Target population, exposure duration, intended application, and level of peer-review are some of the factors that should be considered in choosing an appropriate exposure guideline. No single set of exposure guidelines will be suited for every chemical or situation, but they provide a starting point for site- specific considerations. Ultimately, it is important to clearly understand what these values represent and what they do not represent so that they are used appropriately. Also, if an available exposure guideline does not adequately reflect the site- and situation-specific nature of the scenario, a toxicologist should be consulted to derive a de novo site-specific exposure guideline.
It is important to differentiate the concepts of short-term or long-term exposures from the resulting acute health effects (effects represented by a short and often relatively severe course) or chronic health effects (effects persisting over a long period of time). A short-term exposure, for example, could result in either an acute or chronic health effect. Acute or chronic exposure guidelines, however, are derived assuming a specific duration of exposure.
In this document, the choice of appropriate exposure guidelines includes the question of whether a long- term exposure could feasibly exist (e.g., due to persistence of chemical hazards). One must, at a minimum, mitigate the potential for any acute effects that might arise from short-term exposures. However, one should also evaluate whether there is the potential for long-term health effects that might not be addressed by mitigating acute/short-term exposures.
Not all acute exposure guidelines are developed for the same purpose and care must be taken when interpreting the results of an acute impact assessment relative to the exposure guideline exceeded. In particular, the emergency response exposure guidelines (e.g., AEGLs or ERPGs) are derived to address rare, short-term exposure situations and often use lower uncertainty factor values for that purpose. In contrast, the acute CA-RELs developed by the State of California address continuous or short-term emissions of airborne toxicants to which the public living or working in communities surrounding industrial facilities is at risk of being exposed. Acute CA-RELs are based on the most sensitive, relevant, adverse health effect reported in the medical and toxicological literature. These values are designed to protect the most sensitive persons in the population by incorporating relatively protective uncertainty factors. The uncertainty factors incorporated to address data gaps and other factors are similar to those used in developing chronic toxicity values. Other considerations regarding the definition of a particular exposure guideline and the basis for its derivation (i.e., whether it is intended to account for single exposure events [AEGLs] or protect against the possibility of repeating exposures [Acute CA-RELs]), and the health effect severity level (e.g., cases where an AEGL-1 or ERPG-1 are unavailable) should be factored into the risk characterization as potential uncertainties.