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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 preliminaryidentification 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 ofpotential human health effects.

Generally, a chemical is selected as a contaminant of concern because its maximumconcentration 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 anyenvironmental concentration that exceeds a comparison value would be expected to produceadverse health effects. Whether adverse health outcomes will actually occur as a result ofexposure to environmental contaminants depends on site specific conditions and individuallifestyle 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 LOAELsdetermined in animal or (less often) human studies by cumulative safety margins (variouslycalled safety factors, uncertainty factors, and modifying factors) that typically range from 10 to1,000 or more. By contrast, cancer-based screening values are usually derived by linearextrapolation from animal data obtained at high doses, because human cancer incidence datafor very low levels of exposure do not exist. The resulting screening values (i.e., EMEGs orCREGs) 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., g/L or g/kg
kg = kilogram (1,000 grams)
mg = milligram (0.001 grams)
g = 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., dosesexpressed in mg/kg/day) that are unlikely to be associated with any appreciable risk ofdeleterious noncancer effects over a specified duration of exposure. MRLs are derived foracute (< 14 days), intermediate (15-364 days), and chronic (> 365 days) exposures, and arepublished in ATSDR's Toxicological Profiles for specific chemicals.

Environmental Media Evaluation Guides (EMEGs) are concentrations of a contaminant inwater, soil, or air that are unlikely to be associated with any appreciable risk of deleteriousnoncancer effects over a specified duration of exposure. EMEGs are derived from ATSDRminimal risk levels by factoring in default body weights and ingestion rates. Separate EMEGSare 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 EMEGsin that they are risk-based concentrations derived for carcinogens and non-carcinogens fromRfDs and Cancer Slope Factors, respectively, assuming default values for body weight,exposure duration and frequency, etc. Unlike ATSDR values, however, they are available forfish, as well as for water, soil, and air.

Reference Dose Media Evaluation Guide (RMEG) is the concentration of a contaminant inair, water, or soil that corresponds to EPA's RfD of RfC for that contaminant when defaultvalues 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 unlikelyto cause noncarcinogenic adverse health effects over a lifetime of exposure. Like ATSDR'sMRL, EPA's RfD is a dose expressed in mg/kg/day.

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

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