Mapping indoor radon: individual vs. collective hazard

Indoor radon is considered as an indoor air pollutant due to its carcinogenic effect. Since the main source of indoor radon is the ground beneath the house, we use geogenic Rn as predictor for indoor Rn hazard mapping. In this contribution, we present a model to link geogenic to indoor Rn.

In a first step, we build a random forest model that utilizes observational data (n=6,293) of Rn concentration in soil gas and soil gas permeability across Germany in combination with auxiliary data (geology, soil physical and chemical properties, climate) to create spatially continuous map of a geogenic radon hazard index. Then, in a second step, this is geogenic radon hazard index map is linked to indoor radon data (n=44,629) via a logistic regression model for calculating the probabilities that indoor Rn exceeds 300 Bq/m³. The estimated probability was averaged for every municipality by considering only the estimates within the built-up area. Finally, the mean exceedance probability per municipality was coupled with the respective residential building stock for estimating the number of residential buildings with indoor Rn above 300 Bq/m³ for each municipality.

We found that (1) the municipal-scale maps of 300 Bq/m³ exceedance probability (individual hazard) and affected residential buildings (collective hazard) show contrasting spatial patterns, (2) the estimated number of buildings above 300 Bq/m³ in Germany is 345,000 (1.9 % of all residential buildings), (3) areas where 300 Bq/m³ exceedance is greater than 10 % comprise only 0.8 % of the German building stock but 6.3 % of buildings with indoor Rn exceeding 300 Bq/m³, and (4) most urban areas and most high-radon residential buildings (77 %) are located in low hazard regions.

The implications for Rn protection are twofold: (1) the Rn priority area concept is cost-efficient in a sense that it allows to find the most buildings that exceed a threshold concentration with a given amount of resources, and (2) for an optimal reduction of lung cancer risk areas outside of Rn priority areas must be addressed since most hazardous indoor Rn concentrations occur in low to medium hazard areas.