2.6 Joint analysis of radon and gamma emissions

At each of the 1,351 valid radon reading sites gamma flux rate was estimated based on the kriging. A Pearson product-moment correlation test (α = 0.05; two-tailed) was used to determine if a significant positive correlation existed between predicted gamma flux and indoor radon. Indoor radon values were log-transformed prior to correlation testing, as they were for the ANOVA test, to account for radon’s log normal distribution. The points were also grouped by radon value into below and at or above the EPA radon action level (4.0 pCi/L) to test for categorical relationships. Predicted gamma flux means of the two groups were compared using a Student’s t-test (α = 0.05; two-tailed). A chi-squared test (α = 0.05) was used to compare radon, grouped by EPA action level, and gamma, grouped by observed mean reading (i.e. above or below the mean).

While several studies mentioned above show an important association between aerial gamma data and indoor radon, a previous study found that indoor in situ gamma emission measurements and indoor radon were not correlated (Clouvas et al., 2003). Additionally the EPA notes that adjacent buildings may have very different indoor radon levels (EPA, 2009). So in addition to testing for the kriging surface’s predictive ability on a house by house scale, the kriging surface’s ability to predict on a more general, but still sub-county scale was also tested. The predicted gamma flux raster was resized to 3 km square in order to determine the efficacy of predicted gamma flux rate in predicting indoor radon in a more generalized way. After resizing the raster log-transformed indoor radon values were aggregated to each grid square using the mean of all the log-transformed points in each grid cell. A Pearson’s product moment correlation test (α = 0.05; two-tailed) was used to determine if an association between the 9 km² predicted gamma flux cell and aggregated log-transformed radon concentration existed.