Cooling HVAC doesn't draw in outside air but as stated above, it circulates what's in the house.

Heating with natural gas or oil requires combustion air so modern installations have a 4" or larger pipe to the outside to draw outside air in for the furnace to use in combustion. So, in my case, the basement would provide shielding but if the furnace (which is in the basement) was running that intake pipe would suck in outside contaminants. If there were no power that pipe still would need to be well blocked.


Nukalert is one device that can count radiation. (keychain item my wife bought)

Old Civil Defense radiacs that work (but aren't calibrated) can be bought. (got one in the basement)

Even the foil-leaf personal dosimeters like I used in US Naval nuclear power plants can be purchased. (got two along with the charging station in the basement)

Knowing the levels would bring some confidence in choosing an appropriate course of action.