This Manual is designed to support the new efforts that communities and neighborhoods across the nation are making to improve the environmental quality and health of the places where they live and work. Efforts to understand and improve air quality are a key part of this work to create healthy local environments. But many communities have found the work to understand and improve local air quality to be complex and difficult. Even regulatory agencies with expertise may have difficulty answering community questions regarding the combined impacts of releases from multiple sources. While some regulatory programs may consider combined impacts, permitting programs have usually been set up to deal with facilities on an individual basis and they cannot be easily used to answer community concerns about releases from multiple sources. Similarly, the available data on air quality, designed to support national, state, and tribal regulatory programs, is often too general to address the details of local air environments. This Manual was written to help communities overcome these difficulties and find answers to some of the basic questions about air quality that they have not been able to answer in the past.