The World Health Organization (WHO) Air Quality Guidelines (AQG) 2021 for PM2.5 is tightened to be 5 μg/m3. We firstly estimated deaths attributable to human exposure to PM2.5 (DAHP) to be 455 thousand (372-527) in urban China in 2019, of which indoor sources contributed 253 thousand (207-294) deaths. The economic losses related to PM2.5 from indoor sources were 0.98 trillion (0.80-1.14) RMB, accounting for 56% of the total economic losses. We then further projected the DAHP at 328 thousand (260-392) when the outdoor PM2.5 concentration is 5 μg/m3, while PM2.5 from indoor sources still causes 297 thousand (235-355) deaths and 1.27 trillion (1.00, 1.51) in economic losses each year. There are significant health hazards and economic losses caused by indoor PM2.5, even the outdoor air is clean enough. The formulation and implementation of more air pollution policies are therefore in urgent need to control indoor sources of PM2.5.