1. INTRODUCTION

Odour can be defined as the "perception of smell" or in scientific terms as "a sensation resulting from the reception of stimulus by the olfactory sensory system". Whether pleasant or unpleasant, odours are induced by inhaling air-borne volatile organics or inorganics.

With growing population, industrialization and urbanization, the odour problem has been assuming objectionable proportion. Urbanization without proper sanitation facilities is a major cause of odour problems. Rapidly growing industrialization has aggravated the problem through odorous industrial operations. Undesirable odours contribute to air quality concerns and affect human lifestyles. Odour is undoubtedly the most complex of all the air pollution problems.

Unlike conventional air pollutants, odour has distinctly different characteristics, which, to an extent, can be comparable with noise pollution. Similar to noise, nuisance is the primary effect of odour on people. Some of such characteristics are:

Substances of similar or dissimilar chemical constitution may have similar odours. Nature and strength of odour may change on dilution.

• Weak odours are not perceived in presence of strong odours.
• Odours of same strength blend to produce a combination in, which one or both may be unrecognizable.
• Constant intensity of odours causes an individual to quickly loose awareness of the sensation and only noticed when it varies in intensity.
• Fatigue for one odour may not affect the perception of dissimilar odours but will interfere with the perception of similar odours.
• An unfamiliar odour is more likely to cause complaint than a familiar one.
• Two or more odorous substances may cancel the smell of each other.
• Odours travel downwind.
• Person can smell at a distance.
• Many animals have keener sense of olfaction than man.

Likes and dislikes often depend on association of the scent with pleasant or unpleasant experiences.