Health Risk Assessment
Dr. John Risher
Agency for Toxic Substances and Disease Registry

The Agency for Toxic Substances and Disease Registry (ATSDR) is a component of the U.S. Public Health Service, under the Department of Health and Human Services. Among its legislative mandates, it is charged with determining levels of environmental substances identified at hazardous waste sites that represent minimal risk of adverse health effects to exposed populations. These chemical-specific Minimal Risk Levels (MRLs), which are derived for different routes and durations of exposure, are based upon scientific studies of laboratory animals, controlled human clinical studies, and/or epidemiological data from human populations. The process used for deriving MRLs is essentially the same as that used by the U.S. Environmental Protection Agency (EPA) for calculating oral Reference Doses (RfDs) and inhalation Reference Concentrations (RfCs).

The first step in this process is a critical examination of the overall database for the chemical or substance under investigation. From that analysis, a single study or group of similar studies that represent(s) the single effect that is believed to be the most sensitive toxic endpoint is selected as the "critical study." Typically, a no-observed-adverse-effect-level (NOAEL), representing the highest dosage at which no observable adverse effects were seen in the critical study, or a lowest-observed-adverse-effect-level (LOAEL) then serves as a starting point for MRL or RfD/RfC derivation. After appropriate adjustment for duration of exposure and/or dosimetric airway adjustments (for RfCs) , a number of uncertainty factors are applied to recognize various areas of uncertainty in the MRL or RfD/RfC calculation.

Whether the traditional NOAEL/LOAEL approach or the benchmark dose approach is employed, the NOAEL, LOAEL, or benchmark dose is divided by a composite uncertainty factor to arrive at the MRL or RfC. Such values may then be employed by public health officials to make decisions deemed necessary for the protection of the public health.

Complete Abstract List (suitable for printing)