"JimL" wrote in message
...
The local news reports a chemical spill or fire and says 'shelter in
place' rules apply. One of those rules is to turn off your air
conditioner.

But if I know that my air conditioner does NOT have an external
source of air - by that, I mean that the air is just recirculating
inside air, should I turn off my air conditioner?

My window unit and my car both have a control for introducing outside
air - which I can close. My central unit does not. I wish it did but
I can't afford an air exchanger.....

ps. I've used a water column to measure static pressure with the air
on and it is neutral. Neither positive or negative pressure inside
my home....

Actually, a positive pressure inside your home relative to the outside
environment would be a good thing in case of a chemical/biological incident.
Keeps the nasty stuff out.

But snice the average person doesn't habe that capability, I would think
turning off all ventilation (blower too) would prevent a negative pressure
situation - even in a single room - that could pull contaminated air in from
the outside.