I am a retired engineer. I have seen no mention of attempts to mitigate the effects of Covid19 by filtering free-air in public places/transport. I have no experience of virological research and offer this electro-mechanical solution from an engineer’s point of view. Also, I have no evidence that the device would be efficient enough to neutralise Covid19 aerosols. The device would have a small, low speed fan at one end of a cylinder (dimensions should be concomitant with expected aerosol burden) between the fan and the other – open end – of the cylinder; there would be interposed, a magnetron (self-excited microwave oscillator i.e. microwave generator) irradiating the incoming air. The cylinder would probably need a right-angle bend between the input and the magnetron to stop any stray uhf diffraction. Having a fine grill to stop accidental ingress by persons and random access by rodents/insects etc. I haven’t the necessary resources/construction and testing equipment, to test the viability of this device but I feel it could be possible to build and use the device to good effect. Further: I believe such a device could be cheaply manufactured in large quantities. Initially the magnetrons might be taken from the thousands of micro-wave ovens that are disposed of yearly.