Radon is an odorless and invisible radioactive gas that is naturally released from rocks, soil, and water. According to the U.S. Environmental Protection Agency (EPA) and the U.S. Surgeon General, radon is the second leading cause of lung cancer in the United States. Recent epidemiological studies have also identified linkages between radon exposure and cerebrovascular diseases including stroke. The Indoor Radon Abatement Act of 1988 (Public Law 100-551) directed the EPA to identify areas of the United States that have the potential to produce harmful levels of indoor radon, based on geological data and on indoor radon levels in homes and other structures. As part of this effort, the U.S. Geological Survey (USGS) prepared radon potential estimates for the United States, based on the Radon Index (RI), a composite score derived from the semi-quantitative ranking of five factors: geology, soil permeability, aerial gamma radioactivity, home architecture, and screening indoor radon data. The RI scores were grouped into three geologic radon potential (GRP) zones for compatibility with EPA's "Map of Radon Zones". The GRP maps were originally released as page-sized maps for each state in USGS Open-File Report 93-292. These maps were digitized and merged into a national-scale GIS database.