I finally got the bright idea to check the pad in the whole house humidifier. It was pretty ugly. I pulled the unit apart, took the holder, scrubbed it out, and put in the new pad. And for the first time in weeks, my nose is pretty much clear.
Unfortunately, I'm now having a massive reaction that I would normally attribute to a food allergy. But I haven't eaten anything that could trigger it, so I suspect that the big chunk of crud on the pad may have set off a contact dermatitis that is giving me the usual systemic effects that I get from sulfites: itching, blotchy skin and swelling. Wonderful.
I'm well doped up on Benadryl now and the swelling and itching are somewhat under control. *sigh*