Exposure guidelines developed by various federal, state, or professional organizations are derived from data drawn from the epidemiologic and toxicological literature. Default uncertainty factors are often used in the derivation of these exposure guidelines to ensure that they are protective of the population for which they were intended and to account for unknown differences between the population studied and the population to be protected. Other adjustments may also be applied to account for differences in duration of exposure or other variables or to account for unknown information.
There is considerable variation in how organizations define the length of time associated with different exposure durations. One example is the following set of definitions from the EPA’s Risk Assessment Forum,88 which have been adopted for use in this document:
- Acute exposure duration: Exposure by the oral, dermal, or inhalation route for 24 hours or less
- Short-term exposure duration: Repeated exposure by the oral, dermal, or inhalation route for more than 24 hours, up to 30 days
- Long-term (or Chronic) exposure duration: Repeated exposure by the oral, dermal, or inhalation route for more than 30 days, up to approximately 10 percent of the life span in humans (approximately 90 days to 2 years in typically used laboratory animal species)