@ Dream Buster
Your dream may just have been busted.
Airborne toxic exposure comes in 4 forms:
Gas, vapor, mist and fume.
The last two may very well be visible and the first two rarely so.
Sometimes an odor may be present, but not always.
Often a reasonable cost sensing device is just not available and they are they are usually not specific and may be pressure sensitive.
I could name at least 10 compounds that would set off a traditional carbon monoxide detector and yet pose no hazard, never mind a detector that is designed to detect more complex compounds.
If the compound is a fume or a mist an inexpensive collection device and a future lab analysis will usually work.
If the compound is a gas or a vapor real-time measurement MAY be an option, but in most cases they are cross sensitive to other compounds and often to the point that such interference could render them practically useless.
There are hopes though, nanotechnology sensors may become commercially available at a reasonable price in this decade.