I'll toss out the problems I'm having to see what people think.
My basement is mostly finished with one large carpeted room (about 2/3 of the square footage), a smaller unfinished room (where the water heater, heating, sump, etc. are) and a small powder room.
The stairs to the main floor are also carpeted with the same carpet. These stairs turn 90 degree at a landing about 2/3 up from the basement.
My basement is fairly dry, in fact the house is generally very dry, especially in the winter when the heat is on. There are vents in the basement ceiling connecting to the same ducts that feed heat to the first floor. I've had a dehumidifier there in the basement for about 3 months (the bucket fills about every 3-4 days) and an air purifier for a year. I also run a fan to move air around.
What I have been noticing for about 5-6 months or so now is around the top of the stairs down from the first floor and to a lesser extent the basement itself a kind of chlorine/bleach sort of smell. I see no sign of mold and don't notice a lot of dust there. I've recently changed the filters in the heating system.
Thoughts on what could be causing this?