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The Indiana Department of Environmental Management's Office of Air Quality (IDEM OAQ) requires air dispersion modeling to be conducted to demonstrate that a source will not cause or significantly contribute to the violation of air quality standards. Modeling also must comply with the Prevention of Significant Deterioration (PSD) increments, or toxic thresholds. IDEM follows all air quality modeling procedures set forth in the U.S. Environmental Protection Agency (U.S. EPA) Guideline on Air Quality Models for PSD, nonattainment New Source Review (NSR), and State Implementation Plan (SIP) revisions.