PETITIONED HEALTH CONSULTATION
HOLIDAY UTILITIES
TARPON SPRINGS, FLORIDA
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. 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.
