Model-based assessment of geogenic radon exposure in Chaco Canyon and adjacent Ancestral Puebloan residential settings.

Naturally occurring radon-222 and its short-lived alpha-emitting progeny are important sources of environmental radioactivity in enclosed, earth-contact spaces, yet their potential relevance to archaeological built environments has rarely been quantified. Here we evaluate chronic radon exposure in Chaco Canyon, where Ancestral Pueblo people occupied earth-contact and semi-subterranean rooms associated with Pueblo Bonito and other great houses during AD 850-1150. We developed a Chaco-specific source-to-risk model using an ensemble of uranium-miner radon risk functions to link uranium- and radium-bearing geology, soil-gas radon production, diffusion- and advection-driven indoor entry, ventilation, dust/smoke-enhanced progeny exposure, floor-level occupancy, premodern survival, and radon-associated lung-cancer mortality. The model converts indoor radon scenarios to working-level-month exposure and propagates risk through reconstructed birth cohorts and competing mortality. Under the reference population trajectory, modeled exposure produced notable conditional lifetime lung-cancer risk among long-lived residents and an age-structured mortality burden during AD 850-1150. Because survival into older adulthood was limited, realized population-level burden remained demographically constrained even where conditional individual risk was elevated. We do not propose radon as a primary cause of Chacoan depopulation, Pueblo Bonito decline, or regional abandonment. Rather, the results indicate that elevated radon in selected earth-contact microenvironments could have imposed a chronic, elder-concentrated health burden within an already stressed social and ecological system, making geogenic radioactivity a testable contributor to cumulative vulnerability.