One other thing, I am looking for a method to filter the air vents to our underground fallout shelter to filter nuclear radiation. Presently the vent pipes just have a mesh filter similar to a furnace filter. I dont think this is sufficient for nuclear radiation. What do I need for this? (The air vents are just standard sheet metal furnace ducts which go to the outside and have a rain bonnet over them. There is a 12V electric fan on the intake, connected to a car battery and with a 120vac battery charger. There is also with a hand crank in case of complete power outage. (I just ordered some solar cells to bring in power to charge the battery).