EPA proposed a revision of its Guideline on Air Quality Models to emphasize the cumulative impact of multiple sources of pollution on communities. Current methods for establishing background concentration of pollutants may underestimate these cumulative impacts, and thus underestimate local communities’ exposure to health-harming chemicals. The proposed rule would revise EPA’s near-field dispersion modeling system, AERMOD, and revise EPA’s recommendations for developing appropriate background concentrations in cumulative impact analyses. EPA’s Thirteenth Conference on Air Quality Modeling will serve as a public hearing for the proposed revision. Comments on the proposal were due by Dec. 22, 2023.