The results from the Air Quality Assessment we had done several weeks ago are in, and we have identified a likely suspect:
Clorodifluoromethane, also known as Freon R22, has shown up at a whopping 960.2 µg/cu M. It looks like we have an air conditioner leak.
Our "comfort specialist" is on his way to do a check of the equipment. Most likely, the cooling coils in our 12+ year old system have sprung a leak and something will need to be replaced. Maybe just the coils, maybe the whole shootin' match. We'll have to see.
In the meantime, it should be plenty cool this week, so fortunately we can leave the system off.
I see this as a positive thing. Answers are good.
Update: The problem has been confirmed as a leak in the AC. All of the freon has been pulled from the system so it will no longer be able to leak into the house. A new air handler is now on the top of our shopping list. (sigh)