The precision, uses, and limitations of public health guidance values

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Abstract
Government agencies charged with the protection of public health, such as the U.S. Environmental Protection Agency (USEPA), the Agency for Toxic Substances and Disease Registry (ATSDR), and the U.S. Food and Drug Administration (FDA), must have a reference, or comparison value, upon which to base an evaluation of potential health direat posed by any substance or chemical. The basis, or starting points, for such evaluations may have different names or acronyms, but represent more or less the same thing. These values for non‐carcinogenic endpoints are called 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, threshold limits, above which toxicity is likely to occur. The truth, however, is that these values actually represent levels of a potential toxicant that are highly unlikely to represent any threat to human health over a particular/specified duration of daily exposures. The more frequendy 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 toxicity 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 determination of whether further evaluation of a given potential exposure scenario is warranted.

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