounds and metals; and MRLs for PAHS and 2,3,7,8-TCDD and congeners. Soil ingestion of 0.0002 kg/day for a reference child weighing 10 kg was assumed.

Cs (mg/kg) = 10(kg) x RfD or MRL (mg/kg/day) / 0.0002 (kg/day) (1)

2,3,7,8-TCDD Toxic Equivalent

The 2,3,7,8-TCDD toxic equivalent is a weighted concentration of total polychlorinated dibenzo-p-dioxins (PCDDs)in a mixture that compensates for the differences in toxicity among the 2,3,7,8-TCDD analogs. The relative weight of 2,3,7,8-TCDD (tetra) is 1; 2,3,7,8-PeCDD (penta) is 0.5; 2,3,7,8-HxCDD (hexa) is 0.1; 2,3,7,8-HpCDD (hepta) is 0.001; and other PCDDs are 0. Using this convention, the concentration of each isomer in a mixture is multiplied by the appropriate factor (listed above) and the sum of all the weighted PCDDs in the mixture is represented by the 2,3,7,8-TCDD Toxic Equivalent.

Benzo(a)pyrene Toxic Equivalent

The Benzo(a)pyrene toxic equivalent is a weighted concentration of total carcinogenic poly aromatic hydrocarbons in a mixture that compensates for the differences in toxicity among the polyaromatic hydrocarbon analogs. The relative weight of benzo(a)pyrene is 1; benzo(a)anthracene is 0.1; Benzo(b)flouranthene is 0.1; benzo(k)fluoranthene is 0.1; chryene is .01; dibenzo(a,h)anthracene is 1.0; and indeno(1,2,3-c,d)pyrene is 0.1. The benzo(a)pyrene toxic equivalent is calculated in the same manner as described above for the 2,3,7,8-TCDD toxic equivalent (2).


1. Agency for Toxic Substances and Disease Registry. Health Assessment Guidance Manual. Atlanta: ATSDR, March, 1992.

2. U.S. Environmental Protection Agency. New Interim Region IV Guidance for Toxicity Equivalent Factors Methodology for Carcinogenic PAHs. February 11, 1992.

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