What sort of "fumes" are you looking to detect.
Most inadvertently refer to gases and vapors as fumes, but fumes areparticulates.
Though real time measurement is available for fumes it can be utterlyexpensive.
Many gases and vapors are quite easy to detect either specifically or usingcross sensitivity of other types of sensors, but if any accuracy is requiredfrequent calibration must be done.
Some sensors are also pressure sensitive either statically or transientlymaking variable cabin pressures a conflicting issue.
If real time is not required a simple solution for some fumes would be touse an air sampling pump with a collection filter. The filter would have to bereplaced fairly frequently, but cost only a dollar or two. In an event takesplace and become a dispute the collection filter can be sent to a laboratoryfor analysis. This cannot be a catch all an will not pick up gases and vapors.
Again, what compound do you want to detect?