What makes a smell an odour...? Or is it a matter of taste?

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Hi guys (and gals) My question to the collective... Clients problem - nasty odor and breathing problems when ac is turned on. Client suspects 'sweaty sock syndrome'. (Sidebar - we really need to come up with better names in our industry: toxic/black mold; sweaty sock syndrome; etc... not exactly glamorous terms are they? Where's the PR...? ;->) But I digress... so I sample the air from the main duct immediately downstream of her coil. Mold is amost non-existant but I get almost 200,000 cfu/m3 of bacteria. 75% of which are B. cepacia. Hypothesis - nasty biofilm on coil/drain pan. But that aint my question. Had a team meeting this weekend at her house to discuss next steps. the ac had been acting up (stinky) again so she had turned it off a few days previous. While she had us there she turned it on again to let us smell it. Of the 4 of us there she was the only one who could smell

it. Which leads to my question - what exactly is an odor? I was under the assumption it would be a vapor (MVOC). But... I lent her a PAPR unit with a hood and when she wears it when she's having breathing problems from the ac her breathing/coughing problem clears up. Which suggests that shes reacting to a filterable particulate. But she can also no longer detect the odor when wearing the hood. So the odor is a filterable particle also. So is that possible? What, and how, do we actually smell? Is it possible that what she thinks she is smelling she is actually tasting but its registering as a smell? Taste and smell are closely linked... So does anyone know how odor detection actually works and is it particulate in nature or a vapour? I'm dying to know... Struart McCallum