In NW England, certain types of bedrock of Carboniferous age have been found to have a relatively high radon potential, in particular limestones and some mudrocks. These result in frequencies of domestic indoor radon gas concentrations above the Action Level (200 Bq.m-3) of 5% and 3% for the Dinantian and Namurian subdivisions of the Carboniferous, respectively. More particularly, villages built on such bedrocks can have frequencies of homes 10-15% above the Action Level. Surveys based on the geological radon potential can effectively locate small areas with incidences of high indoor radon levels which qualify them for consideration as Affected Areas (>1% frequency).