How to you prevent your HVAC system from sucking in contaminated air from the outside and blowing it into the house? Do we just have to turn of air conditioning? What would you do if your area is very cold or very hot?

Also, if you have to seal your house, the chances are you are in the midst of a blackout. So you can't use HVAC anyway. But that makes me wonder, again: what do you do for ventilation? Let's say you need to cook with your camping stove, or you need to use your kerosene heater?