Use of Physiologically Based Pharmacokinetic Modeling to Support Development of an Acute (24-Hour) Health-Based Inhalation Guideline for Manganese.

The toxicokinetics of manganese (Mn) are controlled through homeostasis because Mn is an essential element. However, at elevated doses, Mn is also neurotoxic and has been associated with respiratory, reproductive, and developmental effects. While health-based criteria have been developed for chronic inhalation exposure to ambient Mn, guidelines for short-term (24-h) environmental exposure are also needed. We reviewed US state, federal, and international health-based inhalation toxicity criteria, and conducted a literature search of recent publications. The studies deemed most appropriate to derive a 24-h guideline have a LOAEL of 1500 μg/m3 for inflammatory airway changes and biochemical measures of oxidative stress in the brain following 90 total hours of exposure in monkeys. We applied a cumulative uncertainty factor of 300 to this point of departure, resulting in a 24-h guideline of 5 μg/m3. To address uncertainty regarding potential neurotoxicity, we used a previously published physiologically based pharmacokinetic model for Mn to predict levels of Mn in the brain target tissue (i.e., globus pallidus) for exposure at 5 μg/m3 for two short-term human exposure scenarios. The PBPK model predictions support a short-term guideline of 5 μg/m3 as protective of both respiratory effects and neurotoxicity, including exposures of infants and children.