For some reason, a stale, musty, moldy smell seems to end up inhabiting whatever space I do. Around this same time, I usually get so congested I don't notice it, but when I do notice it, it is indeed bad and it worries me.

I want to know what it is.

It's happened at three places now: the house where I grew up (my mom constantly told me my room smelled like a dead animal), a college dorm room (my roommates actually complained about the unbearable smell - which didn't start until I moved in mid-year - to our RA, who called a meeting with me since I was the only one who didn't complain. I only noticed it once or twice, while my roommates were having a hard time sleeping because of it); and now I live alone and I've noticed it again, often when I come home, my sinuses are fairly clear and it hits me. I keep a window cracked at all times to alleviate it, and have purchased air freshener to cope.

But I want to know what's causing it. It's happened at three different places and I'm trying to find a cause that could happen at all of them. Most likely culprits are: shoes (my feet sweat a lot), laundry, trash, and food left out/plates not cleaned. I've tried remedying some of these things, and the only help a little. I even tried cleaning the apartment. It only helps a little.

This concerns me. It seems unlikely to be mold, and dust generally doesn't smell. Anyone have any idea what's causing such an embarrassing problem?