THE PRECISION, USES, AND LIMITATIONS OF PUBLIC-HEALTH GUIDANCE VALUES

Government agencies charged with the protection of public health, such as the U.S. Environmental Protection Agency (USEPA), the Agency for T oxic Substances and Disease Registry (ATSDR), and the U.S. Food and Dr ug Administration (FDA), must have a reference, or comparison value, u pon which to base an evaluation of potential health threat posed by an y substance or chemical. The basis, or starting points, for such evalu ations may have different names or acronyms, but represent more or les s the same thing. These values for non-carcinogenic endpoints are call ed oral Reference Doses (RfDs) and inhalation Reference Concentrations (RfCs) by the USEPA, Acceptable Daily Intakes (ADIs) by the FDA, and oral and inhalation Minimal Risk Levels (MRLs) by the ATSDR. Too often , however, RfDs, RfCs, MRLs, and ADIs are construed as rigid, threshol d limits, above which toxicity is likely to occur. The truth, however, is that these values actually represent levels of a potential toxican t that are highly unlikely to represent any threat to human health ove r a particular/specified duration of daily exposures. The more frequen tly these levels are exceeded and the greater the exceedance, the more likely some toxic manifestation is to occur. These guidance/reference values are most definitely not threshold values for the onset of toxi city in any exposed population. Health guidance values must be thought of in the context of their intended role as mere screening or trigger values, in which they serve as a tool for assisting in the determinat ion of whether further evaluation of a given potential exposure scenar io is warranted.