Multi-year Evaluation of a Clean Heating Policy on Residents' Air Pollution Exposures in Beijing, China

China's Clean Heating Policy (CHP), aimed at shifting households from coal to electricity for space heating, represents a major residential energy transition initiative. We evaluated its multi-year impacts on outdoor, indoor, and personal exposures to PM_2.5 and black carbon (BC) across 50 villages and 1,236 households in rural Beijing. Using a difference-in-differences (DiD) design, we observed a substantial (31 μg/m³) reduction in winter (3 month) indoor PM_2.5 (95% CI: -53, -9), but an increase in 24-h indoor BC by 2.6 (0.4, 4.7) μg/m³. CHP-driven reductions in personal exposures were limited, emphasizing the limitations of using single 24-h measurements to estimate "usual" exposure. Outdoor air quality improved in all villages, with no difference between treated versus untreated villages. Exposure-energy trade-off analysis showed that untreated households achieved similar personal PM_2.5 reductions at lower cost, with smaller coal use reductions and less electricity expenditures. The CHP significantly reduced seasonal indoor PM_2.5, but continued burning of biomass, which was accessible at no cost, limited air quality improvements and may have contributed to the observed increase in 24-h indoor BC. This illustrates how behavioral choices, economic feasibility, and selection of exposure metrics influence the measured impact of household energy transitions.