Odors are ubiquitous in our daily lives. Some odors are pleasant (perfumes, flowers, etc.) resulting in a positive response from those experiencing them. Many odors are neutral—causing neither a positive nor a negative response from those in the area. And then there are the repugnant odors that disgust just about everyone.
All odors result from various volatile chemistries, from the very simple to the extremely complex, that stimulate the olfactory nerves in the nose. Many methods have been used in an effort to convert disagreeable odors to something more tolerable—odor masks which conceal the existing odor by overpowering it with a more pleasant odor; odoractants which chemically change the odor to a more agreeable scent (a la antiperspirants); odor neutralizers which chemically destroy the odor in one way or another; and odor dilution.
Noxious odors can be the result of biological activity or industrial activity. For example, the decomposition of dead vegetation or carcasses by microorganisms is biological. The burning of wood in a reduced oxygen atmosphere to produce charcoal is an industrial process. The fetid and putrid smell of the biological degradation process and the obnoxious odor of this industrial process are both generally perceived as repugnant by most people.
Much effort has been expended trying to measure and control unpleasant odors. However, odors are perceived differently by different people and cultures. This wide range of olfactory perception by people, along with the severe limitations of devices to measure odors, have made it extremely difficult to develop a quantifiable workable method to measure odors.