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Particulate matter (PM) pollution has raised serious concerns for public health. Although outdoor individual protection could be achieved by facial masks, indoor air usually relies on expensive and energy-intensive air-filtering devices. Here, we introduce a transparent air filter for indoor air protection through windows that uses natural passive ventilation to effectively protect the indoor air quality. By controlling the surface chemistry to enable strong PM adhesion and also the microstructure of the air filters to increase the capture possibilities, we achieve transparent, high air flow and highly effective air filters of ~90% transparency with >95.00% removal of PM_2.5 under extreme hazardous air-quality conditions (PM_2.5 mass concentration >250 μg m⁻³). A field test in Beijing shows that the polyacrylonitrile transparent air filter has the best PM_2.5 removal efficiency of 98.69% at high transmittance of ~77% during haze …