From Percentiles to Distributions: Nationwide ZCTA-Level Modeling of Indoor Radon Exposure across the Contiguous United States, 2000-2021.

Radon exposure is the leading cause of lung cancer among nonsmokers, posing a significant public health risk. Despite its known dangers, population-wide exposure assessments of indoor radon have been limited by the reliance on area-based summaries, such as average and median concentrations, which ignore the distributional characteristics of small areas. This study developed a national-scale modeling framework to estimate ZIP Code Tabulation Area (ZCTA)-level distributions of indoor radon concentrations across the contiguous United States from 2000 to 2021. We utilized over six million short-term radon measurements, which were aggregated at the ZCTA level to predict percentiles. By further modeling within-ZCTA radon exposure distributions, we improved the quantification of exposure heterogeneity and were able to estimate the proportion and number of residents exceeding policy-relevant exposure thresholds more accurately. The results reveal that approximately half of U.S. ZCTAs have at least one in five households exposed to radon concentrations exceeding the Environmental Protection Agency's action level (148 Bq/m3), with high-exposure areas predominantly located in the Midwest. These findings provide crucial data to help residents recognize their potential radon risk, urge policymakers to design cost-effective interventions, and offer health researchers an essential database for incorporating within-ZCTA radon exposure distributions into health risk assessments.