Groo:

If you have your outside air intake damper closed on a window unit, it will not suck anything in from the outside.

If you have central air and do not have an outside air mixing intake, the central air does not use outside air, but simply keeps recirculating what is in the house.

For information on CO detectors, I would recommend "Consumers Reports", which is printed by non profit Consumers Union. There is a lookalike magazine out there published by a for profit group, so beware!

Bountyhunter