EPA’s Positive Matrix Factorization (PMF) Model is a mathematical receptor model developed by EPA scientists that provides scientific support for the development and review of air and water quality standards, exposure research and environmental forensics. The PMF model can analyze a wide range of environmental sample data: sediments, wet deposition, surface water, ambient air, and indoor air. EPA’s PMF model reduces the large number of variables in complex analytical data sets to combinations of species called source types and source contributions. The source types are identified by comparing them to measured profiles. Source contributions are used to determine how much each source contributed to a sample. In addition, EPA PMF provides robust uncertainty estimates and diagnostics.