For limited times a positive pressure room may be created using compressed air that was stored prior to the threat. This is only workable for very small, very well sealed rooms as the amount of compressed air and the cost of storing it becomes unacceptable quickly. But if you happen to have a large pneumatic tool shop attached to your house you might be able to use that air to pressurize a well sealed closet for a day or so. Don't know the actual math to determine what size a tank would be need for what size of room with what rate of leakage, but I think that there are people on the forum who can guess at or know the correct math or links to such.