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Background: Long-term exposure to particulate matter less than 2.5 μm (PM_2.5) is considered a risk factor for premature death. However, only a few studies have been conducted in areas with moderate PM_2.5 concentrations. Moreover, an aging society may be more susceptible to environmental exposure and future health impacts of PM_2.5. Methods: This study estimates hazard ratios (HRs) for all-cause and cause-specific mortality from long-term exposure to moderate PM_2.5 concentrations in the elderly populations of seven cities in South Korea. We also projected nationwide elderly mortality caused by long-term exposure to PM_2.5, accounting for population aging until 2045. Mortality in 1,720,230 elderly adults aged 65 years and older in 2008 was monitored across 2009-2016 and linked to modeled PM_2.5 concentrations. Results: A total of 421,100 deaths occurred in 2009-2016, and the mean of annual PM_2.5 concentration ranged between 21.1 μg/m³ and 31.9 μg/m³ in most regions. The overall HR for a 10 μg/m³ increase in a 36-month PM_2.5 moving average was 1.024 (95% confidence intervals: 1.009, 1.039). We estimated that 11,833 all-cause nationwide elderly deaths were attributable to PM_2.5 exposure. Annual death tolls may increase to 17,948 by 2045. However, if PM_2.5 is reduced to 5 μg/m³ by 2045, the tolls may decrease to 3,646. Conclusions: The long-term exposure to moderately high levels of PM_2.5 was associated with increased mortality risk among the elderly. Thus, PM_2.5 reduction in response to the projected aging-associated mortality in South Korea is critical.