An air filtering device for protection suits includes a case in which a fan unit is received and a filtering unit is engaged with a hole in a wall of the case and located outside of the case. A duct of the fan unit extends through a top of the case and is in communication with a muffler connected on the top of the case. A hose is connected with an aperture of the muffler. The fan unit can be driven by AC or DC power and the noise from the fan unit can be reduced by the muffler.