The default subslab soil gas (SSSG)-to-indoor air attenuation factor (AF) of 0.03 to assess the vapor intrusion (VI) pathway and generate VI screening levels for SSSG was developed by the United States Environmental Protection Agency (EPA) based on chlorinated volatile organic compound (VOC) indoor air and SSSG data collected in residential buildings and compiled in EPA's 2012 VI database. In their VI guidance published in 2015, EPA recognized that this default AF may be overly conservative for nonresidential buildings. In 2015, the Department of Defense (DoD) began developing a first-of-its-kind VI database to assess AFs at commercial and industrial buildings at DoD installations and support the development of alternative generic AF values. This database was expanded in 2019 to include 76 buildings at 22 DoD installations across the United States and is comparable in size to EPA's VI database. The DoD database includes chlorinated VOC data from SSSG and indoor air samples collected from multiple sampling zones within these buildings. Empirical SSSG-to-indoor air AFs were calculated using the same screening methods employed by EPA to filter out potential background source contributions unrelated to VI, including a source strength screen. Analysis of indoor air-SSSG data pairs found that there is substantially more attenuation occurring from SSSG to indoor air in DoD commercial and industrial buildings relative to residential buildings, and that the DoD buildings' AFs are one to three orders of magnitude lower than EPA's residential-based default of 0.03. The results support the use of a generic SSSG-to-indoor air AF of 10-3 (0.001) to support VI assessment and develop SSSG screening levels at large commercial and industrial buildings as an alternative to the residential default AF.Implications: The use of subslab soil gas (SSSG)-to-indoor air attenuation factors (AFs) is a key component of vapor intrusion (VI) pathway assessments and VI screening level development for SSSG. Currently, the United States Environmental Protection Agency (EPA) and many state regulatory agencies use a default AF of 0.03 based on chlorinated volatile organic compound (VOC) data collected in residential buildings. VI assessment data collected at Department of Defense (DoD) installations indicate that there is significantly more SSSG-to-indoor air attenuation occurring at DoD commercial and industrial buildings. For that reason, the default AF of 0.03 results in SSSG screening levels that are overestimating VI-related risks in these types of building. The DoD data support the use of a generic SSSG-to-indoor air AF of 10-3 (0.001) for conducting VI assessment and developing SSSG screening levels at large commercial and industrial buildings as an alternative to the residential default AF.