Hi,
I really need help on this one. I have a basement room—finished with carpet and bath—but not used until I rented it out to a couple for the summer. In all honesty, I think it started to smell a little before they moved in, but it was only slightly noticeable now it smells like the worst BO ever. I burned incense, aired it out, put lavender on the carpet but nothing seems to help. I read on the internet that sometimes bad paint will smell like bad body odor. Please help… 
Basment?
Smelled a little before the tennants?
Now it’s worse…
I doubt it’s bad paint. Never heard of such a thing.
Perhaps a dead mouse in the wall. That would be my guess. It will go away as it dries out. It’s sort of icky, but that’s life.
How do I put this… Death and decaying rodents have a very distinctive smell. Not sure how to describe it. If you smell it and know it’s death, you don’t forget. It all smells the same.
It could be moisture, mold and such. Hope it’s not. Hope for a dead mouse in the wall.
Dead mouse was my first thought. When we first moved in, it became a problem when spring came. It was horrid. I was luck though. I found it behind the circuit breakers. The day I cleaned it out, it smelled better.
Could a cat be getting in there and peeing?
‘Worst BO ever’ sounds like it fits the description of a dead rat to me. Mice usually dry out in a few dats, but rats stink for longer. It’s a sort of rotten garbage/onion/BO smell.
Is the floor solid concrete, or is it boards? If the latter, there might be a dead animal in the void below it. Are there any large items of furniture that came with the room?
Bad Odor Driving Me Crazy… Nice Combo!!
Also…
And next on K-Rock 102 It’s Bad Odor by those wild Rockers Driving Me Crazy!
If it’s not a dead mouse but just musty, the route to go to get rid of endemic odours is Kill-z paint on the walls, and replace the carpets. Probably not what you wanted to hear, but if the odour is in your walls and in your carpet, it’s probably not coming out. We bought a smoking house by accident last year (we were told it was a non-smoking house, and it was so clean that we couldn’t tell it had been smoked in), and we’re looking at painting the house top to bottom with Kill-z to get the baked-in smoke smell out - closets, cupboards, everything. We’re lucky that there are no carpets in the house to replace.
If you live in a humid climate I would suggest a de-humidifier in the basement. The water vapor will hold in the scent. It might not eliminate the odor, but it should help.
Is there a floor drain?
Traps in drains keep sewer gas-----methane gas---- from migrating into the living space. That goes for all drains/traps; sinks, showers, tubs toilets and even floor drains.
Take a bucket of water and fill/prime all drains/traps.
ETA
If that room was finished by carpeting over a floor drain without sealing it you could have sewer gas migrating up through the carpet.
Can you determine where its coming from? Does it smell like sewer gas?
ETA to the ETA
This is a fairly common occurrence.
The water in the traps of unused drains-----usually floor drains------evaporate and a hellacious odor comes up from the sewer.
If the odor in question is sewer gas from a dry trap, “priming” the trap-----just filling it with water-----usually makes the odor go away quickly.
Many basements have plumbing fixtures/toilet that drain/dump into a basin below the basement slab, where a dedicated sewage pump ejects the sewage/effluent water up (yes UP) to the sewer pipe.
In 9/10 cases that I’ve seen, this basin is not vented to the outside via a vent pipe, because it is usually incredibly difficult to do so. Lacking this outside-run vent pipe, the sewage gases weep out of the basin and fill the air.
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where do the basement plumbing fixtures/toilet drain to?
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even if there is no basin and no sewage pump involved, the fixtures must be vented somewhere. If they aren’t breathing to an outside vent, it’s possible they are just breathing the sewer gases into the basement.
Do you have any wallboard there? Many people recently have had odor problems with wallboard manufactured in China. Lots of lawsuits!