I have tried everything I can think of and have not been able to determine the cause of a strange odor in the home we are renting. I'm hoping that with all the others on these forums, someone will be able to shed some light on what the cause of this odor is or on what type of expert I could hire to determine the cause. Details below:

We moved into this house in the late fall and never noticed any odors until the weather warmed up to over 60 or so. The odor is very hard to describe. It's something you smell at the back of your nose, if that makes sense. It has sort of a chemical smell to it, unpleasant, a little bit rotten. We've ruled out things like a gas leak, a sewer leak, a dead animal rotting somewhere, mold, garbage disposal, and rotting fruits or vegetables since it doesn't smell like any of those distinct smells. It doesn't seem to be related to the heating/AC system since running those things doesn't have any impact on the strength of the smell.

The house is a tri-level. Den and two entryways on the bottom level, kitchen and living room on the main level, and bedrooms on the top level. Flooring in the den and upstairs is carpet. Flooring on the main level is hardwood. Unfinished partial basement underneath the main level.

The odor is always very noticeable in the morning when the windows on the lower level have been closed for the night. It is strongest in the kitchen and in the lower level (entryways and den). There doesn't seem to be any one spot that smells especially strong (like a kitchen cupboard or the flooring on the stairs, etc.). It's a more generalized smell and you definitely notice it when you walk into that area but can't pinpoint exactly where. The basement doesn't have that odor, even when the main floor does. In the morning, I open all the windows and air the house out and the smell fades. It is then very mild or nonexistant until the next morning when it is once again strong.

The landlords (who are also the previous residents) said they never had an issue with odors when they lived here. They came over to smell it themselves and weren't able to shed any light on what it could be. I do think they are being honest, based on their actions and character so far. When the landlords bought the house, it had been used for low income housing and was a dump. However, it's been over six years since the house was cleaned and renovated and I don't think anything would linger that long and suddenly start causing an odor 6 years later.

I'd like to hire some sort of inspector to look at things but I'm not sure who could help with this. A gas guy would only have the tools to detect a gas leak. A mold guy would only be able to detect mold. Is there anyone who could detect a range of smells? We have looked at the pipes in the basement and don't notice anything amiss there.

I'm very concerned about what is causing this odor because it's so unusual and I worry that it could be harmful. We haven't been experiencing any symptoms that might be related to the odor but I do worry about what it could be doing to us long term. Moving out is something we will do eventually but can't do right now. Any ideas you can offer would be very much appreciated! Thank you!