How to screen for 222Rn in large buildings

222Rn measurements have been made in over 10,000 ground-contact rooms in 908 federal buildings. These data were examined statistically to compare parametric distributions that might be useful in the design and execution of future surveys of indoor 222Rn in large buildings. In 152 of the 365 buildings with the most measurements per building, the log-normal distribution was acceptable. Many other distributions were observed in the other 213 buildings, including bi- and tri-modal distributions. In contrast, when data from entire facilities were examined, the data were usually described by a lognormal distribution. Because of the above observations, we propose that any future surveys of indoor 222Rn in large buildings be done in two phases, screening and assessment. During the screening phase, all facilities may be surveyed with a comparatively low sampling density, perhaps, one measurement per 465 m2 (5,000 ft2) of floor space. Based on the results presented here, it is expected that statistical analysis of the screening data will reveal any facilities with substantial evidence of a high incidence of rooms with elevated indoor 222Rn concentrations (e.g., above 150 Bq m-3). In the assessment phase, the identified facilities will be surveyed with a sampling density of one measurement for every ground-contact room or about one per 84 m2 (900 ft2). Using numerical simulation techniques, we have tested the proposed screening phase protocol against data from sixteen facilities. The results of the simulated screenings support the feasibility of a two-phased approach to the task of identifying facilities having rooms with elevated 222Rn concentrations.