Home air quality emerges as a measurable factor in cancer geography, with new evidence quantifying how residential radon levels predict regional lung cancer patterns across an entire nation. This finding matters because radon represents the largest environmental cancer risk that homeowners can directly address through testing and mitigation. The Bulgarian national analysis spanning 2013-2022 reveals compelling dose-response relationships between indoor radon concentrations and lung cancer incidence across different regions. Researchers tracked 43.5 annual cases per 100,000 residents nationally, but regional variation ranged dramatically from 25.5 to 62.4 cases per 100,000. The correlation strengthened markedly when examining dwellings exceeding specific thresholds: moderate correlation existed with average radon levels, but strong to very strong correlations emerged with the percentage of homes surpassing 200 and 300 becquerels per cubic meter respectively. This threshold effect suggests radon acts more like a binary environmental trigger than a linear dose relationship. The research validates WHO's 200 Bq/m³ reference level as epidemiologically meaningful, not merely precautionary. For health-conscious adults, this represents actionable intelligence: radon testing costs under $50 and mitigation systems typically run $800-2500, making this one of the most cost-effective cancer prevention strategies available. The study's limitation lies in its observational design—it demonstrates association without proving direct causation. However, combined with decades of mechanistic research on radon's carcinogenic pathway, the evidence strongly supports proactive home testing, particularly in regions with known geological radon sources or elevated baseline measurements.