Any substance with a vapor pressure has the potential of emitting odours. The human nose can detect about 5,000 smells out of the almost infinite number of odour possibilitieS. Certain odours do not register at all with some people. For example, only 65% can detect the principal odour in human sweat, androsterone. Most can readily detect certain other animal odours.
The ability to smell is a sense that rapidly becomes fatigued. A person working in a place contaminated with a malodour will become totally oblivious to it. Yet it could be highly offensive to someone entering the area. This is one of the reasons for the common attitude of “My dog/cat/carpet/kennel doesn’t smell. I don’t know what you’re talking about. We don’t have any odours!”
Our interest is in odours readily detected by most people, most of the time. How such odours are perceived varies from individual to individual. A given fragrance can be agreeable to some but objectionable to others. The detection of an odour is dependent on the type and concentration of the odorous molecules, the sensitivity of the individual’s nose and also the odour’s culturally conditioned interpretation. If a smell is always present people may get used to it and not notice it.