Because of their long temporal coverage, MERRA-2 data were used to estimate PM_2.5 concentrations over east Asia (EA), and those data were integrated with population information to comprehensively assess exposure from 1990 to 2019. Theil–Sen median trend and geographic distribution analyses were conducted to characterize the spatiotemporal variations in regional PM_2.5 concentrations. The results indicated that anthropogenic aerosol concentrations were much higher in the fast-growing and densely populated eastern and southern parts of China. Before 2000, PM_2.5 concentrations varied little in most areas, and they only notably increased during 1990–1999 in southern China and South Korea. At that time, 88% of EA's population was exposed to lower levels of PM_2.5 pollution (annual PM_2.5 ≤ 35 μg/m³). From 2000 to 2009, the air quality substantially deteriorated over the entire study region, and the rates of increase of the population-weighted PM_2.5 concentration reached 1.6 (1.1–1.9) μg/m³/yr in east China and 1.4 (1.1–1.6) μg/m³/yr in south China. Almost 50% of the population was exposed to the highest level (Level 5) of pollution (annual PM_2.5 > 35 μg/m³). To combat air pollution, the State Council of China promulgated the toughest-ever Air Pollution Prevention and Control Action Plan in 2013, which effectively decreased PM_2.5 concentrations in the heavily polluted east and south China. As the air quality improved, the population exposed to annual PM_2.5 concentrations of <35 μg/m³ returned to 63.5% in 2018. An age-group analysis indicated that, even with decreasing levels of pollution, the older adult (≥65 years old) population exposed to Level 5 pollution peaked in 2019 at 83.1 million. This implies that, as the issue of population aging becomes increasingly important in EA, additional strict control of primary PM_2.5 and precursor emissions will be required to help decrease the pollution-related health risks facing residents of EA.