Air filtration to bring outside air in to a 20 ft square underground tornado shelter

"I am looking for some sort of air scrubber or filter to bring outside air in to a 20 ft square underground tornado shelter with 8ft ceiling. I need to filter outside air and clean inside air for 6 people for several days if need be when used as disaster shelter."

A positive pressure set up like the illustration below using a 6000 EXEC model (http://www.allerairsolutions.com/6000prices.html)



http://www.allerairsolutions.com/pos_neg_pressure.html

Can be powered off a battery with the help of a power inverter same as used in RVs and cars.