Mass transport analysis: inhalation rfc methods framework for interspecies dosimetric adjustment.

Area of Science:

• Environmental toxicology
• Inhalation toxicology
• Physiologically based pharmacokinetic modeling

Background:

• In 1994, the U.S. EPA integrated dosimetry modeling into inhalation reference concentration (RfC) derivation.
• Existing methods needed to accommodate diverse physicochemical and toxicokinetic properties of hazardous air pollutants (HAPs).

Purpose of the Study:

• To present a dosimetry modeling framework for deriving inhalation RfCs for HAPs.
• To establish a categorization scheme for gases based on their properties to guide model selection.

Main Methods:

• Developed a hierarchical framework for dosimetry models, from optimal to rudimentary.
• Created a gas categorization scheme based on solubility, reactivity, and absorption sites (e.g., extrathoracic, airways, pulmonary).
• Applied mass transport theory to understand gas absorption and distribution.

Main Results:

• The framework allows for dosimetric adjustment factors (DAFs) applicable across a range of HAPs.
• Gas categorization facilitates evaluation of portal of entry and systemic effects.
• Mass transport theory provides the basis for predicting absorption and toxicokinetics.

Conclusions:

• The proposed dosimetry framework and gas categorization enhance the scientific basis for RfC derivation.
• This approach accommodates data limitations and varying toxicokinetic properties of HAPs.
• Further development may involve comparisons with compartmental modeling approaches.